Fanworms: Yesterday, Today and Tomorrow

Sabellida Levinsen, 1883 is a large morphologically uniform group of sedentary annelids commonly known as fanworms. These annelids live in tubes made either of calcareous carbonate or mucus with agglutinated sediment. They share the presence of an anterior crown consisting of radioles and the division of the body into thorax and abdomen marked by a chaetal and fecal groove inversion. This study synthesises the current state of knowledge about the diversity of fanworms in the broad sense (morphological, ecological, species richness), the species occurrences in the different biogeographic regions, highlights latest surveys, provides guidelines for identification of members of each group, and describe novel methodologies for species delimitation. As some members of this group are well-known introduced pests, we address information about these species and their current invasive status. In addition, an overview of the current evolutionary hypothesis and history of the classification of members of Sabellida is presented. The main aim of this review is to highlight the knowledge gaps to stimulate research in those directions.


Introduction
Sabellida Levinsen, 1883 is a morphologically uniform clade of sedentary annelids historically given a rank of Order. Sabellida currently includes members of Fabriciidae Rioja, 1923, Sabellidae Latreille, 1825, and Serpulidae Rafinesque, 1815 [1][2][3]. They are commonly known as fanworms, feather-duster worms, or flowers of the sea, because their radioles are arranged in a crown, protruding from the tube made of calcium carbonate or mucus with agglutinated sediment (Figure 1). In addition to the presence of protective tube and the prostomial crown made of radioles with secondary ramifications (generally referred as pinnules, but see [4] for Fabriciidae), which are mainly used for feeding and respiration, all members of Sabellida share the presence of chaetal inversion [thoracic chaetigers with simple chaetae on notopodia and uncini (hooks) on neuropodia, and abdominal chaetigers with opposite arrangement]. Sabellida includes about 1200 species distributed world-wide, from tropical to polar waters and found in all habitats, from freshwater to fully marine conditions, and intertidal to deepest ocean trenches. With the advent of cladistic analyses, close relationships of sabellids and serpulids with Sabellariidae Johnston, 1865, Siboglinidae Caullery, 1914 and Oweniidae Rioja, 1917 have been suggested, and consequently the composition of Sabellida expanded to incorporate these three taxa (e.g., [13]) ( Figure 2). However, subsequent molecular analyses The Sabellida concept and even the group name has changed over time. Since their erection in the early 19th century, sabellids (including fabriciids), building soft sediment tubes, and serpulids, building calcareous tubes, have always been considered related, based on their general morphology and grouped into the section Amphitrites sabelliennes [5], the family Serpulacei [6], family Serpulacea [7][8][9], Serpulidae [10], and finally Sabellida [11,12].
With the advent of cladistic analyses, close relationships of sabellids and serpulids with Sabellariidae Johnston, 1865, Siboglinidae Caullery, 1914 and Oweniidae Rioja, 1917 have been suggested, and consequently the composition of Sabellida expanded to incorporate these three taxa (e.g., [13]) ( Figure 2). However, subsequent molecular analyses using increasing number of taxa and DNA markers revealed that the three late incorporated taxa were neither closely related to fanworms, nor to each other [14][15][16].

Sabellidae
Monophyly of Sabellidae is supported by the presence of dorsal and ventral lips, the presence of vacuolated cells supporting radioles and pinnules, and the dorsal fusion of the radiolar lobes (Table 1, Figure 4) [2,3,24,42,52]. Currently, monophyletic clades Sabellinae and Myxicolinae are recognized within Sabellidae, the latter is composed of Amphiglenini and Myxicolini [3]. The current composition of the Sabellinae (now Sabellidae) has not changed significantly since the early cladistic analyses [42,52], although nine genera have been erected since (Table 3), and Megalomma Johansson, 1925 was replaced by Acromegalomma Gil and Nishi, 2017 because the name was preoccupied by a group of carabid beetles. Fitzhugh [42] provided diagnoses of all genera accepted at the time, with their potential apomorphies. The most recent review of morphology and diagnostic features of genera and species identification is found in [24].   Table 3. Sabellid currently accepted genera that have gone through major or partial revisions and phylogenetic analyses (either considering morphological features, molecular data or both).

Serpulidae
Monophyly of Serpulidae is supported by the presence of calcareous tubes with complex ultrastructures, distinct from the simple structure found in calcareous tubes of the unique sabellid Glomerula piloseta (Perkins, 1991). The serpulid thorax is surrounded by the thoracic membranes, which are absent in sabellids and fabriciids. Most serpulids have an operculum (or several), a modification of the distal part of a radiole, acting as a plug when animals hide in their tubes (Table 1, Figure 5).
Within Clade A, further assessment of AI 'Serpula-group' (Serpula, Crucigera, Hydroides) [77] supported monophyly of Hydroides, but Serpula was recovered as paraphyletic basal grade and Crucigera was polyphyletic. Later studies assessed relationships within the largest serpulid genus Hydroides [78][79][80]. Within AII 'Spirobranchus-group' several studies examined relationships within the genus Spirobranchus [81][82][83][84] and demonstrated sister group relationship between brackish-water genus Ficopomatus and freshwater monotypic Marifugia Absolon and Hrabȇ, 1930 [46].  Within clade B, studies focused on Spirorbinae, classification of which is based [85] on the six distinct types of brooding, two opercular (Pileolariinis and Januini) and four tubular (Romanchellini, Paralaeospirini, Circeini, Spirorbini). It has been repeatedly argued that tube incubation precedes opercular brooding [85][86][87][88], but Thorp and Segrove [89] advocated for an ancestral opercular incubation. Results of the first morphology-based phylogenetic analysis of spirorbins [44] confirmed the ancestry of tube brooding, but suggested that the opercular brooding arose once and the brooding cup of Januini is a simplification of the brooding structure of Pileolariini. Another analysis of morphological data [90] confirmed that opercular brooding is derived, but suggested that the two types arose independently. No molecular spirorbin phylogeny is available to test this arrangement. As neither traditional Serpulinae, nor Filograninae are monophyletic, re-classification based on a comprehensive integrative analysis and re-formulation of the subfamily diagnoses are needed. Meanwhile Spirorbinae is accepted as nested within Serpulidae, but other serpulid genera are not assigned into subfamilies. Most serpulid genera have not been revised (Table 4). Table 4. Serpulid currently accepted genera that have undergone major or partial revisions and phylogenetic analyses (using morphological features, molecular data or both).

Genera
Number  (Table S1). This revised dataset differs from that of Pamungkas et al. [91], who listed 21 genera and 91 species, but erroneously counted the genera Eriographis Grube, 1850, Leiobranchus Quatrefages, 1850, Leptochone Claparède, 1870 and Tuba Renier, 1804, all of them already synonymised with the sabellid genus Myxicola (Fitzhugh 1989). Discrepancy in the number of species is due to synonymisations and new combinations, as recently updated in WoRMS.
The first fabriciid genus, Fabricia Blainville, 1828, was established to accommodate Tubularia stellaria Müller, 1774, a species with only 12 chaetigers and smaller than the sabellids described at that time [90]. In mid-19th century the genus Manayunkia was erected and Fabriciola was established only in the 20th century. In the second half of the 20th century, 11 genera were established and four genera were erected at the beginning of the 21st century ( Figure 6). It is remarkable that among the 17 currently valid Fabriciidae genera, eight were established by Fitzhugh alone (Bansella Fitzhugh, 2010, Brifacia Fitzhugh, 1998, Fabricinuda Fitzhugh, 1990a, Novafabricia Fitzhugh, 1990b, Parafabricia Fitzhugh, 1992, Pseudoaugeneriella, Pseudofabriciola and Raficiba Fitzhugh, 2001 or with collaborators (Echinofabricia and Rubifabriciola) [27].
According to Pamungkas et al. [91], Serpulidae comprises 576 species in 77 genera, as they mistakenly included several fossil taxon names in the count of extant species. Serpulidae now comprises 562 species in 69 genera, which includes 48 genera with 374 extant species of Serpulinae sensu lato and 23 genera with 188 extant species of Spirorbinae (Table S3). These numbers, however, do not include those of fossil taxa (180 species, in 53 genera), not considered here. Out of 69 species of the genus Spirorbis Daudin, 1800, 46 were described before Bailey [85] re-classified spirorbins according to the incubation methods and never revised, so some of them upon a revision can be re-assigned to other genera.

Taxonomists and Species Discovery
Kirk Fitzhugh is the most productive author in terms of the number of discovered fabriciid species, with 27 species described alone and four in collaborations. Other authorities of fabriciid species include Karl Banse, Gesa Hartmann-Schröder and Greg Rouse, with six species described by each, all as single authors (Table S1).
The most productive serpulid taxonomist is Harry ten Hove who so far described 49 species, including seven alone. Gottfried Pillai described 46 species, including 31 species described alone and 15 in collaboration with ten Hove. Phyllis and Wynn Knight-Jones described 41 species, mostly spirorbins. Katherine Bush described 33 species, includ-

Identification Keys, Diversity Assessments, and Recent Regional Taxonomic Studies
The key to all polychaete genera by Fauchald [12] includes all sabellids and serpulids considered valid at the time, but it is outdated and not recommended for taxonomic work anymore. A key to Fabriciidae genera recognized until 1998 was provided by Fitzhugh [57]. In that study, the currently accepted Echinofabricia was named Genus A. The most recently updated key to all fabriciid genera is that by Tovar-Hernández and Fitzhugh, in press. The keys to Sabellidae genera were provided by Fitzhugh [42], Tovar-Hernández [108] and most recently Tovar-Hernández and Fitzhugh [109]. The review by ten Hove and Kupriyanova [110] includes diagnoses and a key to all serpulid genera (excluding spirorbins) valid at the time.
Revision of literature reveals that intensive fieldwork and continuous taxonomic studies by a single scientist in a specific area have had a great impact in biodiversity knowledge of a region. However, large geographic regions have been scarcely studied, not only in difficult to access deep-sea environments, but even in the intertidal and subtidal zone either because not enough work has been put into taxonomic surveys, or members of Sabellida were not among targeted groups. It is expected that our understanding of species diversity will improve after efforts (financial, logistical, technological and taxonomic expertise) are devoted to fill those gaps. Herein, information about the most recent regional surveys and taxonomic revisions is provided, and the number of species described in these areas given as a rough approximation of their biodiversity knowledge.
The regions with the overall highest number of type localities of described species are the coastal areas of Europe, both coasts of North America, and Central and Western Indo-Pacific, while the areas with lower number of original descriptions are the majority of Africa, South America, as well as Tropical Eastern Pacific and Eastern Indo-Pacific ( Figure 7).
The current state of biodiversity knowledge and a summary of the most recent (defined here as last 20 years) comprehensive checklists, faunistic and taxonomic regional studies (excluding single species descriptions) are organised below by marine realms. References to main comprehensive taxonomic studies are provided as recommendations for getting started with faunas of each realm.

Arctic
This realm covers the Arctic Ocean down to Newfoundland in the western Atlantic, including the northern half of Iceland, northern Russia, from the White Sea to the Bering strait, and all northern Alaska and Canada. One fabriciid, 16 sabellid species, and 16 serpulids have been described from the Arctic, most of them from the Western sector of the Arctic Ocean ( Figure 7, Tables S1-S3). Knight-Jones et al. [111] reviewed species of Pseudopotamilla from Iceland, Greenland and the Canadian Arctic. Jirkov's book [112] on Arctic polychaetes, that includes diagnoses, illustrations, and keys to sabellids (including fabriciids) and serpulids (including spirorbins), as well as the recent comprehensive illustrated revisions with taxonomic keys to all Arctic Serpulidae (including Spirorbinae) by Rzhavsky et al. [113,114] are recommended for studies in this region.

Temperate Northern Atlantic
This realm is delimited in the north with the Arctic realm, and in the south reaches the coasts of Florida, including the northern half of the Gulf of Mexico, and is delimited in the east by the Cape Verde archipelago and the coasts of Mauritania. It also includes the Mediterranean and the Black Sea. Sixteen fabriciids, 124 sabellids, and 108 serpulids have been described from this realm (Figure 7). Of these, one fabriciid, 18 sabellid, and 12 serpulid species were from the Western coasts of the Atlantic Ocean and three sabellids were from northwest Africa. All the rest were described from European waters and the Mediterranean (Tables S1-S3).  The current state of biodiversity knowledge and a summary of the most recent (defined here as last 20 years) comprehensive checklists, faunistic and taxonomic regional studies (excluding single species descriptions) are organised below by marine realms. References to main comprehensive taxonomic studies are provided as recommendations for getting started with faunas of each realm. The book by Fauvel on sedentary polychaetes of France [115] provide descriptions and illustrations of the common species of the north eastern Atlantic and western Mediterranean and is still been widely used despite being outdated. Relatively more updated sources of serpulid diversity data in the Mediterranean are books by Zibrowius [116] and Bianchi [117] that include keys, descriptions and illustrations. The illustrated key by Knight-Jones [118] to the British Isles and North-West Europe is recommended as an initial source of data on Sabellidae, Fabriciidae and Serpulidae. The incoming book on Fauna Ibérica includes chapters on Fabriciidae and Sabellidae and will be a new standard reference for the region [119,120].
Despite the overall high number of species described from this region, only a few taxonomic and faunistic studies have been carried out on Sabellida in the last 20 years in the Atlantic provinces of this realm. In particular, fabriciids were studied by Bick [121], species of Euchone by Bick and Randel [122], Chone by Tovar-Hernández et al. [123,124] and Pseudopotamilla by Knight-Jones et al. [111]. However, the notable exception has been the Mediterranean, where much taxonomic activity, with particular emphasis on introduced species, has taken place recently. Some representative contributions dealing with Mediterranean fanworms include those by Çinar (on serpulids [125], on nonindigenous species [126]), the general annelid checklist of polychaetes from Turkey  [129], on Acromegalomma [130]), the general papers on annelid diversity of the Adriatic Sea [131,132] and the fabriciids and sabellids of the Adriatic [133], the checklist of Iberian species [134] and the most recent one by Tilic et al. [103] dealing with Amphiglena. A checklist of the polychaetes from the Black Sea includes three fabriciids, six sabellids, and 11 serpulids [135].

Temperate Northern Pacific
The western side of this realm is demarcated by the Bering Strait in the north, and an imaginary line from Taiwan to the south of Baja California Peninsula, in the south. The number of fabriciids described in this realm is 11, eight of them from the Eastern side (Table S1). The number of described sabellids is 83, 37 of which from the Western side, and 46 from the Eastern side (Table S2). Out of 92 serpulid species described from this region, 54 are from the western side (Table S3).
The catalogue of sedentary polychaetes from California [136], still widely used for the region, is outdated and is not recommended. The monograph of polychaetes of the Russian Far-East [136] and its English translation [137] that remains the main source of keys and information on polychaetes of the region, including Sabellida, is also outdated and thus should be used with caution.The revision of spirorbins from the east Pacific coast [137] is still the most recent source of information on this group.
The most recent literature-based annotated checklist of polychaetes from Pacific coasts of Russia lists 37 sabellids (including fabriciids) and 40 serpulids [138]. Sabellids from Japan were recently reported by Nishi et al. [95,139] and Yoshihara et al. [140]. Taxonomic studies on serpulids of Japan are summarized in the book by Imajima [141] that provides an illustrated key to 55 species of Serpulidae, but does not include sabellids. The most recent account of Chinese polychaetes [142] provides diagnoses and keys to 64 species of sabellids and 98 serpulid species, and is recommended as a source of biodiversity data and taxonomic keys for China. The complementary revision [143] includes a checklist of most annelid groups from the South China Sea, and lists three fabriciid, 33 sabellid and 72 serpulid species. Recent revisionary studies, including taxonomic keys, from the Pacific coast of North America have reported seven species of sabellids and more than 40 serpulids in the following contributions dealing with species of Hydroides from Northern Mexico [144], serpulids from the Eastern Pacific [145,146], sabellids and serpulids from northern Mexico [147,148], and sabellid Acromegalomma [149] and Notaulax species also from Northern Mexico [101].

Tropical Atlantic
This realm is delimited in the north by the Temperate Northern Atlantic realm and in the south by an imaginary line from Rio de Janeiro in the west to the southern border of Angola in the east. It also includes the southern half of the Gulf of Mexico and the Caribbean. Thirteen fabriciid species have been described from western side of the realm, specifically from the Caribbean, and none from the eastern Atlantic (Table S1). Out of 56 sabellids described in the region, only one came from the African coasts (Table S2) and of the 72 serpulids, only 10 were described from Africa (Table S3).
Recent studies of the Caribbean fanworms included those describing fabriciids [27,155]; revisions of species in Chone [156] and Branchiomma [157]; records and new species of sabellids [155,158]; and selected serpulids, such as Hydroides [159], Serpula and Spiraserpula [160]. The tropical coasts of South America have been scarcely studied. The checklist of polychaetes of Brazilian Tropical Atlantic region reports 11 sabellid and 24 serpulid species for the area [161][162][163][164], with no fabriciids registered so far. However, several of those are records of species described from Europe, North America and South Africa, demanding further study. Additionally, Amaral et al. [163] checklist treated many already synonymised species as valid.
The illustrated key of Sabellidae and Fabriciidae by Tovar-Hernández and Fitzhugh [105] includes all species currently known for the Caribbean, whereas Caribbean Serpulidae are available in [165].

Western Indo-Pacific
This large realm covers most of East coast of Africa, Madagascar, Arabian (Persian) Gulf, the Red Sea, shelf of Bay of Bengal and Andaman Sea. The number of species described in this realm is 93, including 11 fabriciids, 30 sabellids, and 52 serpulids (Figure 7, Tables S1-S3). Out of 52 serpulids, only eight were described from African coasts.
An influential book on Indian polychaetes [166], unfortunately, lists European species (and even illustrates specimens collected in France) and therefore, is not recommended as an identification tool beyond the generic level. The most recent checklist by Sivadas and Carvalho [167] includes two fabriciids, 11 sabellids and 34 serpulids from India and critically evaluated annelid species richness in the region, stressing that native species diversity of India is severely underestimated. The relatively large number of serpulids described from Sri Lanka is due to the intense work of Pillai [40,168,169]. These publications still remain as the only source of faunal information for that region.
An annotated literature-based checklist by Wehe and Fiege [170] is the best compilation of annelid diversity in the area surrounding the Arabian Peninsula. The most recent checklist of intertidal polychaetes of Kuwait by Al-Kadari et al. [171], based on newly collected material, reported seven species of Sabellidae and 12 Serpulidae. A monograph on Serpulidae from the Suez Canal by Ben-Eliahu and ten Hove [172] included 16 species. In the Red Sea, Perry at al. [81] reviewed of serpulids of the genus Spirobranchus and sabellids have not been studied since Knight-Jones [173].

Central Indo-Pacific
This realm comprising the largest number of ecoregions (40) includes part of the coast of South-East Asia, from Taiwan to Malaysia, down to Tropical Australia from Coral Bay, in the West, to Brisbane, in the East. The eastern boundary of this region is delimited by an imaginary line from Fiji up to the south of Japan. The realm includes the Coral Triangle recognized as the global centre of marine biodiversity [174], and fanworms are also diverse in this region. Ten fabriciids, 56 sabellids, and 74 serpulids have been described from this realm (Figure 7), mainly from the Philippines and tropical Australia (Tables S1-S3). Other than newly described species, 78 species of Sabellidae have been reported only from the Gulf of Thailand, Indonesian Archipelago and the Philippine Seas [98,169], and at least 11 taxa are awaiting formal description [175,176]. Serpulidae from Hong Kong were most recently revised by Sun et al. [177], who provided illustrations, diagnoses and taxonomic keys. Tropical Australian sabellids belonging to 12 genera have been documented in a series of recent studies [68,69,[72][73][74]178]. Serpulids from Kimberley (Western Australia) were revised by Pillai [106] and those from Lizard Island (Queensland) by Kupriyanova et al. [179], whereas the revision of the genus Hydroides in Australia [180] Diversity 2021, 13, 130 20 of 73 includes both tropical and temperate species. The most comprehensive treatment of Australian sabellids and serpulids is still the interactive key by Wilson et al. [181], but it is outdated in the light of the recent studies. The digital guide [182,183] allows distinguishing 38 native and non-indigenous species of Serpulidae and 14 Sabellidae from Australia, and includes a glossary with main diagnostic features for members of both groups.

Eastern Indo-Pacific
This small in terms of the coastline length Pacific realm includes Hawaii, Marshall, Gilbert and Ellis Islands, Central and Southeast Polynesia, Marquesas, Eastern Island, and the shelf around them. It hosts type localities of 18 fanworms, including two fabriciids, eight sabellids, and eight serpulids ( Figure 7, Tables S1-S3).
Fauna of this realm is poorly known beyond that of Hawaii. Out of eight serpulids described from this realm, five were described from Hawaii. The latest studies include those on serpulids of Hawaii (Hydroides [140], a key and records excluding spirorbins [145], records of 16 species with a key [142], and serpulids from Cross Seamounts in the Hawaiian chain [184]. Out of eight sabellids, five were described from Hawaii, and two species of Branchiomma were reported most recently [185]. A recent study of serpulids from atolls of Marshall Islands [186] reported 29 serpulids (including spirorbins). Small number of publications dealing with members of Sabellida highlights the need for taxonomic work in the area.

Tropical Eastern Pacific
This realm is delimited in the North by the Cape San Lucas, Baja California, by the northern border of Peru in the South and includes the Galapagos Islands in the West. No fabriciids have been described in this region (Table S1) and the number of described sabellids is 12, five of them originally reported from coast of Panama and the rest from further north ( Figure 7, Table S2). The number of described serpulids is 30, most of them from Galapagos, Panama and Mexico ( Figure 7, Table S3).
Recent studies of Sabellida from the region include those dedicated to Panamanian sabellids [187], some Acromegalomma [149] and Chone [124], and those focused on sabellids and serpulids from Mexico [145,148,188]. Three sabellids and two serpulid species introduced in the Galapagos Islands were reported by Keppel et al. [189].

Temperate South America
The realm covers both Pacific and Atlantic coasts of South America, from Peru to Rio de Janeiro, respectively. Only one fabriciid was described from this region (Table S1). Twenty-nine sabellids have been described (Figure 7), mainly from Chile and Argentina (Table S2), and 21 serpulids were described at a variety of localities from Brazil to Peru ( Figure 7, Table S3).
The publication by Zibrowius [150] remains the most comprehensive study on Brazilian serpulids in both Tropical Atlantic and Temperate South America realms. More recent publications reported three species of fabriciids, 29 of sabellids and 22 species of serpulids for the Brazilian part of the Temperate South America realm [163] and 27 species of serpulids and 17 sabellids for Argentina [190]. Tovar-Hernández et al. [99] studied sabellids mainly from Argentina and Chile. A key to Sabellidae and Serpulidae from continental Chile is available in [191].

Temperate South Africa
This realm includes the coastline of Namibia and South Africa as well as Amsterdam and St. Paul Islands. It hosts type localities of 38 species of Sabellida, including four fabriciids, 19 sabellids and 15 serpulids (Figure 7, Tables S1-S3). Of these, 30 have been described in the littoral zone in South Africa, indicating that less taxonomic effort has been devoted to other areas within this region. The recent papers re-described two serpulid species based on type material (e.g., [192,193]. Although South African polychaetes, including fanworms, were summarized in the influential book by Day [194], most of Sabellida included in the monograph are 'cosmopolitan species' with European type localities (e.g., of 27 serpulids only six have type localities in South Africa), so native Sabellida species diversity is severely underestimated. Spirorbins from South Africa have not been reviewed since studies of Knight-Jones [195] and Knight-Jones and Knight-Jones [196].

Temperate Australasia
The realm includes coasts of Southern Australia and New Zealand hosting type localities of 86 species of Sabellida. This number includes six fabriciids from Australia (Table S1), 37 sabellids (28 described from the Australia and nine from New Zealand (Table S2) and 41 serpulids (28 from Australia and 15 from New Zealand, Table S3) ( Figure 7).
Spirorbins from southern Australia have not been studied since they were reviewed by Knight-Jones et al. [197]. In New Zealand, spirorbins were studied by Vine [198], who reported 24 species, nine of them new to science, while a list of sabellids and serpulids was provided by Glasby and Read [199].
The most recent contributions from the region are descriptions of two spirorbin species from Kerguelen and Bouvet Islands [211]; species of Perkinsiana [212] and reports of 19 still undescribed sabellids from Falkland Islands [213],demonstrating the underestimated diversity. There is no contribution summarizing Sabellida species diversity of this region.

Diagnostic Characters and Techniques Used for Species Discrimination
Most species within Sabellida are characterised by a unique combination of morphological features. The most useful morphological characters used for Sabellidae species identification are summarised in Capa et al. [24]; for Fabriciidae, see Bick [4]; for Serpulidae, see ten Hove and Kupriyanova [110] and Kupriyanova et al. [214]. Since detailed information is provided in these thorough revisions, only succinct identification guidelines are given below for each family.

Shortcuts to Identification of Fabriciidae
The small body size (most species are less than 5 mm long) and the absence of significant diagnostic characters make the identification of Fabriciidae difficult. All fabriciid species possess a radiolar crown with three pairs of radioles (Figure 8a-c). The branches are formed by successive longitudinal splitting of the radioles. The symmetrical branching of the radioles leads to bi-pectinated radioles, as found in most genera (Figure 8c), whereas pectinated radioles result in asymmetrical branching (Figure 8b), as in Manayunkia and Monroika only [4]. Ventral filamentous appendages, present in some genera, are associated with the radiolar crown ( Figure 8b). These appendages have been described as nonvascularized (e.g., in all species of Fabriciola, Pseudofabricia Cantone, 1972 and Rubifabriciola) or vascularized (e.g., in all species of Augeneriella, Echinofabricia, Manayunkia, Monroika and Pseudoaugeneriella, and also some species of Fabricinuda and Pseudofabriciola) [57]. These appendages are branched only among species of Augeneriella.  Peristomial eyes (Figure 8f) are developed among most species of Fabriciidae, black in most fabriciids, or red, as in Echinofabricia and Rubifabriciola [27].
The presence of thoracic pseudospatulate and transitional (=pilose, after Jones 1974) chaetae is of taxonomic significance [4] (Figure 8g,k). However, the distribution of pseudospatulate chaetae is not consistent within the genera. These chaetae occur on The thoracic uncini are characterized by a long manubrium (homologous to handle in sabellids) and a main fang surmounted by a series of smaller teeth (Figure 8d). A slightly offset medium-sized tooth occurs between the large main fang and the smaller apical teeth in Augeneriella, Fabricia, Fabricinuda, Monroika, Novafabricia, Parafabricia, Pseudofabricia, Pseudoaugeneriella and some species of Pseudofabriciola [42]. The apical teeth can also be approximately of the same size in Echinofabricia, or may gradually decrease in size away from the main fang as in Fabriciola, Manayunkia and some species of Pseudofabriciola [57]. The abdominal uncini usually exhibit multiple rows of equal-sized teeth (Figure 8h). Only members of Novafabricia chilensis (Hartmann-Schröder, 1962) and N. gerdi (Hartmann-Schröder, 1974) have uncini with a single row of teeth.
The pygidium is triangular or bluntly rounded in most species, but has a ventral depression in Pseudofabriciola analis Fitzhugh, Giangrande and Simboura, 1994. A pair of black or dark brown pygidial eyes is present in most species of Fabriciidae (Figure 8j). They are red in Echinofabricia (disappearing after fixation) and Rubifabriciola (persisting after fixation), but are always absent in all species of Manayunkia and Monroika, Fabriciola parvus Rouse, 1993 and two undescribed deep-sea species [220]. Unique among members of Sabellida, emergent spicules are present in the epithelium of Echinofabricia species [27].

Shortcuts to Identification of Sabellidae
Sabellids are relatively easily to visually identify to the generic level because genera are provided with unique and conspicuous diagnostic features ( Figure 9). The diversity of radiolar eyes within members of Sabellidae is remarkable (e.g., Figure 9a), and the eye number, type and arrangement offer a very powerful taxonomic aid for genera and species identification [24,69,221]. The large compound eyes located in the tips of dorsal radioles are unequivocally characteristic of members of Acromegalomma, whereas the large single and bulging compound eyes arranged, in a longitudinal row on the outer margin of the radioles, are typical of members of Pseudopotamilla Bush, 1904 (Figure 9a). Anamobaea Krøyer, 1856 and Notaulax Tauber, 1879 are easily recognized due to the presence of long radiolar lobes with dorsal and ventral flanges. Other generic synapomorphies related to the radiolar morphology are the dichotomously branching radioles, found only in Schizobranchia Bush, 1905 and Eudistylia Bush, 1905 (most likely due to a regeneration processes [111,222]), and the external paired radiolar appendages, called stylodes, in members of Branchiomma (Figure 9e). Euchone is recognisable by the presence of a typical pre-pygidial depression with lateral wings, but this character is only visible in adults. Species of Claviramus Fitzhugh, 2002 have radiolar tips with expanded flanges, rolled inwards or bilobed, this feature is easily seen if radioles are complete. Potamethus Chamberlin, 1919 is recognizable due the very long collar (2-4 times the length of next thoracic segment). Other genera are recognised by unique traits, which are not evident to a naked eye and require optic aids. These are the typical companion chaetae of members of Parasabella, the absence of posterior peristomial ring collar in members of Amphiglena, or the presence of a broad, oblique glandular (clitellum-like) ring on third abdominal segment in Euchonoides. Internal structures, such as the rows of vacuolated cells supporting radioles, dorsal lips and radiolar appendages, are of taxonomic significance and often used for species discrimination [64,102,122,[223][224][225][226].

Serpulidae Diagnostic Characters
Within Serpulidae, body symmetry separates serpulins from spirorbins, as serpulins are bilaterally symmetrical, while spirorbins are curved in the direction of the tube coil. Specific identification has been based on a combination of characters such as morphology of the operculum and opercular peduncle, degree of development of the collar and thoracic membranes, and chaetal structures ( Figure 10). Tube morphology and ultrastructure are important for identification of both extant and fossil taxa [227] (Figure 10a-c). Serpulid genera have been described on the basis of unique characters or on unique combinations of characters (or absence of characters), rather than on presence of shared derived characters. Mentioned here morphological characters used for serpulid identification have been described in details and illustrated in ten Hove and Kupriyanova [110], Wong et al. [228], Kupriyanova et al. [214].
The operculum that is present in most serpulins and in all spirorbins is one of the most important diagnostic characters (Figure 10d-h). The shape of the operculum varies significantly, ranging from soft transparent vesicles to complex structures reinforced with chitinous or calcareous endplates and spines. The distinct funnel-shaped opercula of Crucigera and Serpula are composed of numerous radii (Figure 10d), while in Hydroides the funnel is topped with a verticil of chitinous spines (Figure 10e). The operculum-bearing radiole can be identical to others (e.g., Filograna, Apomatus Philippi, 1844), but usually is modified into a smooth peduncle (Figure 10j). In cross-section, the peduncle is commonly cylindrical, but it is flat ribbon-like in members of the genus Metavermilia Bush, 1905. Below the operculum, the peduncle may bear diagnostic distal wings (e.g., Spirobranchus) ( Figure 10h).

Data and Techniques Used for Species Identification and Systematics Initial Collection, Observation and Fixation in the Field
To identify individuals to the species level, specimens are first examined under a stereomicroscope. If conditions in the field permit, they should be examined and photographed alive to document colouration. Removing individuals from their tubes, specially serpulids, without any tube or specimen damage is rarely possible unless 0.05% phenol-seawater solution is used for several hours [229][230][231]. This method does not work in spirorbins (Bick pers. obs.) and it is unknown whether DNA is affected by phenol. It is important to examine and photograph intact tubes if they are broken to extract animals.
Fixation and preservation of specimens vary depending on further purposes of samples. Specimens aimed for a morphological study only are commonly fixed in a 4% solution of formaldehyde in sea water for 24-48 h, if possible after relaxation of individuals (in magnesium chloride). Animals are then rinsed in distilled water and preserved in 70-80% ethanol. For scanning electron microscopy (SEM) osmium tetraoxide is preferable as a fixative.
Specimens aimed for genetic sequencing should avoid all contact with formaldehyde as it degrades DNA, impeding amplification of the usual size fragments to be sequenced. Best procedures for DNA sequencing include fixation of fresh specimens in high concentration ethanol (the higher the better) and storing samples at 4-6 • C, or at least not in direct sunlight at room temperature. The ethanol should be changed at least once, preferably more often. RNA sequencing may need other protocols such as fixing with RNAlater. For integrative (morphological and molecular) studies, a tissue sample taken from a specimen should be fixed in ethanol and stored in a fridge or a freezer, while the rest of the specimen should be fixed in formalin and preserved in ethanol. Further reading on fixation and preservation of samples is found at Rouse and Pleijel [232].

Morphological Studies of Preserved Specimens
When examining freshly preserved or museum material lacking natural pigmentation, staining with methyl (or methylene) green (blue) helps to increase contrast and to reveal glandular patterns, including thoracic ventral shields and glandular girdles [24]. Examination of chaetae requires higher magnification (>100x), therefore, chaetae and notoand neuropodia are dissected from the specimen, placed on a slide in a drop of ethanol, glycerin or permanent media, and covered with a cover glass. Applying gentle pressure on the cover glass ensures that uncini and chaetae lay in a lateral position. Using SEM is essential to reveal details of external features, such as ciliation, chaetal morphology and body wall microstructure, as well as anatomical features not easily distinguished in small specimens (appendages of the radiolar crown, for example). SEM is also an indispensable tool to examine tube ultrastructure, important for taxonomy of serpulids (reviewed by Ippolitov et al. [227]).
Structures of the radiolar crown, e.g., the rows of vacuolated cells supporting the radioles, dorsal and ventral lips, dorsal radiolar and pinnular as well as ventral radiolar appendages, and a parallel lamella in Sabellidae, dorsal lips and ventral filamentous appendages in Fabriciidae, are examined after fine sections with a sharp blade or, better, histological semi-thin sections are made and mounted on temporary or permanent slides, and stained with solutions, such as Mallory or Cason (e.g., [226]).
Morphometric characters such as counts (e.g., numbers of radioles or pinnules), measurements and proportions of soft body parts (e.g., thorax to abdomen ratio, length of dorsal lips) have traditionally been considered diagnostic for some taxa, but individuals may show sexual dimorphism [216], size and age-related variability ( [70,233] or their size may be affected by anaesthetization and fixation techniques [234], which needs to be considered when comparing material.
For drawings to be made to scale, a camera Lucida attached to both the stereo-and the compound scopes is used. Traditional ink drawing (pencil sketching followed by India ink tracing) is currently supplemented or replaced by digital tracing of scanned pencil sketches, using a drawing pad (e.g., [235]). Advances in digital photography and universal availability of microscope-mounted digital cameras and Z-stacking software also resulted in photographs of live or preserved specimens, rather than line drawings being used in species descriptions. Use of SEM micrographs helps to illustrate both chaetal and soft body diagnostic features characters with precision and objectivity. Micro-computed tomography techniques have been proved useful in studies of internal anatomy in sabellids and serpulids ( [49,236,237], and may offer taxonomically useful information.

Genetic Data
Genetic methods have been used in studies of Sabellida for nearly 20 years. The earliest publication by Patty et al. [238] used the C1 regions of 28S (123 bp) of 16 species to assess evolutionary relationships among Sabellidae. Kupriyanova et al. [45] published the first phylogeny of Serpulidae based on analyses of 18S rDNA, 28S rDNA, and morphological characters of 29 taxa. Combination of molecular (18S rDNA, the D1 region of 28S rDNA, and histone H3) and morphological datasets have been used to assess for the first time the relationships within Fabriciidae [27]. Other studies of Sabellida have used a limited number of molecular markers commonly used in polychaete systematics [239]. Currently, the number of sequences in GenBank is 246 for Fabriciidae, 814 for Sabellidae, and 2880 for Serpulidae, figures that indicate the relative larger effort put into molecular studies in serpulids compared to the other two families (Figure 11). In addition, "BOLD Systems [36] includes 443 barcodes (fragments of COI gene) of Serpulidae, 349 of these records are mined from GenBank and 19 diferent BOLD records are also shared with GenBank. Similarly, BOLD holds 692 COI sabellid barcodes, 399 of which are mined from GenBank and 148 BOLD records that were also uploaded in GenBank. Among the unidentified 105 sabellids there is an unknown number of sequences that belong to fabriciids as this database still follows the old clasitication. Sumarising, there are 75 serpulid and 145 sabellid/fabriciid COI sequences available in BOLD, in addition to those found in GenBank. Diversity 2021, 13, x 31 of 74 Figure 11. Number of sequences available in Genbank for Fabriciidae, Sabellidae; and Serpulidae. Figure 11. Number of sequences available in Genbank for Fabriciidae, Sabellidae; and Serpulidae.
The universal DNA barcoding fragment of COI gene is by far the most popular marker for the Fabriciidae, accounting for 54% (133 sequences) of the sequences available for this group (Figure 11), and Sabellidae, 38% (387 sequences) ( Figure 11). For Serpulidae, despite all efforts (reviewed in Sun et al. [78]), no COI sequences had been available until Carr et al. [240] reported six. However, Sun et al. [76], who developed genus-specific primers to generate COI sequences for 11 species of Hydroides, showed that "serpulid" sequences in Carr et al. [240] are likely from bacteria. Progress in COI barcoding in serpulids is mainly a result of new primer development [77]. Currently, the number of COI sequences for members of Serpulidae in Genbank is 564 (not including those mentioned problematic sequences [240], Figure 11).
Manayunkia is the fabriciid genus with most sequences in GenBank (179). The Sabellidae genera with most available sequences are Branchiomma (259), followed by Sabella (86) and Sabellastarte (61). Among the serpulids, Hydroides is the genus with the highest number of sequences in GenBank (1366), followed by Spirobranchus (601); all the rest have at least one order of magnitude less sequences available ( Figure 12).
Although Sabellida are still behind other annelids in terms of genomic approach, several studies recently reported mitochondrial genomes and used transcriptomes for resolving systematics and evolutionary questions within this group. The first Sabellida mitochondrial genome was published for the serpulid Spirobranchus giganteus (Pallas, 1776) [241]. Mitochondrial genome sequences of ten Hydroides species have been reported in Sun et al. [242]. The mitochondrial genomes of Sabella spallanzanii (Gmelin, 1791) and freshwater fabriciid Manayunkia occidentalis were recently published [243,244]. The phylogeny by Tilic et al. [3] includes transcriptome sequences of 20 species of Sabellida (three fabriciids, 15 sabellids, and two serpulids), containing up to 3015 orthologous genes. Several other studies have also dealt with expressed sequence tag (EST) libraries and transcriptomes to address molecular mechanisms of larval settlement, or gene order and loss [245] in serpulids, adding up to 4205 sequences of mRNAs of larval cDNA library. With current fast adoption of the genomic approach, the number of sequences is expected to raise dramatically in the near future.

Species Delimitation and Identification
Application of molecular methods in combination with traditional morphological techniques or alone have expanded in the last two decades with regard to species delimitation in annelids in general, and in Sabellida, in particular. These methods are sustained by the definition of species as independently evolving entities (metapopulations), that are genetically (and often phenotypically) distinct [246,247]. Thus, species are expected to be reciprocally monophyletic clusters, morphologically distinct and/or genetically divergent, as a result of evolutionary forces applied to closely related lineages. Molecular-based approaches have not only improved species delimitation by providing additional evidence to morphological taxonomy, but also helped to reveal cryptic (only genetically distinct) species [248].
In Fabriciidae, even though species appear morphologically similar due to the animal's small size and the diagnostic features being difficult to recognize by non-specialists, molecular approach to species identification and delimitation is still uncommon. Only one species, Manayunkia occidentalis, has been described based mainly on genetic data [216].
In Sabellidae, boundaries between species within the genera Amphiglena, Branchiomma, Parasabella, Pseudobranchiomma Jones, 1962, Sabellastarte and Sabellomma Nogueira, Fitzhugh and Rossi, 2010 were assessed with molecular and morphological data [70,[72][73][74]. Results revealed cryptic diversity hidden in species complexes and helped to assess the diagnostic features traditionally used for morphological species identification [2,27,[72][73][74]103,140]. Although Sabellida are still behind other annelids in terms of genomic approach, several studies recently reported mitochondrial genomes and used transcriptomes for resolving systematics and evolutionary questions within this group. The first Sabellida mitochondrial genome was published for the serpulid Spirobranchus giganteus (Pallas, 1776) In Serpulidae, the study of Halt et al. [249] was the first to name a new species in Sabellida without morphological indicators, after analyses of DNA revealed two cryptic species with non-overlapping distributions within Galeolaria caespitosa Lamarck, 1818. Another study revealed three genetic species with overlapping distributions within Ficopomatus enigmaticus (Fauvel, 1923), two cryptic and one morphologically distinct [204]. A combination of molecular and morphological data helped to partially resolve species complexes within the genera Hydroides [203,250,251] and Spirobranchus [84,193,252].
The idea of 'DNA barcoding' is that a species can be uniquely characterised by a short DNA fragment and then identified by comparing such a fragment from an unknown specimen to a reference DNA sequence [253]. Initially, a 650 base pair fragment of the mitochondrial COI was proposed as a standard barcoding gene for animals [253] because of its variability among closely related taxa and supposed ease of amplification. Later, however, a number of other mitochondrial markers (cytb, e.g., [81][82][83][84]193,204,249,252] and nuclear (ITS, [82,249]) have been used, especially in serpulids where amplification of COI proved to be challenging. The first attempts to use DNA data alone while ignoring any morphological and biogeographic evidence to identify potentially invasive serpulids [254], were rather a failure. The authors mistakenly claimed discovery of an Australian species Spirobranchus taeniatus (Lamarck, 1818) (mostly likely Spirobranchus triqueter) and North American Serpula columbiana Johnson, 1901 (almost certainly Serpula vermicularis) attached to drifting marine litter in the Mediterranean, after comparing (minimum 97% nucleotide identity was accepted) partial sequences of the conservative (thus unsuitable for specieslevel barcoding) 18S gene with the limited set of sequences available in GenBank. Similarly, Langeneck et al. [255] criticised another paper by the same authors (Rech et al. [256]) who identified specimens associated with floating debris in the Lagoon of Venice as Hydroides sanctaecrucis, suggesting its presence in the Mediterranean might have been overlooked due to misidentification as common H. dianthus. Rech et al. [256] again did not examine the morphology of the specimens and used 18S sequences for identification, accepting an identity ≥97% with sequences of H. sanctaecrucis in GenBank. However, 18S rDNA sequences are ill-suited for molecular identification because they have identity close to 100% in closely related species. The low sequence identity shows that the specimens in Rech et al. [256] study did not belong to H. sanctaecrucis, and the species name should be removed from checklists of species non-indigenous for the Mediterranean [255].

Ecology Fabriciidae
A review of the ecology and biology of Fabriciidae was published recently [4]. Most fabriciids occur in intertidal and subtidal zones, mainly in sheltered areas on sandy, muddy or rocky sediments, in mangroves, on red and green algal mats, and in seagrass beds, with low benthic species richness.
Fabriciids are mainly distributed in marine and brackish ecosystems worldwide, but species of the genus Manayunkia are also common in freshwater, and even hypersaline lakes, where they survive salinities of 82 psu for several months [257]. The abundance of some species tends to be very high in habitats with low biodiversity. The highest abundances of Fabricia stellaris and Manayunkia aestuarina (over 10 6 ind. m −2 ) have been reported in physiologically stressful conditions, such as sediments with a high organic matter content and waters of highly variable salinities [258][259][260][261]. A reduction in organic matter content from 1.8% to 1.0% in the Baltic Sea was followed by a reduction in fabriciid abundance from 16 000 to 6000 ind. m −2 [262]. Giangrande et al. [263] found five fabriciid species in a coastal Mediterranean system naturally acidified by carbon dioxide vent emissions. Among these, Parafabricia mazzellae Giangrande, Gambi, Micheli and Kroeker, 2014 and Brifacia aragonensis Giangrande, Gambi, Micheli and Kroeker, 2014 were most abundant even in the extremely low pH zone (pH 6.6-7.2).
Fabriciidae species produce flexible tubes consisting of the finest sediment particles stabilized by mucus. Detritus might also be deposited on the outside of the tubes. Fabriciids are not obligatory tube dwellers and they can voluntarily leave their tubes and build new ones. When outside, they crawl with the posterior end in front, while the radiolar crown is folded up and dragged behind [264,265].
Fabriciids are suspension-feeders like other Sabellida, but Manayunkia spp. are deposit feeders. In addition to detritus, they ingest bacteria, heterotrophic protozoa, cyanophyceans and diatoms. The sizes of the ingested particles range from 1-2 µm to 2-7 µm and occasionally reach up to 20 µm [265].

Sabellidae
A review of ecology and biology of sabellids was recently published [24]. Sabellids are able to inhabit either hard or soft sediments. Species of Amphiglena, Bispira, Perkinsiana, Pseudobranchiomma, Sabellomma, Sabellastarte and Stylomma Knight-Jones, 1997 mainly inhabit littoral hard substrates, as epibionts of algae, or associated with biogenic structures, including live coral or rubble [2,68,73,74,269,270]. Some species of Perkinsiana and Pseudopotamilla associated with dead coral and limestone sediments are capable of actively boring into the calcium carbonate [271,272].
Sabellids have been only recently found in chemosynthesis-based environments, such as hydrothermal vents, methane seeps and organic falls, and their diversity in such habitats is poorly understood. Bispira wireni (Johansson, 1922) was reported from a hydrothermal vent from Okinawa, Japan [280]. Jasmineira sp. and an undetermined sabellid colonized bone and sunken wood in the southwestern Indian Ridge [281]. Unidentified sabellids have been reported from methane seeps in the Gulf of Mexico [282] and Chile [283]. Recently, an undescribed species of Bispira was found at a deep-sea cold seep off the Pacific coast of Costa Rica [284].
With the exception of Glomerula piloseta, that inhabits calcareous tubes, all sabellids build tubes by secreting the mucous base and enforcing it with different size particles they attach, including mud, sand, feces or biogenic fragments [24]. Smaller species are more liable to leave the tubes if disturbed and can build new ones [295], but larger species, even if capable to build new tubes, tend to inhabit the same one for longer periods or their whole lives [270,296,297].
Until very recently, sabellids have been found mostly in areas of high productivity and assumed to be obligatory suspension feeders [298]. However, a sabellid-bacterial symbiosis, fueled by methane, between a still undescribed species of Bispira and methane-oxidizing Methylococcales bacteria, has recently been reported from a methane seep [284]. This makes Bispira a new addition to the list of annelids (including Siboglinidae and two new serpulids of the genus Laminatubus, see below) relaying on chemosynthetic symbionts for nutrition.
Associations of sabellids with other organisms relate to their ability to bore into calcium carbonate. The seven species of Caobangia are commensals or parasites of freshwater gastropods and bivalves in rivers of southeastern Asia [289,299]. Terebrasabella heterouncinata Fitzhugh and Rouse, 1999 bores into the shells of marine gastropods, including abalones and limpets, in South Africa and California [66,300,301]. Notaulax montiporicola Tovar-Hernández and ten Hove, 2020 associated with the living coral Montipora nodosa (Dana, 1846) does not bore into coral, but uses crevices to settle and allows coral tissue to grow around its tube [101].

Serpulidae
A review of ecology and biology of serpulids was recently published [214]. Serpulids are typical on hard substrates in all marine environments. Inhabitants of areas with predominantly soft-sediments always attach to rocky outcrops, stones and shells, and can deal with high sedimentation rate by building their tubes upwards to avoid being buried in the sediment [302]. Many serpulids are notorious opportunistic foulers, capable of colonising any available hard substrates. The ability to settle and build large aggregations on human-made surfaces makes serpulids important and troublesome members of fouling communities. However, some show high habitat selectivity, resulting from non-random larval settlement and juvenile survival (reviewed by Kupriyanova et al. [303]). A few unusual serpulids are pre-adapted to living unattached on soft substrates in subtidalshelf (Ditrupa) [110] or bathyal-abyssal (Bathyditrupa and Spirodiscus) environments [107]. However, larvae of free-living Ditrupa need to attach initially to small particles during settlement and metamorphosis to start building the tube [304].
Serpulids are some of recognizable animals to inhabit the periphery of seeps and hydrothermal vents. Laminatubus alvini ten Hove and Zibrowius, 1986 and Protis hydrothermica ten Hove and Zibrowius, 1986 were the first serpulids to be formally described from vent communities of East Pacific Rise and Laminatubus is known from other seeps (e.g., Pescadero Transform Fault, Gulf of California) and vents, e.g., Alarcon Rise, Gulf of California [305]. Hyalopomatus mironovi  and Protis sp. were reported from hydrothermal vents of North Fiji [306]. Less is known about seep-associated serpulids, as more species have been reported from fossil than modern hydrocarbon seeps [307]. Serpulids (tentatively identified as members of Neovermilia Day, 1961) have been reported from cold seep communities in Nankai Trough [308], the Peruvian active margin [309], the Terevaka ridge [310], the Peru Trench, Middle American Trench off Mexico [311], and the Barbados prism [312].
Although serpulids are predominately marine, some species of Hydroides tolerate mixohaline conditions (e.g., [313], for H. elegans (Haswell, 1883)), while representatives of Ficopomatus can cope with a wide range of salinities and are common in brackish-water environments world-wide [110]. Marifugia cavatica Absolon and Hrabȇ, 1930, closely related to Ficopomatus, is the only known truly fresh-water serpulid, inhabitant of subterranean caves of the Dinaric Alps [46].
Serpulids produce their calcareous tubes using a pair of calcium carbonate secreting glands located on the collar. As obligate tube dwellers, they never leave their tubes and cannot build new ones if removed. Adult serpulids lie in the tube with their dorsum facing the substrate and locomotion is limited to partial emergence from and withdrawal into the tube [314]. Hiding behaviour is a common antipredator tactic, and animals may adjust the durations of such behaviour to current benefits and costs [315].
As suspension-feeders, serpulids show varying abilities in particle sorting and clearance rates. High planktonic biomass removal (>50% of initial standing stock) and significant differences in clearance for different components of the community by reef-building Ficopomatus enigmaticus indicate that the serpulid can regulate planktonic biomass and promote changes in plankton community structure [316]. Ditrupa arietina (Müller, 1776) lives unattached in soft sediments and ingests diatoms, haptophytes, bacteria and cyanobacteria ranging from 1 to 50 µm in size, and the origin of the food is both planktonic and benthic [317]. Recently metanotrophy (similar to that found for members of Bispira sp.) has been reported, as a result of symbiosis between two species of the genus Laminatubus (L. joicebrooskae Rouse and Kupriyanova, 2021 and L. paulbrooksi Rouse and Kupriyanova, 2021) and methane-oxidizing Methylococcales bacteria, from a seep off Costa Rica [284].
While a large number of serpulids is found in coral reefs on coral rubble (e.g., [318]: Fiji, [319]: Okinawa, Japan, [179]: Queensland, Australia), only some taxa, such as Floriprotis Uchida, 1978, Pseudovermilia, Spirobranchus and Vermiliopsis spp., are found in association with live corals. Some Spirobranchus species are reported as obligate associates of corals to the extent that their successful settlement occurs only on live corals (e.g., [320][321][322]), although recent observations indicate that while Spirobranchus larvae have a preference for live corals, they will survive on other substrates [83]. Many serpulids form epizootic associations with other invertebrates, mostly molluscs, crustaceans, bryozoans, and sponges. For example, Hydroides spongicola Benedict, 1887 occurs symbiotically in the chemically aggressive do-not-touch-me sponge Neofibularia nolitangere (Duchassin and Michelotti, 1864), while Circeis paguri Knight-Jones and Knight-Jones, 1977 is associated with hermit crabs (reviewed in [303]). Spirorbins are commonly found in specific epiphytic associations with macrophytes and their settlement can be stimulated by algal extracts [323,324].

Biogeography, Distribution and Bathymetry
Members of the Sabellida are found world-wide and, like most polychaetes, for much of the 20th century were assumed to have naturally wide, even cosmopolitan, distributions (e.g., [166,194,325]). Darling and Carlton [326] use the term eucosmopolitan to refer to the species with naturally broad distribution (found in two or more oceans). However, recent studies overwhelmingly show that 'cosmopolitan' taxa represent complexes of either morphospecies or cryptic species (reviewed in [327]). All evidence to date suggests that polychaetes have restricted natural geographic and bathymetric distributions, thus taxa reported with wide ranges should be treated as potential species complexes. Two general exceptions to the rule of restricted distributions are deep-sea and invasive species. Ranges of deep-sea polychaetes are expected to be wider than those found in shallow seas as a result of stable environmental conditions over wide distances, a traditional view (e.g., [328]) also supported by recent studies (e.g., [329,330]). Annelids that are easily translocated by anthropogenic means can establish and become invasive in remote localities, consequently expanding their ranges [327]).
Biodiversity patterns may be influenced not only by intrinsic ecological and historical factors, but also by 'extrinsic factors' sensu Giangrande and Licciano [331]. When a group is studied by a few specialists working in a particular area, species distribution may correlate with that of the specialists ('author effect'). The concentration of taxonomic expertise in some regions may increase the number of species in those areas compared to less studied areas ('concentration effect').

Fabriciidae
Fabriciids have been described from all marine realms except for the Tropical Eastern Pacific (Table S1). Temperate Northern Atlantic is the province with the highest number of fabriciids (16 species), followed by Tropical Atlantic (13 species), Temperate Northern Pacific and Western Indo-Pacific (11 species each), Central Indo-Pacific (10 species) and Temperate Australasia (six species), Temperate Southern Africa (four species), Eastern Indo-Pacific (two species), and Actic, Southern Ocean, and Temperate Southern America, each with a single species (Table S1). Most of the 4739 georeferenced occurrences in GBIF are also from the Temperate North Atlantic Realm and the Tropical Atlantic ( Figure 13) and refer to members of Fabricia, Fabricinuda and Manayunkia [34]. There are records from neither Tropical Eastern Pacific nor from Western Indo-Pacific, the west coast of South America, and the west coast of Africa. The historical records until 1999, compiled by Giangrande and Licciano [331], showed 55.8% of the total fabriciid species are found in the tropics. Interestingly, the currently available information, based on species type localities show sthat most species were described from the Atlantic Ocean, at all latitudes. Explanation to latest results could be 'concentration effect', with taxonomic expertise accumulated at both sides of the Atlantic, and poor state of knowledge elsewhere. show sthat most species were described from the Atlantic Ocean, at all latitudes. Explanation to latest results could be 'concentration effect', with taxonomic expertise accumulated at both sides of the Atlantic, and poor state of knowledge elsewhere. The species of most genera are distributed almost worldwide. The adaptability of Fabriciidae to different environments and the wide distribution of some taxa can be shown by the example of the genus Manayunkia. The ten extant Manayunkia species occur worldwide in marine, brackish and freshwater habitats, as well as hypersaline lakes. Their common ancestor was most likely already present in marine habitats [4]. There are ten species adapted to freshwater conditions: M. speciosa and M. occidentalis (Nearctic); M. zenkewitschii, M. baicalensis (Nusbaum, 1901) and M. godlewskii (Palearctic) [216,217]. One species, M. athalassia Hutchings, Dekker and Geddes, 1981, was found in hypersaline lakes in Australia [257], another species, M. mizu Rouse, 1996, in marine habitats [219,332] and three species, M. aestuarina, M. caspica Annenkova, 1928), M. brasiliensis Banse, 1956, in brackish environments [333,334] (Nogueira pers. obs.). The species of most genera are distributed almost worldwide. The adaptability of Fabriciidae to different environments and the wide distribution of some taxa can be shown by the example of the genus Manayunkia. The ten extant Manayunkia species occur worldwide in marine, brackish and freshwater habitats, as well as hypersaline lakes. Their common ancestor was most likely already present in marine habitats [4]. There are ten species adapted to freshwater conditions: M. speciosa and M. occidentalis (Nearctic); M. zenkewitschii, M. baicalensis (Nusbaum, 1901) and M. godlewskii (Palearctic) [216,217]. One species, M. athalassia Hutchings, Dekker and Geddes, 1981, was found in hypersaline lakes in Australia [257], another species, M. mizu Rouse, 1996, in marine habitats [219,332] and three species, M. aestuarina, M. caspica Annenkova, 1928), M. brasiliensis Banse, 1956, in brackish environments [333,334] (Nogueira pers. obs.).
Most Fabriciidae occur in intertidal and subtidal waters. Only certain Pseudofabriciola species (e.g., P. californica Fitzhugh, 1991, P. filamentosa (Day, 1963), P. filaris Fitzhugh, 2002, and P. longipyga Fitzhugh, Giangrande and Simboura, 1994), Fabricinuda longilabrum  and Raficiba barryi occur between depths of 50 m and 335 m [53,58,63,335]. The record of Fabricia sabella (Ehrenberg, 1937) reported by Hartman (1965) from 1000 m off New England needs a revision because the nominal F. stellaris is a brackish-water species from the Baltic Sea. In the southwest Atlantic, exceptionally, two yet undescribed species provisionally assigned to the genera Fabriciola and Novafabricia have been found at 4500 m [220] and one other of Fabriciola from the Okhotsk Sea below 2000 m ( Table 5).

Sabellidae
Sabellids have been described from all marine realms and all seven members of Caobangia are known so far are from the Indo-Malay limnic realm. In the current analysis, the Temperate Northern Atlantic is the realm with the highest number of sabellids described (125 species), which represents 24% of the sabellids described worldwide, followed by the Temperate Northern Pacific (79 species, 15%), Central Indo-Pacific (58 species, 11%), Tropical Atlantic (56 species, 11%), Temperate Australasia (37), Western Indo-Pacific (20 species), Temperate South America (28 species), Southern Ocean (24 species), Temperate Southern Africa (18), Arctic (16 species), Tropical Eastern Pacific (15 species), Eastern Indo-Pacific (8), 23 species non-marine and eight species with unknown type locality (Table S2). Most of the 117 073 georeferenced occurrences in GBIF are from the Temperate Northern Atlantic and the Tropical Atlantic realms, and records belong to members of genera Euchone, Jasmineira, Chone and Sabella [34] (Figure 13). Other realms with large representation of sabellids records are Tropical Atlantic, the Arctic and Temperate Australasia. The number of described species and number of records is higher in the Atlantic than in any other ocean, the result that contradicts the patterns showing an increase in sabellid species richness towards the tropics, and mainly in the Indo-Pacific [331].
Some genera with few species, such as Anamobaea or Stylomma, and the freshwater Caobangia, are exclusive of tropical environments [69,158,229], and others, such as Branchiomma, Bispira, Acromegalomma, Notaulax, Sabellastarte, and Sabellonga, have either tropical or temperate distribution [149,157,[336][337][338]. A few genera show a significant preference for colder waters and are either well represented at greater depths or in higher latitudes (e.g., members of Chone, Euchone, and Jasmineira). The Antarctic region is richer in number of genera and species than the Arctic [24] (Figure 7), and genera, such as Perkinsiana, are mainly distributed in the Southern Ocean [212].
Currently, 36 named species have published records below 1000 m, 28 of them are reported from below 2000 m (Table 5)

Non-Indigenous and Invasive Species
Shallow-water Sabellida, due to their sedentary tubicolous lifestyle, are common members of biofouling communities and are easily translocated by anthropogenic means, i. e., on ship hulls and floating marine debris [147]. Distribution as larvae in ballast water has been suggested [359][360][361][362] and is the most plausible hypothesis for Sabella spallanzanii [363,364], but larvae of sabellids and serpulids have never been reported from ballast water samples. If become established in remote localities, such translocated taxa significantly expand their ranges. However, some reported broad distributions are a reflection of uncertain taxonomic status. Such species remain as widely distributed or even 'cosmopolitan' only as long taxonomic uncertainty persists, and a molecular investigation usually split hem into a number of geographically restricted species.
According to the literature survey of polychaetes reported outside their natural ranges, Serpulidae and Sabellidae account for 22% of the total number of non-native polychaetes world-wide [365]. However, a critical assessment of non-indigenous species records is needed. The number of such species is a function of the research effort put into distinguishing non-native and native taxa, which in turn depends on the knowledge of native diversity and the state of taxonomy of a group. Integrative taxonomic revisions of species with reportedly global distributions are important because such taxa may include cryptic invaders that are particularly difficult to track because they are often assumed to be native species or wrongly assigned to other invasive species [366].
Several criteria for distinguishing non-indigenous from native species have been proposed as lines of indirect evidence, such as a new record for an area, a new localised occurrence showing a population explosion, species with disjoint distribution, with insufficient natural dispersal capabilities to account for observed distribution range, or species associated with means of human-made transport, e.g., found on ship hulls (e.g., [172,367,368]). In the last two decades, molecular genetics tools have proved indispensable direct lines of evidence for assessing the status of a reportedly invasive taxon. The population genetics studies allow testing if distant populations belong to same species, and also if genetic variability of suspected new arrivals is lower than that observed in the native range, which indicates a recent translocation (e.g., [72,74,204,294,[368][369][370]). Molecular data can also help to determine the origin of non-native populations. For example, haplotype variability analysis suggests that although serpulid Hydroides dianthus (Verrill, 1873) was originally described from New England, its native range may be the Mediterranean [250].
A good example of a sabellid with an uncertain invasive status is Laonome calida described from the Calliope River, Australia, and later reported as introduced in the Netherlands [294], Odra River and the Sea of Azov [371,372]. Simultaneously, the morphologically similar Laonome xeprovala was described from the Baltic Sea, and DNA sequences from specimens from the Netherlands and Sea of Azov showed that they belong to the same species [102]. Neither L. calida nor L. xeprovala had been reported from Europe before 2014, and increased occurrences suggest a recent invasion [373]. Molecular analyses, however, are needed to determine whether the European populations belong to the same species found in Australia (implying that L. xeprovala could be a junior synonym of L. calida) or a distinct non-indigenous species of unknown origin is found in Europe (in which case L. xeprovala would be valid). Similarly, Branchiomma species are easily translocated outside of their native ranges [72,129,147,[374][375][376][377][378][379], but members of this genus are so morphologically homogenous that species identification using morphological characters only is problematic. Therefore, assessing the identity of Branchiomma spp. with invasive potential requires a comprehensive generic revision, including DNA-based species delimitation analyses [72,379]. Other records still to be confirmed by molecular studies are the sabellids Euchone variabilis Hutchings and Murray, 1984, Laonome triangularis, Desdemona aniara and the three species of Pseudobranchiomma reported from Australia [74,126,380,381]. Table 6. List of translocated species, with presumned origin indicated by provinces or ecoregions (sensu Spalding et al. [31]). Those species that are included as unresolved, require confirmation and two are not cosmopolitan (Branchiomma curtum and Parasabella pallida). NA: not applicable.

Species Type Locality Presumed Origin Category
New Occurrences (Outside of Expected Natural Range)

Molecular Markers Comments
Acromegalomma claparedei (Gravier, 1906 [72,126,129,147,185,189,376,377,379,[386][387][388]. Some records need confirmation [185].  Caribbean records may be erroneous [391]. Both syntypes of B. curtum and Caribbean specimens were juveniles produced by fission [392]. See comments in [391].  Specimens from the Baltic Sea, the Netherlands and the Sea of Azov possessed identical genotypes, but unknown origin [102]. Confirmation that it is not the same as L. calida is needed. It was reported in Australia near an international port as sp.

Peudobranchiomma orientalis (McIntosh, 1885)
Hong Kong Unknown Non-indigenous Australia none As cf. P. orientalis in Capa and Murray [74]. Ethanol fixed specimens are need for molecular analysis. As cf. P. schizogenica in Capa and Murray [74] from Australia, also reported in Galapagos [183].  [420]. Styan et al. [204] revealed three species (not formally described) with overlapping ranges in Australia, one of which is morphologically distinct from the other two. Grosse et al. [421] found two other species within the complex Spain.  [422][423][424][425]. Likely a species complex [204].   One of the best examples of a sabellid with the invasive status confirmed through a combination of morphological and genetic data is Sabella spallanzanii. This large conspicuous Mediterranean native was introduced to Australia in 1965 and to New Zealand in 2008 [364,435,436]. Analyses of COI sequences from the native and non-indigenous populations proved that the New Zealand incursion originated from Australia rather than from the Mediterranean [370]. Other examples of sabellids with invasive status (but ofunknown origin) confirmed with DNA analyses are Parasabella crassichaeta Capa and Murray, 2015 and Pseudobranchiomma cf. schizogenica Tovar-Hernández and Dean, 2014, reported from both Hawaii and Australia [73,74]. Out of seven nominal species of the serpulid genus Hydroides reported as translocated outside of their natural range, five are confirmed invaders ( Table 6). One of them, H. elegans, is the best-known cryptogenic polychaete, reported from most sub-tropical locations world-wide [110], and biofouling as the major mode of its dispersal was supported by DNA data [430]. Ficopomatus enigmaticus is another cryptogenic serpulid, because, although it was described from France, its native range is enigmatic (hence the name), likely to be southern Australia [420]. This typical species has invaded warm-temperate estuaries world-wide, as confirmed by DNA studies [204,437,438].
The Mediterranean Sea leads the rank when it comes to reported introductions, with 13 serpulid and 10 sabellid non-indigenous species reported, mainly as a result of Lessepsian migration from the Red Sea [125,365,440,441]. In this region, 11 species of sabellids and serpulids, mainly of the genera Branchiomma, Ficopomatus, and Hydroides, have been listed among the top 100 worst invasive species, based on their economic and ecological impacts [442]. However, taxonomic and invasive status of many of these taxa needs to be revised.
3.6. Fanworms Are Important: Some Applications 3.6.1. Nuisance Fouling Species Several serpulid species, predominantly of the genera Hydroides, Ficopomatus, and Spirobranchus, are capable of colonizing a wide range of natural and artificial substrates and settling gregariously, which makes them economically and ecologically important fouling nuisance species.
Serpulid foulers constitute a significant financial burden to due to costs associated with the removal of tubes from artificial structures. Millions of dollars are spent annually to prevent the fouling of marine organisms, especially of Hydroides, on human-made structures [443]. Dense tube aggregates attach to underwater seawater intake pipes of power plants reducing water flow and causing blockages. Fouled docks require cleaning maintenance in harbours around the world. Fouling interferes with navigation and shipping industries by decreasing ship speed, while increasing the weight and drag of buoys [444,445].
In marine aquaculture the key impact is the direct fouling of stock causing physical damage, biological competition and environmental modification, while infrastructure, such as aquaculture nets and cages, is also damaged. The conservative estimates of economic loss to the aquaculture industry are 5-10% of production costs attributed to biofouling [446].
Manayunkia speciosa and/or M. occidentalis (see [216]) are obligate hosts of the myxozoan parasites Ceratonova shasta and Parvicapsula minibicornis, which cause ceratomyxosis in salmon and trout in North America [267,268,447]. Management actions, such as flow manipulations to increase the mortality of M. speciosa and disturbance of its habitat, have been implemented [268].

Non-Indigenous and Invasive Species
Non-native to a region species translocated to another region can expand and have significant impact on human health, economic interests or environmental values. Such translocations of fouling species of Sabellida are well documented (e.g., [448][449][450][451][452]). Countries around the world have established biosecurity systems, aimed to prevent the introduction and/or spread of non-indigenous organisms. Some Sabellida have been listed in individual countries' Laws and Regulations, indicating its status as unwanted non-indigenous species (invasive, pests, parasites, pathogens). For example, in New Zealand, Sabella spallanzanii has been registered as a notifiable organism, subject to targeted surveillance work, including study of population dynamics and reproduction, under the New Zealand Biosecurity Act 1993 [364]. In Australia, non-indigenous marine species already found and those not yet found but have demonstrated significant impacts elsewhere are ranked according to their invasive impact and potential.  [453]. Ficopomatus enigmaticus is the only annelid registered in the Spanish Catalogue of Exotic Invasive Species [421]. In Brazil, the only species of Sabellida reported as invasive is Branchiomma luctuosum (Grube, 1870) [401].

Indicators of Pollution
Manayunkia speciosa is an indicator of moderate organic pollution, but is intolerant of severe pollution or anoxic sediments [447,454,455]. Decrease of the organic content of the sediment from 1.8% to 1.0% leads to reduction in abundance of its congener, M. aestuarina, from 16 000 to 6000 ind.m 2 in the Baltic Sea [262]. Euchonoides moeone was proposed as an indicator of sediment organic enrichment in a sewage outfall in Hawaii [279].
Sabella spallanzanii can trap anthropogenic micro-particles and glue these to their tubes, and it has been proposed as an indicator of microlitter pollution in sheltered and polluted environments such as ports [456]. Larvae of Hydroides elegans have been used as indicators for biomonitoring and ecotoxicology tests (e.g., [457][458][459]).
Some sabellids and serpulids have been suggested as bioindicators of heavy metal pollution. For example, the tube of Sabella spallanzanii is an important compartment in metal retention and suitable for evaluation of the pollution by traced elements [460], while Branchiomma bairdi and B. luctuosum, invasive sabellids in the Mediterranean, can accumulate high concentrations of arsenic (As), cadmium (Cd), chromium (Cr) and lead (Pb), considered to be priority toxic or ubiquitous persistent, bioaccumulative and toxic (PBT) substances under the EU Water Framework Directive [461]. Some studies have focused on the effects of heavy metals on larval development and metamorphosis using serpulid larvae (Hydroides elegans: [462,463]); Galeolaria caespitosa: [464]).

Bioremeditators
As suspension feeders, Sabella spallanzanii, Branchiomma luctuosum and B. bairdi have been tested as bioremediators for aquaculture waste-water treatment in polluted coastal areas [273,274,[465][466][467][468][469][470]. However, these three taxa are invasive in some areas, and may pose a threat to native ecosystems. Nevertheless, the use of non-indigenous species as bioremediator may allow to transform a potential risk into a benefit, with high potential commercial gain and economic feasibility [470]. Due to their important role in organic sediment bioremediation, the Food and Agriculture Organization of the United Nations (FAO, Roma, Italy) recommended Sabella as one of organisms with most potential for the development of integrated multi-trophic aquaculture systems [471].
Ficopomatus enigmaticus is a dominant species in estuaries and lagoons, where it can affect the community structure and contribute to the invertebrate biomass [472,473]. Due to its ability to build extensive reefs, F. enigmaticus is considered an ecosystem engineer that can modify estuarine ecosystem, changing water flow, sedimentation rates, and creating a structured hard substrate habitat in a soft-sediment environment. Large aggregations of F. enigmaticus remove suspended particulate matter, reduce excess nutrient loads and improve oxygen levels in enclosed waters, thereby improving the water quality and environmental conditions for other benthic species (reviewed in [420]). Davies et al. [474] stressed that because of the fundamental role F. enigmaticus played in the maintenance of water quality of an enclosed system near Cape Town, South Africa, eradication of this non-indigenous species should not be a management option.

Models Organisms in Research
Sabellids are used as models in regeneration biology, most notably in studies examining the developmental basis and functional ecology of regeneration [222,[475][476][477]. Members of the genus Myxicola are known for the giant axon [478] that directly innervates the worm's muscles, presumably aiding in super-fast retraction into the tubes [479]. The outsized nerves make this species a model organism for studies of neuroanatomy, neuroactivity and electrophysiology [480][481][482]. Myxicola's giant axons were also used for testing the effects of the anticonvulsant Carbamazepine on the ionic conductance [483]. Moreover, the mucus of Myxicola infundibulum (Renier in Meneghini, 1847), with natural antibacterial and antioxidant compounds, showed potential for drug prospection [470].
Hydroides elegans is easily adapted for laboratory research because of its rapid generation time (three weeks) and ease of propagation. The adults spawn and eggs easily fertilise, their larvae become metamorphically competent in several days and readily settle in the laboratory. Thus, H. elegans has been declared an important model organism [484] and has been used routinely during last two decades in hundreds of experimental embryological, larval ecology and biofouling studies, including tests of newly formulated marine coatings (e.g., [485][486][487][488][489][490]). Other Hydroides species, such as H. ezoensis and H. dianthus, have also acted as model organisms for larval ecology research (e.g., studies of mechanisms of gregarious settlement by [491][492][493]. Spirobranchus lamarcki (Quatrefages, 1866) has provided an important model system for molecular and embryological work, including studies on the organization and expression of its developmental genes (e.g., [494][495][496][497]). Recently H. elegans and S. triqueter have served as models in ocean acidification and biomineralization research (e.g., [498][499][500]).

Objects of Ornamental Trade
Sabellida includes some of the most beautiful marine invertebrates due to their colourful radiolar crowns. They are listed among the ten most imported ornamental invertebrates [501]) and are amongst the most photographed polychaetes found in marine guides and featured on postcards, stamps, calendars, T-shirts and even tattoos. Largest sabellids (Anamobaea, Bispira, Notaulax, Sabella and Sabellastarte) and serpulids such as Christmas tree worms (Spirobranchus) and coco worms (Protula) are popular in the aquarium trade. The vast majority of ornamental sabellids and serpulids are tropical species, although a market for cold-water species has been growing [502,503]. Efforts to culture sabellids (e.g., Sabellastarte spectabilis (Grube, 1878) [504][505][506], Sabella pavonina Savigny, 1822 [270,507] and Bispira brunnea (Treadwell, 1917) [508] are well under way. Aquaculture can provide environmental benefits by reducing collecting pressure on highly traded species.

Future Perspectives in Fanworm Research
As it is clear from this review, knowledge of Sabellida biodiversity is incomplete and the reported species numbers appear to be an underestimation of the true diversity. This review highlights that some of the lesser known coastal and continental shelf areas including Hudson complex, the Atlantic coast of South America (especially the tropical Atlantic region, excluding the Caribbean Sea), the coastlines along the Arabic Sea and Gulf of Bengal, and the Far East of Russia. However, Africa, with the exception of South Africa and Morocco, is by far the most neglected continent when it comes to taxonomic studies.
More surveys into deep-sea (abyssal and hadal), chemosynthesis-based (hydrothermal vents, methane seeps and organic falls) and freshwater habitats are needed for a better understanding of the Sabellida diversity and adaptations to these habitats. The fact that undescribed species have been collected in recent deep-sea cruises along several worldwide regions (e.g., [284,509,510]) provides evidences for deep sea fanworms still awaiting to be discovered. Studies of symbiotic/commensal relationships with other organisms, e.g., molluscs, corals, sponges, or examinations of bacterial microbiomes may reveal not only new taxa, but also new ecological relationships and trophic networks (e.g., [284,490]).
Importantly, the diversity of some remote areas, including deep sea environments, is poorly known not only because of the obvious logistical difficulties with collecting, but also due the insufficient number of experts and their unbalanced distribution across the globe known as 'taxonomic impediment' (e.g., [94,511]). We need to train and sustain more systematists able to discover, describe, identify and classify species and also to increase efforts directed to manage and curate existing research collections [355].
As many more species are yet to be discovered, either in the field or in museum collections, particular attention should be paid to setting a high standard for the new species descriptions, which would include use of modern microscopic techniques (e.g., SEM, Phase Contrast), assessment of intra-and interspecific variability, and preparation of quality informative illustrations (digital drawings and high-quality digital photographs of stained fixed or live specimens, when possible). Exploration of both new characters, e.g., ultrastructure of calcareous serpulid tubes that proved useful for species delimitation [107,227,512], and new techniques to examine existing morphological characters, e.g., tomography and 3D reconstructions (e.g., [237]) should significantly improve species descriptions in the future and aid species delimitations.
Although multivariate morphometrics have been used to analyse differences among annelid species and populations (e.g., [513,514]), this technique is not very common for species delimitation because body shapes of these soft-bodied organisms vary depending on the fixation or anaesthetization methods [234]. We suggest that application of morphometrics to chaetal or opercular traits should be explored. However, regardless of availability DNA data, morphological studies must include statistical assessment of intraspecific variability and its sources (such as size-dependent, ontogenetic, environmental) (e.g., [72,278,515]).
Understanding of true species diversity of Sabellida requires world-wide revisionary studies of existing species and their distribution ranges. As early species descriptions are often very short and sometimes poorly illustrated, further re-descriptions of older species (especially described before mid-20th century) are needed, ideally based on topotypical material (as, e.g., done for serpulid Spirobranchus kraussii by Simon et al. [193]). For the species with lost or lacking types, neotypes should designated (e.g., as for Pseudopotamilla reniformis (Bruguière, 1789) by Knight-Jones et al. [111]), preferably accompanied by DNA sequence data (e.g., as done for Hydroides brachyacantha by Sun et al. [197]. It is imperative that the type material (holotype, type series, additional specimens showing intraspecific variability, and DNA extractions) is always deposited in properly curated permanent museum collection(s) where it is maintained in optimal conditions [516].
Contrary to previous conceptions that the ocean has no boundaries and that polychaetes more often than other organisms have cosmopolitan distributions [327], it now became clear that the genetic and species diversity of marine invertebrates is highly structured geographically and significant species diversity is hidden in former 'cosmopolitan polychaete species'. Therefore, resolution of 'cosmopolitan species' should be one of the main goals of revisionary studies of Sabellida. This goal is only achieved with application of fast-developing molecular tools such as DNA sequencing and genomics/transcriptomics. Molecular tools and analytical methods are indispensable to further improve our understanding of the species diversity, but also to trace the pathways and origins of invasive species, to determine biogeographic boundaries between species, and to provide reliable phylogenic hypotheses. Robust well-resolved phylogenies with significant taxon coverage using transcriptome and mitochondrial genome data are important to address important character evolution questions (e.g., photoreceptor evolution and evolution of the reproductive and larval strategies in Sabellidae and Serpulidae). Finally, in the future molecular identification of species by non-specialists might replace morphology-based identifications only if reliable databases of reference sequences supported by voucher specimen depositories are built.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
Fabriciidae and Sabellidae species with doubtful identity requiring further investigation (inquirenda), indeterminable or incorrect assignation. Fabriciidae