On the Systematics and Biodiversity of the Palaeoannelida

: Palaeoannelida Weigert and Bleidorn, 2016 is an old clade branching off at the base of the Annelida radiation. It includes two morphologically and ecological divergent groups of sedentary burrowers and tube-dwellers: Magelonidae Cunningham and Ramage, 1888, and Oweniidae Rioja, 1917. Magelonids are characterised by a ﬂattened, shovel-shaped prostomium and a pair of ventral papillated palps. Oweniids have simpliﬁed bodies lacking parapodia or appendages and are easily distinguished by the presence of oval patches of packed uncini, each with two distal curved teeth. The present review aims to summarise available information about the diversity of forms and life strategies displayed in the group, providing some guidelines for species identiﬁcation and the techniques commonly used for their study. In addition, the assumed geographic distributions of some taxa are critically discussed. A brief introduction about the evolutionary relationships, systematics, and taxonomic history is given for both Magelonidae and Oweniidae. The motivation of this review is to highlight the main knowledge gaps from a taxonomic, methodological, and geographic perspective, aiming at stimulating further research into members of this clade.


Introduction
The term Palaeoannelida was proposed by Weigert and Bleidorn [1] for the clade formed by Oweniidae Rioja, 1917 and Magelonidae Cunningham and Ramage, 1888, branching of at the base of the annelid tree and sister to all the rest of annelids (see also [2,3]). The basal position of these two groups was assessed after analyses of phylogenomic data [4][5][6]. Previous analyses of just a few molecular markers had unveiled Oweniidae diverging early in the annelid radiation but only weakly supported (e.g., [7][8][9][10][11][12][13][14][15]). The position of these sedentary organisms at the base of the annelid tree generated some controversy with the fossil record, as most Cambrian annelid fossils showed morphologies corresponding to an epibenthic lifestyle. However, a recent finding of a tubicolous annelid dated in the early Cambrian [16], which was proposed within the Magelonidae, has suggested that a diversity of life modes, including sedentary and errant forms, may have inhabited the oceans at that time. The relationship between Magelonidae and Oweniidae, whilst supported by some morphological features such as the presence of a monociliated epidermis and lack of nuchal organs [17], still needs further work.
Magelonidae are known colloquially as the 'shovel head worms' due to their uniquely flattened and spade shaped prostomia ( Figure 1) utilised in burrowing. The family is relatively small, containing 72 species worldwide [18]. Based on the diversity of species reported from relatively small geographic areas, the number of species is likely to be drastically underestimated [19]. The first species to be described was Magelona papillicornis F. Müller, 1858 from Brazil, with the group being raised to the rank of family by Cunningham and Ramage [20]. Their unusual morphology has often led to difficulties in relating them to other annelid groups. However, in spite of this they are easily recognised by their characteristic prostomia, and two uniquely papillated and ventrally inserted palps ( Figures 1A and 2A,B), characters which support the monophyly of the group [21] (and Mortimer et al., in preparation A). Magelonid species are relatively uniform in appearance and this has posed issues with species identification and the understanding of generic delineations within the family.  In this contribution we aim to provide an updated revision on current biodiversity knowledge in Palaeoannelida, focusing on species richness and distribution in different world sea regions, updates in taxonomy, classification, and systematics after recent advances in the knowledge of their anatomy, and evolutionary relationships and consequent classifications. We would like to emphasise major gaps in knowledge and where efforts should be made in terms of biodiversity surveys, analytical efforts, and the strengthening of taxonomic skills in order to increase our knowledge about the species inhabiting the planet.

Methods
A thorough literature review was performed in order to acquire information about species diversity, type locality, and geographic, bathymetric and ecological constrains of members of Palaeoannelida. Moreover, some recent advances in their anatomical study after Capa et al. [57] were incorporated due to their systematic significance. To accomplish this aim, the World register of Marine Species [58,85] database has been of great use, and some amendments herein made, (e.g., accepted synonymies according to latest references) have also been coordinated with WoRMS for its update. Tables with all considered currently valid nominal species alongside their type localities, depth (from original description) and marine realms (sensu Spalding et al. [86]) are provided (Tables A1 and A2). Additionally, a revision about methodologies and techniques used for species identification and characterisation is made, with the latest trends. There have been many difficulties relating magelonids to other annelid groups; a problem mirrored in the Oweniidae. For many years, magelonids have been considered among the spioniform polychaetes [19] and after phylogentic analyses of morphological data were placed in the Spionida (together with Apistobranchidae, Chaetopteridae, Longosomatidae, Poecilochaetidae, Spionidae, Trochochaetidae, Uncispionidae clades [87]), principally based on the presence of a pair of grooved palps and spiomorph parapodia [87], or related to Oweniidae [17]. Various other placements have since been suggested, for example, related to Polydora Bosc, 1802 (Spionidae), Cirriformia Hartman, 1936 and Dodecaceria Örsted, 1843 (Cirratulidae), or Fauveliopsis McIntosh, 1922 (Fauveliopsidae) [10] utilising combined morphological and DNA sequences of nuclear markers. Further, phylogenomic analyses placed Magelonidae as sister to Oweniidae branching off at the base of the annelid tree [4,6,88].
Although a sister-group relationship of Magelonidae and Oweniidae has been outlined, this and their position within Annelida are still the subject of intense debate [57]. Clarification is needed in regard to the extent to which the prostomium is fused to the peristomium in magelonids (see below) and also in the nature of the 'buccal organs' in the two families (see below in relation to the burrowing organ). It has been suggested that the head structures in the Palaeoannelida clade are heterogeneous, with Magelona possessing papillated peristomial palps whilst grooved prostomial palps are present in Owenia [89]. Further work is clearly needed, but perhaps further studies assessing similarities and differences between the two families may shed light on the subject.
There is only one cladistic analysis on the Magelonidae to date [21], which confirmed the monophyly of the group but from which no further proposals were made. A forth coming account of inferences of phylogenetic hypotheses within the Magelonidae (Mortimer et al., in preparation A) may add clarity. A fossil polychaete from the early Cambrian (Dannychaeta tucolus Chen, Parry, Vinther, Zhai, Hou and Ma, 2020) has recently been described within the Magelonidae based on phylogenetic analyses [16].

Taxonomic History
Taxonomically, Magelonidae received little attention prior to the 1930s (Figure 4), however, the number of taxonomic papers increased in the 1940s and 1950s with works by prolific polychaete researchers such as Hartman [90,91]. With the 1960s, the group saw a rapid expansion of taxonomic work, not only by notable magelonid workers such as Jones [29] but works by Gallardo [92], Eliason [93], Hartman [94,95], Glémarec [96], Kitamori [97], Reish [98], Hartmann-Schröder [99], Day [100] and Harmelin [101] (Table A1). This increase is likely to have been influenced by the publication of Jones, which set out a standard to which magelonid works should attain. The 1970s largely saw works by Jones, who by that time had been established as the magelonid expert [31][32][33]. His research on magelonids continued until 1979 where he switched his attention to the Vestimentifera (now Siboglinidae) [48]. The major taxonomic works of the 1980s and 1990s were those concentrating on Southeastern Brazil [46] and Thailand [34]. Since 2000, the number of researchers producing taxonomic works on the group, and the number of species being described has once again increased. Workers such as Mortimer and Mackie [35,36], Fiege et al. [48], Hernández-Alcántara and Solís-Weiss [43], Mortimer et al. [38,49] and Magalhães et al. [102] (see Table A1 for all references) contributing further to the taxonomic knowledge of the group. Certainly, with renewed interest in the group, it is likely that this trend in increasing publications will continue. However, outside of the key works by Jones there have been very few major revisions of the family [21,22,48]. A forthcoming account of inferences of phylogenetic hypotheses within Magelonidae (Mortimer et al., in preparation A) will add further information on the key morphological characters. The only worldwide key to species was produced in 1963 [29]. However, over 50 species have been described since that time (WoRMS , Table A1). Whilst keys to local regions can be found in several papers: California [45], Eastern USA [95], Gulf of Mexico [42][43][44], Brazil [46], Europe [18,48,49], Japan [97,103] Viet Nam [92], Thailand [34], Western Indian Ocean [37,38], Seychelles [36], South Africa [104], updated versions are warranted in most regions.

Taxonomic Characters and External Morphology
Magelonids are generally fragile, being long and slender and for this reason they can be extremely difficult to collect whole. Over 60% of species have been described from posteriorly incomplete specimens. They are often pale in colour (white to cream), although the gut, which frequently has a darkish green hue, can often be seen through the body wall. Several species (M. alleni, Magelona cincta Ehlers, 1908, Magelona equilamellae Harmelin, 1964, Magelona japonica Okuda, 1937 and Magelona variolamellata Bolívar and Lana, 1986) are known to carry dark pigmentation occurring as a distinct band between chaetigers 5-8 [52,53] (Figure 3C), and pigmentation in other parts of the body such as the palps has additionally been reported, e.g., Magelona mirabilis (Johnston, 1865) [48,51]. In addition, specimens belonging to two undescribed species recently found off West Africa carry dark pigmentation of the parapodia, and distinct stripy pigmentation from head to toe, respectively (Mortimer et al., in preparation B).
The magelonid body is divided into distinct regions: the head (comprising the prostomium and the peristomium), the thorax (an achaetous first segment, and either eight or nine chaetigers), the abdomen comprising many chaetigers (approximately 50-160 chaetigers, see [22], although for most species this is unknown), and the pygidium ( Figure 2C,D). The precise location and organisation of the peristomium has been debated [10,19]. Early works suggested the prostomium "merges posteriorly with the achaetous peristomium" [30], suggesting that the peristomium is represented by the achaetous first segment. However, magelonid larvae have been shown to possess two anterior segments bearing chaetae which subsequently become achaetous [105]. Subsequent workers regarded the peristomium to be limited to the buccal region [87]. Personal observations (KMJ) suggest that the peristomium is visible on the ventrum (see [22]: figure 4.2.5, posterior to buccal region) and this and the achaetous 'first segment' (Figure 1) warrant further investigation. Magelonids have long been regarded to possess an eversible ventral proboscis (e.g., [30,87]), however this heart shaped sac is not connected to the buccal region, plays no part in feeding and is involved only in burrowing ( Figure 2B). The term burrowing organ has now been applied to this structure to avoid future confusions [18,22]. Between the thorax and abdomen, there is an often-marked constriction in the body ( Figure 3D,E), and the two flanking regions are additionally discernible by a change in chaetal type (Figures 2C,D and 3) from capillary chaetae ( Figure 5) to hooded hooks ( Figure 6). Magelonids possess biramous parapodia, often carrying foliaceous flattened structures, for which the term 'lateral lamellae' was coined [29]. However, these structures may be filiform to foliaceous and may be prechaetal, postchaetal, subchaetal or lateral in position. It is the identification of these features, particularly in the thoracic region which is a major diagnostic feature in separating species, and it is for this reason that all thoracic parapodia must be fully described and illustrated [21,31].
Several 'crucial morphological' characters for separation of species have been suggested [29,45]: (1) dentition of abdominal hooded hooks; (2) presence or absence of prostomial horns (distal projections of the anterior margin); (3) presence or absence of medial lamellae (DML, VML) in the posterior region (cirriform lobes adjacent to hooded hooks see Figures 6B and 7B,C); (4) presence or absence of specialised chaetae of the ninth chaetiger (mucronate, pennoned); (5) morphology of thoracic lamellae; (6) the relative dimensions of the prostomium (L:W ratio); (7) presence or absence of superior dorsal lobes on thoracic chaetigers; (8) presence and distribution of lateral abdominal pouches and (9) the presence or absence of postchaetal expansions behind chaetal rows in the abdomen. With the description of further taxa, several other characters are additionally noted: (10) pigmentation patterns and (11) overall body size and proportions ( Figure 3).
Magelonid prostomia can be broadly divided into three categories: length approximately equal to width ( Figure 1B,C), length greater than width ( Figure 1A), or width greater than length ( Figure 1E). The overall shape can aid identification, but care must be taken as the appearance can be modified if lateral or anterior margins become compressed. Prostomial horns may be present or absent. When present, they may be distinct ( Figure 1B Figure 1D) or crenulated (e.g., Magelona crenulifrons Gallardo, 1968) ( Figure 1B,E). The prostomium may carry one ( Figure 2D) or two pairs of dorsal muscular ridges ( Figure 1A,B), and distinct patterned areas either side of the ridges (often as raised oblong or arched lines) may be present or absent (Figure 1).
The biramous parapodia of the thoracic region carry lamellae which may be equal in terms of size and shape in both rami, or noticeably larger in the notopodia. In some species the neuropodial lamellae may be marginally larger, however, this situation is generally infrequent. The lamellae may be filiform (e.g., Magelona filiformis Wilson, 1959) ( Figure 5A) to foliaceous (e.g., notopodia of Magelona sinbadi Mortimer, Cassà, Martin and Gil, 2012) in shape ( Figure 5B,D). The notopodial lamellae are generally postchaetal to subchaetal and encircle the chaetal bundle, confluent with a much lower prechaetal lamellae, almost cuff-like ( Figure 5C). The upper edges of the lamellae may be smooth (e.g., M. equilamellae), crenulate (e.g., M. johnstoni) or bi-lobed (e.g., Magelona obockensis Gravier, 1905). At the top of the notopodia and in a slightly prechaetal position superior dorsal lobes (SDL) may be present ( Figure 5B-D). These are smaller than the lamellae, and often digitiform in shape, although in some taxa they may be foliaceous. When present, they may occur on all thoracic chaetigers, from chaetigers 1-8 or only present in the posterior thorax (e.g., chaetigers [4][5][6][7][8]. The neuropodial lamellae of the thoracic region may be prechaetal, subchaetal or postchaetal, the edges of which are generally smooth. Many species have filiform subchaetal lamellae attached to low pre-and post-chaetal ridges ( Figure 5A,E). In these species the lamellae may occur in the same position along the thorax, or vary, often starting in a slightly prechaetal position, becoming ventral in the mid thorax and becoming postchaetal by the posterior thorax. The postchaetal portion may be more expanded in some species, particularly on the eighth and ninth chaetigers, where it is often triangular and of a similar length to the subchaetal lamellae ( Figure 5D). The parapodia of the eighth and ninth chaetigers are particularly important to observe. In some species, the thoracic lamellae are similar on all thoracic chaetigers (e.g., M. symmetrica), whilst in others the first seven are similar with the lamellae of chaetigers eight and nine varying (e.g., M. montera), and in others it is the lamellae of chaetiger nine which varies in comparison to the remaining thoracic lamellae (e.g., M. mirabilis).
The abdomen which starts at the 9th chaetiger for Octomagelona ( Figure 2D) and the 10th chaetiger for Magelona ( Figure 2C) is comprised of numerous chaetigers (~50-160 chaetigers). Parapodia are biramous, and as in the thorax, carry lateral lamellae (e.g., LL in Figure 7C) which are generally symmetrical in terms of size and shape between the two rami. However, in a few species such as M. alleni and Magelona korena Okuda, 1937 the notopodial lamellae are somewhat larger than those of the neuropodia. Lamellar shape varies but can be generally separated into those which are rounded ( Figure 6B), carrying a basal constriction and those which are slender triangular, with no basal constriction (Figures 3D, 5A and 7C). Above the lamellae in the notopodia ( Figure 6A), and below the lamellae in the neuropodia ( Figure 7B), hooded hooks occur in a single row (N.B. the hooks of M. equilamellae were noted to occur in two rows towards the middle of the ramus [18]). In each ramus, hooks may be orientated in one direction, laterally towards the lamella ( Figures 3D and 6A), or in two groups vis-à-vis ( Figure 6B). Behind the chaetal row, a postchaetal expansion of the lamella may be present ( Figure 6B). At the inner margins of the chaetal rows, small triangular to digitiform processes (dorsal (DML) and ventral medial lobes (VML) of Jones) may be present ( Figures 6B and 7B,C).
Abdominal hooded hooks may be bidentate (with one secondary tooth above the main fang) ( Figure 6C), tridentate (with two secondary teeth) ( Figure 6D) or polydentate ( Figure 6E,F). Quadridentate, pentadentate and hexodont hooks have all been reported to occur within the family. Enlarged hooks may be present, the form of which varies from spines, re-curved hooks or enlarged 'ordinary' hooks (e.g., Magelona falcifera Mortimer and Mackie, 2003 or Magelona spinifera (Hernández-Alcántara and Solís-Weiss, 2000)). Some species have a small hook emerging at the base of the lateral lamellae (e.g., M. filiformis see [18]: 104), whilst several curved support chaetae (aciculae) may also be present (e.g., Magelona conversa Mortimer and Mackie, 2003).   Lateral abdominal pouches are recorded in approximately half of all known species, although their function is currently unknown [51]. Whilst pouches may be divided roughly into two types [48], greater variation in morphology has been noted [22,37,51]. Pouches may be anteriorly (comprising of a dorsal and ventral flap, with a convoluted membrane in between) ( Figures 3D and 7B) or posteriorly open (simple and pocket-like flaps) ( Figure 7C). The former type generally occurs in pairs on either side of segments at the start of the abdomen, whilst the latter can be paired or unpaired, and may occur from median abdominal chaetigers, or only in the extreme posterior of the animal [51]. Posteriorly open pouches may occur on consecutive segments or alternating from segment to segment and from one side of the body to the other. They may be smooth or medially split.
For the 40% of species in which the pygidium is described, morphology is relatively uniform, possessing two slender cirri, placed laterally, with the anus being ventral ( Figure 7A). However, variation has recently been highlighted [18,52]  Several species have shown species-specific staining patterns using methyl green or methyl blue [34,[39][40][41]50] and these methods can be extremely useful in identifying specimens in bulk samples.

Internal Morphology
In comparison to observations on the external morphology of magelonids the internal morphology has received less attention. Much of the early work on the anatomy of magelonids came with the publications of McIntosh [23][24][25][26][27][28] and Jones [30]. The latter author providing details of early studies, including the circulatory system [106], lateral organs [29], musculature [107,108], nervous system [109] and including his own observations on the nervous system, muscular system, septa, and circulatory system. Monociliated epidermal cells in the larvae of M. mirabilis were reported [110], although figure 1A of that publication is unlikely to be M. mirabilis. The closed circulatory system consists of an anterior dorsal vessel, divided into a series of chambers set apart by valves. A heavily muscularised portion of the dorsal vessel is largely responsible for the movement of blood around the animal [30]. The circulatory system is vital in the eversion of the burrowing organ ( [30], although note comment above about terminology) which can be clearly observed in live animals ( [22]: figure 4.2.4). Notes on the unusual pale pink blood which contains numerous corpuscles have been made [111], the blood pigment is hemerythrin [112]. Palps contain a single blood vessel, clearly observed in live material ( [51]: figure 1F) and it has been suggested that the palps have a secondary respiratory function [26,30].
Various authors have reviewed the structure and evolution of the nervous system [2,113,114]. The central nervous system of the Magelonidae has been considered to be simple in comparison to those of errant polychaetes [114]. The brain is composed of an anterior compact neuropil and posteriorly encircles the prostomial coelomic cavities. Thereafter, two lateral medullary cords branch off and fuse caudally. The ventral nerve cord comprises of two parallel cords of neurite bundles and paired neuropils which fuse between the 9th and 10th chaetigers [2]. Nuchal organs, ganglia and mushroom bodies are all considered to be absent [114], although the former are considered to occur in larvae [19]. Although eyes have been recorded as absent in adult magelonids, a species recently described from Hawaii, Magelona cinthyae Magalhães, Bailey-Brock and Watling, 2018 has distinct eyespots on the posterior prostomium.
The musculature of a species approaching M. mirabilis was investigated by phalloidin labelling and confocal laser scanning microscopy [115]. The prostomial muscles are anterior extensions of the ventral longitudinal muscles of the body and a complex of circumbuccal muscles are described ventral to the mouth opening. However, it is perhaps worth noting that this region is at the base of the burrowing organ rather than connected to the buccal region itself and the terminology should be reviewed. The palps possess muscles along their entire lengths, comprised of longitudinal fibres, which the authors [115] suggested would restrict movement to contraction and slight coiling. Whilst some authors have noted this to be the case for some species, including M. mirabilis [51,116] motile palps have been reported for other species [51,52] and will be discussed below. Each palp is moved by two palp retractor muscles. Dorsally and ventrally muscle strands run longitudinally, however, there is a distinct change in musculature between the thoracic and abdominal regions. Oblique and cross-striated muscles in M. papillicornis (likely to be M. mirabilis or M. johnstoni, see [48]) were investigated by Wissocq [117].

Species Diversity and Distribution
There are 72 magelonid species currently considered to be valid ( [58], Table A1). Species have mainly been described from temperate and tropical environments, and the number described from the Temperate Northern Pacific, Temperate Northern Atlantic and Western Indo-Pacific is comparably larger than any other marine realm (Table A1, Figures 8 and 9A). There are no species described from the Arctic or the Southern Ocean (Table A1, Figures 8 and 9) although records from the Global Biodiversity Information Facility (GBIF [118] suggest magelonids have at least been found in these two regions but remain unverified at this time. These type locality occurrences perhaps reflect the influence of workers such as Jones [29], Hartman [95] and Wilson [119,120] in the Temperate North Atlantic, Hartman [90,94], and Jones [31,33] in the Temperate North Pacific, and Nateewathana and Hylleberg [34], Mortimer and Mackie [35,36] and Mortimer et al. [38] in the Western Indo-Pacific (Table A1), rather than differences in actual species numbers occurring in each region. However, further work is needed to corroborate this hypothesis. The diversity of magelonid species is generally high in relatively small geographical areas, e.g., 13 species in the Gulf of Mexico [42], nine species in European waters [18], 11 in the Arabian Gulf [38] and over 20 off west African waters (Mortimer et al., in preparation B). It was suggested that the high diversity of magelonid species observed off Phuket Island, Andaman Sea may be a direct result of sediment disturbance associated with monsoons [121], the authors hypothesising that catastrophic events affect interspecific competition by removing dominant species. However, areas with similar species diversities can be seen in non-monsoon affected areas. Abundance data on populations of Magelona in Monterey Bay indicate that population densities can be extremely variable form year to year [122].  [58] listed under the marine realms (sensu [86]) from which they were described. Numbers indicate the number of species described from each bioregion; (B) number of species described from each depth range (depth in meters, data from the original descriptions of 61 species for which the data was provided).
Magelonids show a preference for shallow waters and data taken from the original descriptions of 61 magelonid species (for which data was provided) shows the number of species at depths of less than 20 m to be much greater than at any other depth range, with the number of species dropping off substantially after 60 m ( Figure 9B). Although this information only includes data from the original descriptions, it does indicate the group's preference for shallower waters. Whilst the data may be biased by the ease of sampling onshore or in shallower waters it is likely that these depth patterns contrast with other annelid groups. However, further work is certainly needed to clarify depth ranges of individual species. Whilst some magelonid species are found both intertidally and subtidally, some are known to be distinct offshore species (e.g., M. minuta; [22]). Magelona phyllisae Jones, 1963 (note incorrect spelling M. physilia) has been noted to occur in low oxygen habitats [123] in northern Chile. Sensitivity to hypoxic conditions was also noted in the "dead zone" of the Gulf of Mexico for a species approaching M. phyllisae [124].
Very little information about the distribution ranges of magelonid species exists, and those already documented need validation. Magelona papillicornis was believed to be a widespread species, occurring from regions such as America, Africa, Europe, New Zealand, and India [32]. It is now recognised that many of these records are erroneous and have since been referred to several other species such as Magelona debeerei Clarke, Paterson, Florence and Gibbons, 2010, M. mirabilis and M. johnstoni [32,48]. Current maps from GBIF [118] highlight many of these erroneous records still and should be verified to prevent further confusion. Recent work off the western coast of Africa (Mortimer et al., in preparation B) involving morphological and molecular work suggests that whilst some species have quite restricted distributions others may be far greater. Certainly, further work is needed but this relies heavily on increased taxonomic work to resolve the identity of species in many areas. Undoubtedly, species recorded large distances from the type locality should be treated with caution at this time, e.g., American and Brazilian species recorded off India [125].

Biology and Ecology
Magelonids are common in sands and muds, both intertidally and subtidally; most species occurring in waters less than 100 m deep. The group's preference for fine sediments has been linked to avoidance of sharp fragments that might damage the burrowing organ so vital for moving within sediments [27]. Whilst the broad sediment preferences are recognised for the family, very little is known about specific species habitats. Investigations into the distribution of four species (M. alleni, M. filiformis, M. mirabilis and M. johnstoni) in the German Bight showed sediment (median grain size diameter or mud content) to be the most important predictor in habitat suitability, but with salinity and water depth also of importance [126]. However, variations in habitat preference were observed between species (e.g., M. filiformis in sediments with less than 10% mud, whilst M. alleni occurred in sediments with elevated mud contents), something mirrored in observations in the UK, with M. minuta and M. alleni (personal observations, KMJ). Within the waters off the Arabian Peninsula individual species were noted to occur in sediments with distinct granulometric characteristics [38].
Whilst most magelonid species are believed to burrow more or less continuously through sediments [51,127], at least 10 species (e.g., M. alleni, M. cincta, Magelona polydentata Jones, 1963, M. variolamellata) are known to build distinct tubes ( Figure 7D), which are often multi-layered paper-like tubes covered in sediment [22,52,53]. Individuals of M. alleni, have been observed to live for over a year in tube-lined burrows under tank conditions, being highly sedentary in comparison to previously observed species (Personal observations, KMJ).
Magelonids have been primarily described as surface deposit feeders [30,42,51,52], although suspension feeding [19,51,128,129], subsurface feeding and carnivory [116] have additionally been suggested. Varying feeding modes have been observed to predominate in different species observed simultaneously in the same laboratory setting [52]. Fauchald and Jumars [127] considered feeding to be selective, with selectivity increasing in nutrient-poor conditions. However, in contrast M. alleni was reported to be predominately non-selective [52] and magelonids feeding upon large volumes of sediment have been reported [116]. The feeding process has been described in detail for several species [30,51,52,130] and involves palps above the sediment surface, collecting particles ( Figure 10A,B), which are then transferred along the palp to the mouth by cooperative movements of the papillae. A mucus thread has been postulated to aid transport of particles along the palps [30,130] and additionally reported to be involved in excretion in the tubicolous species M. alleni [52]. Given papillae involvement in particle transfer, differences in palp morphology between species, e.g., those species which possess slender palps with few papillae, in comparison to those that are broad carrying numerous rows of long papillae may indicate differences in feeding strategies and diet. Magelonids have been reported to consume crustaceans, crustacean larvae, sediment, detritus, diatoms, algal cysts, spores, tintinnids, molluscs, worms and other small animals [26,30,51,52,128,129,[131][132][133]. Despite the number of papers recording diet, knowledge of species-specific diets is generally lacking, although interspecific variation is likely [51,52].
Very little is known about the reproductive biology of the group [19,22], although sexes are separate. Eggs are easily observed in the abdomen of live animals and range from 50 to 150 µm in size [22]. Magelona sacculata was recorded to possess small eggs, with a high fecundity, varying from 8 to 12,000 eggs [122]. Eggs first occurred generally between chaetigers 10 and 20, and in the most fecund individuals extended to the last chaetiger. The family are believed to have ect-aquasperm and fertilisation is thought to be external [22,134,135]. Studies on the life history and population dynamics of M. sacculata suggest it reaches maturity in approximately two months, spawning in its first year with death occurring at or shortly after [122]. Lateral sinuous movements of the thorax within the water column are believed to be associated with reproduction ( Figure 10C,D), although further evidence is needed. Both planktotrophic and lecithotrophic larvae have been recorded to occur within the group [105,135,136], and it has been suggested that the planktotrophic larvae of M. sacculata are long-lived, surviving for at least two months [122]. Although little is known about larval development, several species have been described and illustrated [22,105,137]. The dispersal potential of four species of Magelona in the Southern California Bight was estimated to be high based on known and inferred lifehistory information [138]. Magelonid larvae (Magelona sp.) have been observed to perform reverse diel vertical migrations off Central Peru [139].
Magelonids as potential indicator species of marine pollution, have been suggested [140], based primarily on the observation that non-polluted mangrove areas in Brazil were characterised by the magelonid M. papillicornis [141].

Family Oweniidae Rioja, 1917 3.2.1. Systematics
The monophyly of Oweniidae is supported by embryos which do not go through the typical spiralian development, by having a unique mitraria larvae, a pair of protonephridia resembling those of deuterostomes, lacking the typical external cuticle of most annelids, possessing a monociliated epidermis and a nerve cord that is intraepidermal in some species [17,57,65,[142][143][144][145][146][147][148]. Although robust and unambiguous characters, some of the aforementioned attributes are not easily observed. As a practical aid, members of this family are easily recognised by their elongate and cylindrical bodies, without protruding parapodia or appendages (except for those of the head, present in some species), and by bearing oval patches of transverse irregular rows of packed uncini provided with two distal teeth.
Due to the morphological and ontogenic peculiarities of members of this family, the relationships between oweniids and other annelids have been object of debate for decades (see [57] for a summary about the systematic history of the family).
Monophyly of Oweniidae and the four currently considered valid genera has been assessed based on morphological information [17] but a phylogeny including molecular data has never been published for members of the family. The monophyly of Owenia is supported by the presence of their characteristic ramified tentacular crown on the anterior end and neuropodial uncini with teeth arranged side by side [17]. The monophyly of Myriowenia is also based on the presence of head appendages, but in this case, corresponding to a pair of grooved palps, as well as the arrangement of uncinal teeth, one on top of the other [17]. Members of Galathowenia are characterised by an anteriorly truncated head and the presence of a ventral cleft (e.g., [17,59,62,68,80,149,150]). The monophyly of Myriochele, as currently circumscribed, is supported by the presence of acicular chaetae with an elongated tapering distal end and a smooth surface and by the shape of the head, which is similar in width to the segments with a rounded anterior margin [17].
According to the latest phylogenetic hypothesis, oweniids possessing palps (Myriowenia and Owenia) branch off at the base of the family tree, with Galathowenia and Myriochele as sister groups [17,57] (Figure 11). If this hypothesis is corroborated, it would mean that the presence of palps is the plesiomorphic condition and have subsequently been lost in Galathowenia and Myriochele.

Taxonomic History
The first described oweniid was Owenia fusiformis Delle Chiaje, 1844. The genera Owenia and Myriochele are the most speciose containing 19 and 20 species respectively [85]. Galathowenia comprises 12 species with only one species described in the 19th century (Galathowenia australis (Grube, 1866), as Psammocollus). Finally, Myriowenia comprises only four species and was erected well into the 20th century.
Taxonomic studies on the family are closely related to significant historical events ( Figures 12A and 13). For instance, a number of species were described from the mid-19th century to early-20th century (i.e., three Owenia, five Myriochele, one Galathowenia), followed by almost non-existent progress extending to the end of WWII (in which only Myriochele picta Southern, 1921 and Galathowenia oculata (Zachs, 1933) were described). From 1945, studies increased substantially; Myriowenia was erected by Hartman in 1960 and further species were described belonging to Myriochele and Galathowenia. However, the number of Owenia species did not increase in the same way and this is perhaps due to the belief that O. fusiformis was cosmopolitan. Finally, the 21st century is witnessing a substantial increase in number of new oweniids described, influenced heavily by the reconsideration of the status and past records of both O. fusiformis and M. heeri.
The taxonomic bibliography of the group includes several regional studies that include information about genera/species, keys and/or comparative tables of distinguishing characters. Among the most relevant are Fauvel [151] on Atlantic and Mediterranean French coasts, Ushakov [152] in the USRR, Hartman [153] in Antarctica, Day [154] in South Africa, Milligan [72] in the Gulf of Mexico, Nilsen and Holthe [65] in Scandinavia, Imajima and Morita [76] in Japan, Hartmann-Schröder [155] in Germany, and more recently Blake [71] in California, Cantone and Di Pietro [61] and Parapar [62,63] in Antarctica, Parapar [67,68] in Iceland, Gil [156] from European coasts and Capa et al. [17] in Australia.

Taxonomic Characters
The shape of the body is consistent across members of the family, i.e., elongated, cylindrical and fairly rigid [71,157]. The prostomium and peristomium are fused forming the head [65,83] and its shape differs across the genera: distally truncated without palps (Galathowenia), distally rounded without palps (Myriochele), rounded and provided with a pair of grooved tentacles (Myriowenia) and with multi-lobed tentacles, in the shape of a tentacular crown (Owenia) (Figures 14 and 15). These head structures are generally used for particle selection and tube building (e.g., [158][159][160][161]), and are homologous between them and seemingly also to the palps of magelonids [161]. However, we consider further work is needed to assess the homology of the head appendages of members of these two groups.
The mouth and lips are differently developed among members of Oweniidae; Myriochele, Myriowenia and Owenia are provided with a button-hole mouth with shallow lips ( Figure 14B-D); while Galathowenia bears a cleft directed backwards from the mouth, forming two large lateral lips ( Figures 14A and 15A) (see [17,56]). An epithelial fold between the head and the first segment, often referred to as a collar is common in some oweniid species [60,68,71,76,79] (Figures 14E and 15D).
Posterior to the head, the trunk, consists of several chaetigers (up to 30 in species of Galathowenia and Myriochele, and up to 60 in species of Owenia and Myriowenia; [17]). The anterior three chaetigers are provided with uniramous parapodia whilst the following chaetigers are biramous (Figures 14 and 15). These regions have been previously referred to as thorax and abdomen, respectively (e.g., [65,71]), names that have recently been discouraged (e.g., [17,67,80]). The number and relative lengths of uniramous chaetigers is species specific and is one of the main taxonomic features used for species discrimination, as was shown by Parapar et al. [68] in the review of Myriochele (and the then accepted Myrioglobula) in Icelandic waters. It is common to find species descriptions including a formula indicating the relative length of uniramous (or else three anterior) segments, for example, for Galathowenia annae Capa, Parapar and Hutchings, 2012 the following formula is given RLUS = 1:2:1, where the second chaetiger is twice the length of the first and third [17]. Genera specific patterns can also be observed, especially in members of Myriowenia and Owenia, which always possess three uniramous chaetigers, while Galathowenia have two or, more commonly, three uniramous segments. Species of Myriochele show a wide range of variability for this feature from species with a single uniramous chaetiger (e.g., Myriochele islandica ), others with two (e.g., Myriochele olgae Blake, 2000) and others with even three uniramous chaetigers (e.g., M. heeri). In some oweniids additional segments are added before the pygidium, thus chaetiger number is highly dependent on age. Biramous chaetigers are unequal in length, the anteriormost being longer and usually those towards the pygidium becoming progressively shorter (e.g., [17,67,80]).  Oweniids possess ciliated, longitudinal lateral grooves, running between the notoand neuropodia, homologous to the lateral organs of other annelids ( Figure 14B, [17,161]). In addition, glandular longitudinal grooves (lacking cilia) are present on ventral body surfaces, while there is a non-glandular and non-ciliated groove running along the middorsum [17,62], however, none of these characters are used in taxonomic descriptions.
Parapodia are conspicuously reduced in oweniids compared to other annelids, including the sister group Magelonidae. They lack structures such as cirri, branchiae or epidermal papillae ( Figure 16A). The notochaetae emerge directly from the body wall and neurochaetae from low glandular transverse tori [162]. Notochaetae include fine distally tapering capillaries, known as "capillary chaetae" in Galathowenia, Myriowenia and Owenia ( Figure 16A,B) [17,77], that coexist with the shorter and comparatively more robust chaetae, with a narrow distal tip, known as "acicular chaetae ", in Myriochele [17,62,65,67,68,76] ( Figure 16A). Capillary chaetae show external structures under SEM like densely packed scales. Morphometric analyses of these scales have shown value for identification of species of Owenia (e.g., [163,164]), although some authors found the intra-and interspecific variation to overlap [77]. Neurochaetae come in the shape of dentate hooks with long shafts (i.e., uncini) provided with two distal teeth, homologous to the capitium of other annelids but lacking instead a true rostrum ( Figure 14A,B, Figure 15A-C and Figure 16C,D) [57]. Uncini are arranged in multiple transverse rows per segments [146] forming the uncinal patches ( Figure 16A). Uncinal morphology has been long used for generic assignment. Species of Galathowenia possess teeth arranged in an oblique row (with one tooth higher than the other). Teeth are generally arranged vertically (one on top of the other) in members of Myriochele and Myriowenia, and horizontally (teeth occurring side by side) in Owenia species [57]. Morphometric analyses with the aid of SEM have also been performed within members of Owenia in order to find distinct traits to separate species among this morphologically homogenous group [77,163,164]. Oweniids display a variability of pygidial shapes. They may be simple, bilobed, multilobed or pointed ( Figure 17) [57] showing intrageneric variability but being species specific [62]. Even if reported, the presence of true pygidial cirri has not been confirmed yet and the structures reported for Myriowenia gosnoldi Hartman, 1965 may be related to body regeneration after breakage [17]. With genera being fairly homogenous morphologically there has been a reconsideration of the number and identity of morphological characters used in oweniid taxonomy and characterisation of species compared with early studies (Figure 18). For instance, the unique combination of approximately eight characters were needed to identify species prior to 1970, which were visible under a stereo or optic microscope (but see [165]). In contrast, there are roughly twice as many characters needed to correctly identify oweniids to species currently (e.g., [65]). While the same characters are considered across the family due to the relative homogeneity in the oweniid body shape, the anterior end shows more variation among genera and is therefore more useful for species characterisation. However, for all members of Oweniidae, the use of a SEM is essential, and as mentioned before, very often morphometric analyses are needed for species discrimination [77,81,163,164]. Branching patterns of the head tentacles have also been shown to bear taxonomic information for some Owenia species [78].
The use of methyl blue stain and the different colouration body patterns in Owenia is being increasingly used (e.g., [71,164]) although its use as a taxonomic character in the characterisation of the species is not generally used in recent works (e.g., [74,78]). Iconography has also shown much variation across the literature through time (Figure 19), even showing evident differences in artistic quality. For instance, drawings and illustrations by Rioja [167] and Day [154] are generally superior to those found in some of the later works. In the case of M. heeri, accurate and artistic illustrations can, however, be found in early work (e.g., [169]).

Internal Morphology
The first studies on internal anatomy date back to the beginning of the 20th century [158] but are still mostly limited to a handful of species such as those referred as O. fusiformis and M. heeri [158,[170][171][172], the microanatomy of the central nervous system has received more attention due to its relevance in understanding the phylogeny of this group, and of annelids in general (see below).
The body wall in Oweniidae is composed of longitudinal muscles only, which are developed and grouped into bundles that gradually decrease in thickness [173,174]. The coelomic lining of the body wall is composed of a layer of monociliated peritoneal cells that cover the muscle cells except near the ventral and dorsal mesenteries, where longitudinal muscles form the lining [173,175].
The use of histological sectioning (HIS) (Figure 20A,B) and micro-computed X-ray tomography (micro-CT, or µCT) (Figures 20C,E and 21) has allowed for a re-examination and better assessment of the annelid internal anatomy [176]. Internal coelomic space is occupied mostly by the gut (Figures 20A,B and 21B-D), which is composed of three regions: foregut (esophagus and stomach), midgut (intestine) and hindgut (rectum), which can be easily distinguished histologically [177]. The anteriormost region of the gut of Owenia, Myriochele and Myriowenia has been described or illustrated as having a ventral pharyngeal organ [59,65,68,155,[158][159][160][161]178]; this organ has often been reported incorrectly in previous work as a proboscis (e.g., [71]). Correspondingly, protractible ciliary folds on the dorsolateral walls of the foregut were observed in Galathowenia, Myriochele and Myriowenia [17,68,178]. The buccal region and the esophagus are lined by ciliated epithelial cells interspersed with glandular cells that secrete digestive enzymes; stomach walls are thick and involuted ( Figure 20B) containing two types of numerous glandular epithelial cells [160]. The intestine lacks glandular cells and serves purely for absorption. The rectum is thin-walled and less convoluted; faecal pellets are mostly composed of sand grains and other material that are released when the animal reverses into its tube [160]. The gut musculature is responsible for the movement of material through the gut and is helped by contraction of the body wall muscles [57].
The nervous system shows some unusual features when compared to other annelids and is currently considered in phylogenetic studies [2]. It is largely intraepidermal with a well-developed nerve plexus, including many peripheral nerves but lacks segmental ganglia [179,180]. The absence of nuchal organs [172] a condition shared with the Magelonidae, could be regarded as plesiomorphic among the annelids and supporting their basal position at the base of the annelid tree [4]. However, further investigations are needed to truly verify their absence [57,181]. Eyespots are usually present in the head of adults and are closely associated to the brain [182] but it is still unclear whether they develop from the prostomium or the peristomium [17]. Helm et al. [2,183] compared the development of oweniid neuroarchitecture with that of other annelids based on histology, SEM and immunohistochemistry and found that development and metamorphosis of the mitraria larva is mostly similar to that of other annelids irrespective of the drastic changes in body shape during metamorphosis. The central nervous system in Oweniidae is medullary, with a ring-shaped and basiepidermal brain lacking higher brain centers, ganglia associated to ventral nerve cord or complex sensory organs ( Figure 21A-C) [147,148,161]. In Owenia borealis Koh, Bhaud and Jirkov, 2003, the glandular structures located in the first two segments are parapodial organs (Figures 20B,C and 21D) which contain secretory cells producing the tube materials. Their relationship with the nephridial sacs located in first chaetigers of the abdominal region remains obscure and warrants further investigation [148].   [161]; (D) from Rimskaya-Korsakova et al. [148]. All modified from the original. Abbreviations: Br-brain ring, Clob-crown lobes, Gf-giant fibre, Lmc-lateral medullary cord, M-mouth, Np-neuropil, Pgo-a/p-parapodial glandular organ-anterior/posterior, So-somata, Stm-stomach, Tnot-thoracic notochaetae, Vmcventral medullary cord. In (D) first two thoracic chaetigers are indicated.

Species Diversity and Distribution
The family Oweniidae is currently composed of 55 valid species in four genera, 16 species in genus Galathowenia, 18 in Myriochele, 19 in Owenia (plus three taxon inquirendum) and two in Myriowenia [57,85]. The first species to be described was Owenia fusiformis Delle Chiaje, 1844 from Sicily and the group was raised to the rank of family by Rioja [184].
Oweniids have colonised a wide range of marine habitats across the world, from the intertidal fringe to the deep sea [185][186][187] (Figure 12B), occurring in all marine ecoregions (sensu [86]). Literature reviews show Galathowenia, Myriochele and Owenia to have broad distributions; Myriowenia appears restricted to the Atlantic and Pacific coasts of North and South America (Figure 22). There are, however, apparent gaps in the knowledge of diversity and distribution for all genera. According to the type localities of all described species, the temperate East Northern Atlantic (with nine species described thus far) appears to be as well-known as temperate Australasia (eastern coast of Australia); however, in contrast, data are still lacking for Eastern Indo-Pacific, Temperate South America and South Africa, and Western Indo-Pacific ( Figure 22). According to the international network and data infrastructure GBIF [118] there are up to 112,695 records for this family. Records are heterogeneously distributed, with many reports concentrated in Atlantic and Pacific North America, the Arctic and European Atlantic, the Australian Indo-Pacific and New Zealand; reports are fewer in comparison for the Subantarctic Scotia Arc, the Mediterranean Sea and some European and North American areas. These records correspond mostly to the genera Galathowenia (47%), Myriochele (17%) and Owenia (34%) ( Figure 23A-D), and particularly to a few numbers of species considered previously as cosmopolitan (see below). Myriowenia records account for just 0.1% of all references and those are restricted to North American Atlantic and Australian Pacific ( Figure 23C). Regarding Galathowenia, the most reported species are Galathowenia fragilis (Nilsen and Holthe, 1985) and G. oculata, particularly from North America and Europe ( Figure 23A); a similar distribution pattern is found for Myriochele, mainly represented (83% of records) by M. heeri, Myriochele danielsseni Hansen, 1879 (both mostly in Europe), and, to a lesser extent, by M. olgae. Furthermore, Galathowenia and Myriochele are the only genera reported so far from subantarctic latitudes. Owenia is also often reported from the northern Hemisphere but also shows many records from Australia and New Zealand ( Figure 21D); O. fusiformis accounts for up to 80% of the reports while Owenia collaris Hartman, 1955 is the most reported taxon from the North America Pacific. However, it is likely that further sampling and examination of oweniid collections from across Australasia and Pacific regions would reveal new undescribed taxa. Several species such as O. fusiformis, G. oculata and M. heeri, have been traditionally reported as cosmopolitan or broadly distributed; recent available data suggests that most of these species fit better within the concept of complexes of morphologically similar species showing narrow ranges of distribution in regard to environmental features (e.g., [63,66,67,75,77,79,80,[188][189][190][191][192][193]). However, the only study that has included molecular information to assess this hypothesis was that of Jolly et al. [194] who analysed several populations of Owenia in the northeast Atlantic, considering COI sequences and concluding that several species exist there that are, in turn, distinct from those occurring in the Mediterranean Sea.

Biology and Ecology
Knowledge about reproductive and developmental strategies is restricted to species of Owenia (summarised in [57]). Mature animals are dioecious [83]. It has been indicated that reproduction occurs once or several times a year depending on the geographical location and environmental conditions. For instance, some populations of O. fusiformis in NE Atlantic were found showing discrete seasonal spawning (e.g., [159,195]) while others breed throughout the year [196]. However, this disparity of reproductive modes should be considered with caution as some of the studied animals could have been misidentified as O. fusiformis and, in fact, belong to several different Owenia species inhabiting these waters (e.g., [66,194]). Maturity was reported after the first year of benthic life [195] and fecundity seems correlated with the size of females, but number of spawned eggs can reach up to 85,000 [195,197].
Owenia collaris and O. fusiformis embryos undergo holoblastic spiral cleavage to form a typical coeloblastula [83,198], where the anus is developed from the blastopore and the mouth is later formed from a secondary opening [83,159]. This development pattern shares more similarities to that seen in deuterostomes [83]. In addition, the larval cilial bands are formed later in the ontogenetic stages compared to other annelids, these are also short and simple and are differently arranged to the typical trochophore [83]. This mitraria larva develops completely 24 h after fertilisation in Owenia, having already a defined mouth and anus and the ability to swim. Several morphological changes take place in the larvae for about four weeks, and then metamorphosis starts with the protrusion of the trunk segments from the larval hyposphere, the reduction of the episphere as part of the head, and the disintegration of the ciliary bands [83]. Early juveniles have a head (fused prostomium and peristomium), a uniramous segment with two sets of chaetae, six or seven biramous segments each with a single pair of chaetae and one pair of uncinal tori, and a pygidium. Soon after metamorphosis the juvenile readily builds the tube by gathering materials with the anterior end; after two weeks, pairs of buds start to form the prostomial tentacles typical of adult Owenia. As noted above, Helm et al. [183] show that development and metamorphosis of the mitraria has many parallels to other annelids irrespective of the drastic changes in body shape during metamorphosis.
Asexual reproduction has been suggested in several species of Galathowenia, Myriochele and Owenia due to the presence of breakage grooves in anterior segments [17,57,62,67,199,200], bodies already broken at this level [62,63] or anterior/posterior ends showing different degrees of regeneration [57].
All oweniids are tube dwellers; the tube is flexible and made after mucous secretions binding foreign particles (i.e., sediment grains, foraminiferan thecae, shell fragments) [160,201,202]. Branchial tentacles and buccal lips are involved in tube building (e.g., [158][159][160]). It has been argued as to whether the materials utilised for the tube are preselected, and whether there is any preference for type and size of particles used [64,66,164] or whether the animal simply relies on any available material [67,68]. It appears that tube-building patterns depend on the species under consideration, development and the availability of materials. Experimental work with Owenia caissara Silva and Lana, 2017 has demonstrated that the animal is unable to rebuild the tube if completely released from it [164]; in turn, those partly removed were able to rebuild it using a wide range of sediment particles including filamentous debris, at least in laboratory conditions.
Oweniids are suspension or surface deposit feeders that collect particles at the sedimentwater interface [71,160]. Species of Owenia, with a tentacular crown provided with cilia, create water currents that direct suspended particles into the crown and the mouth; Owenia and other oweniids bend over the sediment and collect sand and detritus directly from the bottom surface. Particles are sorted by the highly muscular lips helped by mucus [203]; the shape of the upper lip can be modified in order to facilitate sorting of materials, similarly to that observed for terebellids [204]. Feeding behaviour in Myriowenia has not been studied, though they may gather particles from sediment surface using their long palps [71].
Oweniids are quite common in subtidal soft seabeds, occurring across a wide range of depth, but very little is known about specific species habitats. Data taken from original descriptions of all oweniid species (i.e., Table A2) shows that: (1) only two species have been reported from the intertidal and the number of species at depths of less than 200 m is about 50% of the total; (2) about 20% of species show a wide depth range, from shallow waters to the continental slope realm, reaching 2000 m deep or more; (3) few species (e.g., Myriochele malmgreni) seem to be typical of deep waters. The second group includes the widely distributed G. fragilis, G. oculata, Galathowenia scotiae (Hartman, 1978) and M. heeri, but also recently described species such as O. borealis (Table A2); this suggests again the existence of species complexes or rather frequent misidentifications attributed to those species. In contrast to oweniids (see above), magelonids show a preference for shallow waters, with only handful of species that occur from 400-1000 m. Further work is certainly warranted to clarify depth ranges for most species.
Most ecological knowledge comes from studies done at European latitudes and about O. fusiformis, a species that has been attributed in the past to a cosmopolitan distribution but actually represents a complex of sibling species (e.g., [82,[205][206][207]). Owenia fusiformis may reach high densities in sandy sediments, of up to 1000 indiv./m 2 [82] while those of the small-sized G. oculata range up to 500-700 indiv./m 2 [205,208]; an increase in the abundance of the latter has also been reported during early stages of eutrophication related to mariculture [209]. In the Northeast Atlantic, O. fusiformis may also be the dominant species in terms of biomass in shallow sandy-muddy sediments [210]. In general, the very presence of dense aggregations of oweniid tubes may serve for sediment stabilisation [211][212][213], and allows for an increase in local benthic diversity by favouring larval settlement and providing shelter for other invertebrates as other tube-building polychaetes do [213]. The ability of O. fusiformis to switch between deposit-and filter-feeding has been suggested as being a key factor for successful colonisation of habitats subjected to variations in hydrodynamism [206,214]. Finally, oweniids may serve as a relevant source of food for demersal fishes and flatfishes [155]; for instance, Owenia is among usual prey items for opportunistic preying fishes depending on the season [215].

Conclusions and Future Perspectives
Despite some efforts to increase the known biodiversity information of Magelonidae and Oweniidae (especially since the 60s in the former group and since 2000 in the latter), there are still large gaps in our taxonomic knowledge and the number of species world-wide, is most likely under reported, and in need of reappraisal. Additionally, the evolutionary relationships and subsequent classification for members of these two groups need to be assessed combining different sources of data.
Several marine realms and world seas should be further explored and sampled. Thus, Australia, South America, the Western Pacific and Africa at the very least for Magelonidae, while in East Atlantic, North West and West Indo-Pacific and also South America for Oweniidae. Whilst the forthcoming review of magelonids of Western Africa (Mortimer et al., in preparation B) should help in part to resolve the situation in that region, more work is needed, such as the description of the aforementioned undescribed Octomagelona species.
Most of our understanding of the biology, reproductive biology, anatomy and behaviour of both families have come from a relatively small number of studies on one or two species. Further work based on traditional and modern microscopy imaging technique (e.g., micro-CT, SEM) in combination with molecular methods (e.g., molecular phylogenies and species delimitation analyses) are needed to look at both assessing current species diversity and intraspecific variability. This is especially useful in morphologically homogenous groups, such as magelonids and oweniids, and crucial for a precise and effective assessment of their diversity, their distribution patterns and specific ecological preferences, and relevant for assessing putative cosmopolitan or broadly distributed species.
Although the latest molecular analyses suggest both families are part of the basal radiation of the Annelida (Palaeoannelida), morphological characters do not support a close relationship. Further investigation of potential homologies, understanding the morphological diversity found in each group. Assessment of the relationships within both Oweniidae and Magelonidae are also warranted, ideally integrating different sources of data.  and Cardiff University) for providing the image of magelonid palps feeding utilised in this paper. Authors deeply thank reviewers' comments on the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.     [85] name it as Owenia assimilator Caullery, 1944 but Caullery [165] reports it as O. assimilator Andrews. Gil [156] affirms that Caullery was mistaken, because he was referring to Ammochares aedificator Andrews, 1891, a species that Read and Fauchald [85] report as synonym of Owenia fusiformis.