Limno-Terrestrial Tardigrada of Sub-Antarctic Islands—An Annotated Review

: Research on the limno-terrestrial Tardigrada fauna of the Sub-Antarctic zone began almost 120 years ago. Here we present an overview of the literature data on the presence of tardigrades on sub-Antarctic islands, including the substrates on which they have been found. From 32 published sources, we found original data on the occurrence of 49 currently valid species on six sub-Antarctic islands/island groups. Of these, 9 species (18%) were originally described from this zone, another 13 species (26%) were described from Continental or Maritime Antarctica, almost half of these species (22 species—45%) were originally described from European localities, and the remaining 5 species (10%) were originally described from South America, Africa, or Australia. The validity of the records of individual species is discussed. We consider the presence of 29 species in the Sub-Antarctic to be doubtful. We ascertained a total of 90 combinations of species and islands or island groups. More than half (64%) of these will require conﬁrmation in the future because we currently consider them doubtful. We can conclude that the tardigrade fauna of the sub-Antarctic islands is only very superﬁcially known, and the occurrence of most species in this zone must be veriﬁed.


Introduction
Tardigrades are microscopic hydrophilic metazoans with a bilaterally symmetrical body and four pairs of legs that typically terminate in claws.The length of tardigrades is between 50 µm for the smallest juveniles to over 1200 µm for the largest adults of some species (e.g., [1,2]).
Tardigrades live in diverse marine, freshwater, and terrestrial habitats throughout the world, from oceanic depths to the tops of mountains [1].
Most of the known non-marine tardigrade species live on terrestrial substrates such as mosses, liverworts, cushion-shaped plants, lichens, and leaf litter.Some of these species also occur in freshwater habitats, while others are found exclusively therein [1,3].
The phylum Tardigrada is distributed all over the world.Although tardigrades represent organisms with passive dispersal that should not have distribution barriers, many limno-terrestrial species have a limited range of distribution [2].
Tardigrades reproduce only via eggs.Limno-terrestrial environments are most commonly colonized by populations of female eutardigrades and heterotardigrades using parthenogenesis as the means of reproduction [2].
Tardigrades can survive adverse periods in their environment by means of two types of dormancies: quiescence and diapause.Quiescence is represented by four forms of cryptobiosis: anhydrobiosis, cryobiosis, anoxybiosis, and osmobiosis [4].Cryobiosis and anhydrobiosis are of particular importance for the survival of limno-terrestrial tardigrades in sub-Antarctic conditions.
Cryobiosis is induced by low temperatures and enables tardigrades to survive freezing and thawing.However, the rate of cooling must be slow, enabling frozen specimens to survive for several years.Cryobiotic ability is common in terrestrial species, but it is very rare, or even absent, in freshwater species [1,2].
Anhydrobiosis is induced by the loss of water from evaporation, which is why this ability is typical of terrestrial tardigrades.As the surrounding water evaporates, the entire body contracts, forming a barrel-shaped 'tun'.At the same time, the tardigrade loses more than 95% of its free and bound water and significantly reduces or suspends its metabolism.Adults and eggs can survive up to 20 years in the anhydrobiotic state.After the addition of water, the animal returns to an active state if the rate of desiccation was slow enough [2].
Encystment and resting eggs are involved in diapause.Encystment is known in some moss-dwelling, soil, and freshwater tardigrades.In nature, cysts can survive for months without completely exhausting their food reserves [2,4].
The number of known species of tardigrades in the world is currently close to 1500, and a few dozen new species are described every year (see [5]).
Antarctica is commonly divided into three main terrestrial biogeographic zones characterized by different climatic characteristics and different ecosystems: Continental Antarctica, Maritime Antarctic, and Sub-Antarctic (e.g., [6]).
Continental Antarctica consists of the mainland and adjacent islands with the exception of the west coast of the Antarctic Peninsula and the islands belonging to the Maritime Antarctic zone.The Continental zone is characterized by a harsh climate with an average monthly temperature not above freezing point and a total annual precipitation of approximately 10-15 cm.It lacks vascular plants, and sparse cryptogams are mainly represented by lichens (e.g., [7]).
The Maritime Antarctic zone comprises the western coast of the Antarctic Peninsula up to approximately 72 • S and adjacent islands and more distant associated islands and archipelagos (South Shetland Islands, South Orkney Islands, South Sandwich Islands, and Bouvetøya).It has a cold, humid oceanic climate with mean monthly temperatures exceeding 0 • C for 1-4 months and an annual precipitation of 35-50 cm.Floristically, the area is home to two species of vascular plants and has a relatively rich and diverse flora of bryophytes and lichens (e.g., [7,8]).
The Sub-Antarctic zone consists of a ring of small isolated oceanic islands surrounding the continent at a relatively high latitude (ca.46-54 • S).The following six small island groups belong to the Sub-Antarctic (Figure 1):
Marion and Prince Edward Islands (Republic of South Africa, Indian Ocean); 3.
The islands of this zone have cool oceanic temperate climates.For at least half the year, the mean monthly temperature is above the freezing point.The precipitation is higher than 90 cm per annum.The vegetation is tundra-like and is without arborescent vascular plants.It is composed mostly of woody herbs, forbs, pteridophytes, and cryptogams.Extensive grass heath, herb fields, and mires cover some lowland areas.Coastal regions consist of tussock grassland [7].The islands of this zone have cool oceanic temperate climates.For at least half the year, the mean monthly temperature is above the freezing point.The precipitation is higher than 90 cm per annum.The vegetation is tundra-like and is without arborescent vascular plants.It is composed mostly of woody herbs, forbs, pteridophytes, and cryptogams.Extensive grass heath, herb fields, and mires cover some lowland areas.Coastal regions consist of tussock grassland [7].
A universally accepted geographic definition of the Sub-Antarctic does not exist, although it is commonly accepted as a meaningful term for this terrestrial biological zone.Based on ecoclimatic criteria (temperature and presence/absence of trees or woody shrubs), the Sub-Antarctic includes only those islands where trees or woody shrubs are absent.These islands represent a transition zone of continuous environmental conditions between the more extreme environments of the islands and archipelagos of Continental and Maritime Antarctica (Balleny, Peter I Øya, South Shetland, South Orkney, South Sandwich Islands, Bouvetøya) and the more temperate conditions of those cold temperate ocean islands and archipelagos with trees or woody shrubs, such as Islas Diego Ramírez, the Falkland Islands (Islas Malvinas), Tristan da Cunha, Gough Island, Amsterdam and Saint Paul Islands (Île Amsterdam and Île Saint-Paul), and the shelf islands of New Zealand-Campbell Island (Motu Ihupuku), Chatham Islands, Antipodes Islands, Auckland Islands, Bounty Islands, and Snares Islands (Tini Heke) (e.g., [12][13][14][15]).It should be mentioned that some authors also considered some of these cold temperate oceanic islands to be sub-Antarctic islands (see, e.g., Tierra del Fuego in [16], Campbell Island and Snares A universally accepted geographic definition of the Sub-Antarctic does not exist, although it is commonly accepted as a meaningful term for this terrestrial biological zone.Based on ecoclimatic criteria (temperature and presence/absence of trees or woody shrubs), the Sub-Antarctic includes only those islands where trees or woody shrubs are absent.These islands represent a transition zone of continuous environmental conditions between the more extreme environments of the islands and archipelagos of Continental and Maritime Antarctica (Balleny, Peter I Øya, South Shetland, South Orkney, South Sandwich Islands, Bouvetøya) and the more temperate conditions of those cold temperate ocean islands and archipelagos with trees or woody shrubs, such as Islas Diego Ramírez, the Falkland Islands (Islas Malvinas), Tristan da Cunha, Gough Island, Amsterdam and Saint Paul Islands (Île Amsterdam and Île Saint-Paul), and the shelf islands of New Zealand-Campbell Island (Motu Ihupuku), Chatham Islands, Antipodes Islands, Auckland Islands, Bounty Islands, and Snares Islands (Tini Heke) (e.g., [12][13][14][15]).It should be mentioned that some authors also considered some of these cold temperate oceanic islands to be sub-Antarctic islands (see, e.g., Tierra del Fuego in [16], Campbell Island and Snares Islands in [17,18], New Zealand s shelf-island groups in [12,19], Falkland Islands and Gough Island in [19], and Saint Paul Island in [20,21]).
Circulation in the Southern Ocean is dominated by the continuous eastward-flowing Antarctic Circumpolar Current, which has recently split into numerous circumpolar frontal systems [22].The transition of warm, light subtropical water from lower latitudes to cold, dense Antarctic water in the south occurs in a step-like manner rather than as gradual change across the breadth of the Southern Ocean.Oceanologists distinguish narrow bands (fronts) in which the surface temperature of the water changes rapidly.Between the fronts lie zones with relatively uniform water mass properties [23].Inter-frontal zones have a steady flow and differ from each other in hydrochemical and hydrophysical characteristics [22].Of the fronts, the most significant one is the Polar Front (Antarctic Convergence), dividing the Southern Ocean into two distinct regions: the sub-Antarctic region to the north and the Antarctic region to the south.The Polar Front has variable width characterized by steep gradients at sea-surface temperature; abrupt changes in phytoplankton abundance, zooplankton distribution, pelagic bird species, and weather conditions; and sometimes by a salinity maximum at the surface [24].The sub-Antarctic and Antarctic regions of the Southern Ocean have different plankton and fish communities.Essentially, the terrestrial Sub-Antarctic zone is situated between the Antarctic and Subtropical Convergence (Figure 1) and consist of the islands in the southern Atlantic, Indian, and Pacific Oceans [25].
Tardigrada research on the sub-Antarctic islands began at the start of the 20th century.The first data were published by Richters [26] and were derived from a collection of moss samples that was received from the German South Polar Expedition.Later, Richters [20] published results from the material gathered during the Swedish South Polar Expedition.Both expeditions were directed at the Antarctic mainland, with a visit to the sub-Antarctic islands as part of their program [20,26].In the early 20th century, Murray [27] also published results from the Sub-Antarctic zone, which were obtained during the British Antarctic Expedition 1907-1909.In addition to the Antarctic material gathered, other samples, mostly mosses, were collected from several distant countries, as this expedition practically circumnavigated the world [27].
Following a long hiatus, Marcus [28] and Ramazzotti [29] contributed to the knowledge of the sub-Antarctic tardigrade fauna, but since the 1970s, data on the distribution and taxonomy of sub-Antarctic limno-terrestrial Tardigrada have been increasing with every decade.Among these works, we mention here some of those that have contributed the largest number of original findings or descriptions of species from the Sub-Antarctic zone; namely [9,[30][31][32][33][34][35][36][37][38].
Collected faunal data from the sub-Antarctic islands (among compilations from the entire world) can already be found in the monographs of Marcus (most recently in [39]), Ramazzotti (most recently in [40]), and, finally, in the monograph of McInnes [41], which was specifically devoted to the global geographical distribution of non-marine tardigrades.In it, the author lists 26 species from the Sub-Antarctic zone.
In view of the numerous nomenclature changes that have occurred since the previous publications [5], and with regard for the new knowledge about the zoogeography of limno-terrestrial tardigrades [42][43][44][45][46], we consider it useful to present this review on the distribution of limno-terrestrial Tardigrada on the islands of the Sub-Antarctic, in which we not only compile all available data on the fauna of non-marine tardigrades of the Sub-Antarctic from the oldest to the present but also express an opinion on the actual presence of each of the individual species reported from these islands.

Materials and Methods
We extracted data on the occurrence of specific Tardigrada species on specific islands/island groups of the Sub-Antarctic zone from the most complete possible spectrum of available potential sources dedicated to tardigrades of the Antarctic region, either exclusively or only marginally, from the oldest to the most current.Regarding monographs, we checked two (the first and the last) by Marcus [39,47], the final monograph by Ramazzotti and Maucci [40], as well as monographs devoted to the global distribution of limno-terrestrial tardigrades [41][42][43][44][45][46].
In our list of reported species from the Sub-Antarctic zone, we did not include taxa higher than species, species complexes, or pairs of species such as, e.g., the pair Hypsibius (Diphascon) alpinus + H. (D.) pinguis in [30] or Macrobiotus hastatus/pullari in [48].
When analyzing sources, we distinguished whether they contained primary data or data taken from another source (secondary data).

Results
Records of 49 limno-terrestrial Tardigrada species from 11 families and 25 genera in six sub-Antarctic islands/island groups (South Georgia, Marion Island from the Marion and Prince Edward Islands, Possession Island (Île de la Possession) from Crozet Islands, Kerguelen Islands, Heard Island from the Heard and McDonald Islands, and Macquarie Island) were found in 51 articles/monographs (32 contributed at least one original datum on the occurrence of a specific species on a specific island/island group, while the rest contained only secondary data).
In the following review of the records, data for each species are arranged as follows: -Currently valid name of species with citation(s) of its description/definition; -The name(s) under which the species was/were reported from the Sub-Antarctic, arranged in order of their appearance; - The type locality of the species at the level of territorial units (state, island); - The list of islands or island groups (if the name of the island was not published) from which the species was reported (references of original findings are in bold); -Substrate(s) from which the species was extracted in the Sub-Antarctic (if given in the source(s)); -Remarks on the findings of the species in the Sub-Antarctic.
Oreella mollis Murray, 1910 [27] Oreella minor Ramazzotti, 1964 [54] Oreella mollis J. Murray, 1910 [8] Oreella mollis Murray, 1910 [55] Terra typica: New South Wales (Australia) [27] Sub-Antarctic: South Georgia [8], [54] and [55] Substratum: moss [54] Remarks: The species has been recorded from the Southern Hemisphere only [43].The other records closest to South Georgia are from Argentina [56] and the Maritime Antarctic (e.g., [57]).Oreella minor was synonymized by Dastych et al. [58].Remarks: The species have also been reported from other localities in the Maritime Antarctic zone (e.g., [8]), as well as from Continental Antarctica (e.g., [63,64]).According to Miller et al. [9], specimens from Macquarie Island differ from the original description, and this species complex requires revision.Antarctic localities from the type locality, the reports of the species from the Sub-Antarctic zone are doubtful and that it is necessary to validate them.However, a redescription of the species on the basis of new material from the type locality will first be necessary.
Since Jennings [30] reported a pair of Diphascon alpinum Murray, 1906 [113] and D. pingue (and not a specific species) from South Georgia, we do not include his report in the above list of species records.
Ramazzotti [29] expressed uncertainty about the identification of this species from the Kerguelen Islands but finally favored H. convergens because this species was already known from these islands (see [39]).Marcus [39] noted the presence of the species on sub-Antarctic islands based on the finding by Richters [26] of Macrobiotus tetradactylus Greeff [120], which Marcus considered at that time to be a synonym of Hypsibius convergens.Similarly, other authors who listed the species from Possession Island [41], from the Kerguelen Islands [40,41,119], and from Heard Island [41] obviously started from [39].Convey and McInnes [8] reported the species from South Georgia, apparently on the basis of the original report in [32], but obviously had doubts about the identity of the species (without any explanation) as they called it Hypsibius cfr convergens (sic).Therefore, Velasco-Castrillón et al. [55] wrote in Supplementary material that the report of Hypsibius convergens from South Georgia in [8] is probably a misidentification.Thus, the only finding of the species in South Georgia (see [32]) is questionable.So, it is clearly necessary to confirm the presence of H. convergens in the Sub-Antarctic.Today, we know that Hypsibius convergens is the nominal species of a species complex with an unknown geographic distribution.It is most likely either a Palearctic or Holarctic species [45].Its integrative redescription based on a population from the type locality is needed [46] in order to verify the presence of the species in the Sub-Antarctic zone.Ramazzotti [29] did not state a substratum from which H. convergens was collected-we completed the probable substratum in the paragraph.
21. Hypsibius dujardini (Doyère, 1840) [49] Hypsibius (Hypsibius) dujardini (Doy.)[ Remarks: Hypsibius dujardini is the nominal species of a complex of similar species with an apparently global distribution (although it has not yet been recorded from the Australian continent) [41,46].Its recent integrative redescription [122] makes it possible to verify its identity outside the neotype locality (Paris), which is currently its only confirmed distribution.It is likely that Hypsibius dujardini sensu stricto is a Palearctic or Holarctic species [45].Within the Antarctic region, so far, it has been reported from the Maritime Antarctic (e.g., [123]) and from the Sub-Antarctic.All these previous findings are doubtful and must be verified.

Hypsibius pallidus Thulin, 1911 [79]
Hypsibius pallidus Thulin, 1911 [8,9,31,32,41,55] Terra typica: Sweden (Europe) [79] Sub-Antarctic: South Georgia [8], [31] and [32,41,55]; Macquarie Island [9] Substratum: mosses [31] Remarks: Hypsibius pallidus is a predominantly European species, but with a Holarctic distribution, and exceptionally, it has also been reported from the Southern Hemisphere [41,43,45].Within the Antarctic region, it has only been reported from the Sub-Antarctic.It belongs to the Hypsibius convergens-dujardini species complex and can easily be confused with other species of the complex.According to Kaczmarek et al. [43], the presence of the species in South America is doubtful.Therefore, we are of the opinion that the presence of the species on the sub-Antarctic islands needs to be verified.
Ursulinius G ąsiorek, Stec, Morek and Michalczyk, 2019 [144] has been reported not only from the Sub-Antarctic, but also from the Maritime Antarctic zone (e.g., [173]).In light of the description of Dactylobiotus caldarellai Pilato and Binda, 1994 [174] from Tierra del Fuego and D. ovimutans Kihm, Kim, McInnes, Zawierucha, Rho, Kang and Park, 2020 [175] from South Shetland Islands (Maritime Antarctic), all records of D. ambiguus from the Southern Hemisphere should be revisited [9].Similarly, according to McInnes and Convey [110], the Antarctic form of Dactylobiotus ambiguus is possibly synonymous with D. caldarellai.Watson [176] (according to Miller et al. [33]), Miller et al. [9], and Selkirk et al. [172] reported the occurrence of Thulinius augusti (Murray, 1907) [171] from Macquarie Island.Later analysis of some of the original samples and original microscopic slides [9] determined that the material did not represent Thulinius augusti but rather Dactylobiotus cf.ambiguus.Miller et al. [9] stated that D. ambiguus had been found in South Georgia and cited Ottesen and Meier [32], but we omit this information here because Ottesen and Meier [32]  Substratum: moss, mud from a pond bottom [32] Remarks: While the species has been reported from every continent, its distribution is predominantly Palearctic [41].Within the Antarctic region, it has only been reported in the Sub-Antarctic zone.Given that the species was originally described from the Palearctic region, and the report of the species from South Georgia was not supported with eggs (at least, the authors did not mention them), we consider it necessary that the presence of D. dispar in South Georgia be confirmed.

Discussion
In addition to the forty-nine currently valid species listed, the following taxa were also reported from the Sub-Antarctic but were not included in the above list:
The taxonomical status of the species is now considered unclear.It is considered a species inquirenda in [39,47].It is currently invalid, and due to the insufficient description of the species based on immature specimens [137], it is not possible to identify it with any currently valid species.

•
Macrobiotus krynauwi Dastych and Harris, 1995 [178]: Reported from South Georgia by Velasco-Castrillón et al. [55] (Supplementary Material, Table S1) based on data in Dastych [31] and Convey and McInnes [8].However, we did not find a mention of this species in Dastych [31].The taxonomical status of the species is now considered unclear.It is considered a species dubia in [39,47].We cannot identify this now invalid species with any of the valid ones because of the insufficient description.
From the list, we also omitted reports of tardigrades from cold temperate oceanic islands (see Introduction), which the recording authors considered to be sub-Antarctic: Île Saint-Paul [20,21], Snares Islands [17], and Campbell Island [17,18].

Conclusions
Of the 49 species reported from the Sub-Antarctic zone, 9 species (18%) were originally described directly from the Sub-Antarctic, 8 species (16%) were described from the Maritime Antarctic, and 5 species (10%) from Continental Antarctica.Almost half of the species (twenty-two species-45%) were originally described from European localities, three species (6%) were described from southern parts of Australia, one species (2%) from South Africa, and one species (2%) from South America.
Of these 49 recorded species, we consider the records of 29 species (59%) to be doubtful, and new or re-evaluated material is necessary to verify their status in this zone (see also Table 1).bold = species whose occurrence on the island is considered probable; bold and underlined = species whose occurence on the island is certain as the island is its type locality.
We ascertained a total of 90 combinations of species and islands/island groups (see also Table 1), of which 40 were not listed either in the review by McInnes [41], mainly because these records were published after 1994, or in the review by Velasco-Castrillón [55], who included in the list of taxa in Table S1 in Supplementary Material only records from a single sub-Antarctic island (South Georgia).Of the 90 species occurrences on specific islands/island groups, 58 (64%) require confirmation as we currently consider them doubtful (see also Table 1).
As can be seen from the above, the fauna of the sub-Antarctic islands is only very superficially known, and the occurrence of most species in this zone must be verified.Furthermore, with a sufficiently detailed survey, the presence of several hitherto unknown species of tardigrades will certainly be found on these islands.

Figure 1 .
Figure 1.Location of sub-Antarctic islands/island groups and major circumpolar oceanic fronts (the locations of other small islands are not drawn, with the exception of the Falkland Islands).AF-Africa; AU-Australia; SA-South America; AC-Antarctic Convergence; STC-Subtropical Convergence (Map from Hofmeyr et al. [10], reproduced with permission from the first author and the publisher; South African Journal of Wildlife Research, published by Southern African Wildlife Management Association; map modified according to Leach et al. [11]).

Figure 1 .
Figure 1.Location of sub-Antarctic islands/island groups and major circumpolar oceanic fronts (the locations of other small islands are not drawn, with the exception of the Falkland Islands).AF-Africa; AU-Australia; SA-South America; AC-Antarctic Convergence; STC-Subtropical Convergence (Map from Hofmeyr et al. [10], reproduced with permission from the first author and the publisher; South African Journal of Wildlife Research, published by Southern African Wildlife Management Association; map modified according to Leach et al. [11]).