Introduction

Hydrozoans are one of the main and most characteristic zoological groups of Antarctic benthic communities, although there are areas of Antarctica where the fauna of hydrozoans is either completely unknown or scarcely known. This is the situation for Adélie Land, a sector of East Antarctica situated between 136° E and 142° E, limited by George V Land to the east and Wilkes Land to the west.

The benthic hydroid fauna off Adélie Land is little known. Naumov and Stepanjants (1972) published the single study devoted to benthic hydroids from this area. These authors reported on 32 species collected on the continental shelf to a depth of 240 m during the XII and XV Expédition Antarctique Française (1961–1963 and 1964–1965, respectively). The material came primarily from Pointe Géologie Archipelago, although ten samples were taken off Cape Géodésie (66°40′S–139°51′E), one off Cape Jules (66°44′S–140°55′E), and two between Français Glacier and Cape Robert (66°26′S–137°57′E). Previously, Briggs (1938) had recorded two species at 66°32′S, 141°37′E, collected during the Australasian Antarctic Expedition (1911–1914). See also Peña Cantero and Fresneda Marzal (2018) for a recent report on the known benthic hydroids from the nearby George V Coast.

The present study focuses on the benthic hydroids collected in the Antarctic Specially Protected Area of Pointe Géologie Archipelago, Dumont d’Urville Sea, Adélie Land (ASPA 120), an area of considerable biological value, containing a high biodiversity, and of great importance for scientific research.

Materials and methods

The material studied comes from the area of Point Géologie Archipelago (Fig. 1), an Antarctic Specially Protected Area (ASPA 120), located between Cape Géodésie and the tongue of Astrolabe Glacier, in Dumont d’Urville Sea, Adélie Land. The material was collected using beam trawls during sampling surveys associated with the REVOLTA project (Radiations EVOLutives en Terre Adélie). Samples containing hydroids were collected at depths ranging from 19 to 172 m (Table 1). Hydroids were fixed in 70% ethanol. The collection is deposited in the Muséum National d’Histoire Naturelle of Paris, France.

Fig. 1
figure 1

Area of study and location of sampling stations: o REVOLTA I, ◊ REVOLTA II, * REVOLTA III, □ REVOLTA IV (see Table 1 for more details)

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Table 1 Samples studied and related data (J refers to Jacobsen)
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In order to have an inventory of the species inhabiting the marine bottoms in the area, a revision of the literature on Antarctic benthic hydroids was carried out. All species recorded off Adélie Land, including those absent from this collection, are included in Appendix (see Online Resource). The material studied for each species is also included in the Appendix, together with information on previous records in the area by Briggs (1938) and Naumov and Stepanjants (1972).

As the collection mainly consists of relatively well-known species, no description is provided, with the exception of Oswaldella antarctica, a species not recorded since its original description in 1904, and Oswaldella occulta, recently described in 2018. On the other hand, as the information on the ecology and distribution of most of the species in this collection has been given in recent publications, I will deal here only with the new data provided by the present study.

In the analysis of geographical and bathymetrical distribution, the patterns considered by Peña Cantero and García Carrascosa (1999) and Peña Cantero (2004), respectively, have been used.

The colonial nature of the hydrozoans and the fact that colonies often come on board in fragmented or poor condition make quantative analyses difficult, if not impossible. As an approach to assess the importance of the species in the communities, they were classified according to their frequency of occurrence in the samples. Four categories were recognised following Peña Cantero and Majón-Cabeza (2014): ubiquitous species (≥ 30% of stations), very common (between 30 and 20%), common (between 20 and 10%) and rare or accidental (≤ 10%).

For the statistical analysis, a presence/absence dataset was built and a similarity matrix was calculated using the Sørensen similarity index, following the methodology used in Soto Àngel and Peña Cantero (2017). From the similarity matrix, a cluster analysis (hierarchical agglomerate by group average) was performed, with the SIMPROF test, to determine the relationship between samples with statistical support of the groups and to investigate the structure of the hydroid assemblages. A SIMPER analysis was carried out to find out the species characterising each assemblage. The PRIMER 6 software package (v.6.1.6) was used for the statistical analyses (Clarke and Warwick 2001).

Results

Biodiversity

35 species were found in the hydroid collection gathered during the REVOLTA campaigns in the area of Pointe Géologie Archipelago (Table 2); one species could not be identified at the species level. Fifteen species represent new records for the area (Table 2), bringing the number of known species to 45, although the presence of some of the previously reported species is uncertain (see Online Resource).

Table 2 Benthic hydroids found in the present study (in bold new records for the area of study)
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The species belong to the subclasses Anthoathecata and Leptothecata, 14 families and 18 genera. Anthoathecates are represented only by Rhizorhagium antarcticum, Hydractinia dendritica and three species of Eudendrium. The species of Leptothecata belong to the families Campanulariidae, Campanulinidae, Haleciidae, Kirchenpaueriidae, Lafoeidae, Phialellidae, Phylactothecidae, Schizotrichidae, Staurothecidae and Symplectoscyphidae.

Symplectoscyphidae is by far the most diversified family with 11 species (31%), followed by Eudendriidae and Kirchenpaueriidae with 3 (9%). Symplectoscyphus, with eight species, is the most speciose genus (23%), followed by Antarctoscyphus, Eudendrium and Oswaldella with 3 (9%). In general, hydroid occurrence is low; 19 species were found in one or two stations. Oswaldella terranovae is the species with the highest occurrence, being present in 22 stations (69%), followed by Campanularia hicksoni found in 9 (28%), Halecium interpolatum in 8 (25%), Symplectoscyphus glacialis in 7 (22%), and Antarctoscyphus elongatus, Campanularia sp. and O. occulta in six stations (19%).

Of the 35 species recorded, only O. terranovae is classified as ubiquitous (≥ 30%), three (C. hicksoni, H. interpolatum and S. glacialis) as very common (between 30 and 20%) and six species (A. elongatus, Antarctoscyphus spiralis, Billardia subrufa, Campanularia sp., O. occulta and Schizotricha nana) as common (between 20 and 10%). The remaining 15 species are considered rare or accidental (≤ 10%).

The station with the greatest number of species was 27bc, with 34.3% of the species represented, followed by 42 with 22.9%, and the stations 43, 45, 50 and 87b, with 20%. Nevertheless, almost half of the stations (15 stations, 46.9%) have a much reduced hydrozoan diversity, with just one or two species present.

Community structure

The results from the cluster analysis show four distinct assemblages; the largest one, moreover, clearly subdivided into two groups (Fig. 2). The SIMPROF test supports (p 0.05) four of the five groups considered. Assemblages A, B and D comprise only three stations each, at depths from 57 to 117 m, 38 to 172 m and 30 to 43 m, respectively. The much larger group, assemblage C, includes 21 stations, from depths between 36 and 133 m.

Fig. 2
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Cluster analysis based on Sørensen similarity index with SIMPROF test

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The study of the species associated with each assemblage allows determining the hydroid fauna that characterises them (Table 3). Assemblage A, with a similarity of 22.22, is defined by Staurotheca antarctica and Symplectoscyphus vanhoeffeni, both contributing uniformly to the total similarity. Group B, with a similarity of 55.56, is characterised by A. elongatus, which is the only species responsible for all similarity. Group D, with the lowest similarity (19.42), is characterised by two species, Symplectoscyphus naumovi, present in all three stations, and B. subrufa, which is present in two. Finally, assemblage C, the largest, with 21 stations, has a similarity of 42.38, and is mainly characterised by O. terranovae, present in all stations, but also by H. interpolatum, C. hicksoni, Campanularia sp., O. occulta and S. nana. As indicated above, Group C is further divided into two large groups, supported by the SIMPROF test. Group C1, composed of 12 stations and with a similarity of 56.59, is defined by O. terranovae, which contributes to more than 95% of the similarity. On the other hand, Group C2, consisting of nine stations with a similarity of 45.05, is characterised by O. terranovae, likewise present in all stations, but also by H. interpolatum, S. glacialis, C. hicksoni, Campanularia sp., S. nana and A. spiralis; all these species contribute to more than 92% of the similarity (Table 3).

Table 3 Internal similarity within the assemblages found, number of species and species composition
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The hydroid assemblages found show a strong dissimilarity amongst them (Tables 4 and 5). The lowest dissimilarity (67.19) is found between groups C1 and C2. Dissimilarity is much greater among the remaining assemblages, reaching 100% between A and B, and A and C1.

Table 4 Percentage of dissimilarity between the assemblages
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Table 5 Percentage of dissimilarity between the assemblages (with C subdivision)
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Taxonomy

Family Kirchenpaueriidae Stechow, 1921

Oswaldella antarctica (Jäderholm, 1904)

(Fig. 3).

Fig. 3
figure 3

Oswaldella antarctica (Jäderholm, 1904). a stem fragment showing cauline apophysis and basal part of hydrocladium; b–d cauline apophyses showing ‘mamelons’ (arrows) in different development; e–f unforked hydrocladial internodes. Scale bar: 200 µm (a, e–f), 100 µm (b–d)

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Description. Basally truncated stem 23 mm high, with a few stolons in the most basal 2 mm. Stem divided into internodes. Stem diameter distinctly increasing distally, from 200 to 460 µm. First ten internodes with a single apophysis; remaining ones with two. Typically, apophyses arranged alternately in one plane. Apophyses relatively long (200–230 µm) and thin (130–140 µm at distal part), making a 45° angle with longitudinal axis of stem. Cauline apophyses with one ‘mamelon’ on the same side and two axillary nematophores. Apophyses followed by typical hydrothecate internodes or by a series of short ahydrothecate internodes, probably associated with regeneration.

Hydrocladia typically bifurcated; a third-order hydrocladium observed once and a few unforked hydrocladia also present. Hydrocladia short, with up to six hydrothecae. Hydrocladia divided into homomerous internodes. Each with one hydrotheca, on middle of internode, one mesial superior nematophore, emerging through a simple hole in perisarc, placed behind free part of adcauline hydrothecal wall, and one mesial inferior nematophore, provided with a tiny, much-reduced nematotheca located on a slightly raised part of internode.

Hydrotheca elongate. Abcauline hydrothecal wall straight; abcauline length slightly increasing along hydrocladium (from 240 µm, at first hydrotheca, to 290 µm, at fifth one). Adcauline hydrothecal wall with distinct free part. Aperture circular, 200–220 µm in diameter; rim even.

Remarks. Oswaldella antarctica was only known for sure from the original description (see Peña Cantero and Vervoort 1995 for a redescription of the type material). While the species had been reported repeatedly, the records have been demonstrated to belong to other species of the genus (Oswaldella blanconae, Oswaldella erratum, Oswaldella incognita, Oswaldella rigida and O. terranovae). The species consequently could be considered very rare. The material studied here undoubtedly belongs to O. antarctica, coinciding with the type material in all details, including the shape of the hydrotheca, even though only a few hydrothecae of the present material are in good condition.

Even though the cauline apophyses are typically arranged alternately in one plane, there are two internodes at the distal part of the stem with opposite apophyses. This, however, is likely related to an issue of regeneration. Both internodes are preceeded by an internode distinctly shorter and deprived of apophysis. In fact, before one of these shorter internodes, the more basal one, the typical internode shows a clear fracture.

Most cauline apophyses are followed by a series of short ahydrothecate internodes, which is probably also associated with regeneration. In fact, the most distal and, consequently, youngest internodes do not have ahydrothecate internodes and are directly followed by a typical hydrothecate internode.

Oswaldella antarctica was only known off Seymour Island (Graham Land), in West Antarctica, from a depth of 150 m (Jäderholm 1904; Peña Cantero and Vervoort 1995). Our material, collected at a depth of 87 m, represents the second record for the species, and the first one from East Antarctica, pointing to a circum-Antarctic distribution.

Oswaldella occulta Peña Cantero and González Molinero, 2018

(Fig. 4).

Fig. 4
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Oswaldella occulta Peña Cantero and González Molinero, 2018. a stem fragment showing two cauline apophyses and one hydrocladium; b forked hydrocladial internode; c–d unforked hydrocladial internodes (c frontal view, d lateral view); e unforked hydrocladial internode and basal part of gonotheca; f distal part of gonotheca. Scale bar: 200 µm

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Remarks. Oswaldella occulta was recently described and it is well characterized, not needing a redescription. However, the abundant material in this collection allows me to add important additional information about some features. The material studied here undoubtedly belongs to this species, coinciding perfectly with it in every detail, as for example in the presence of two axillary nematophores on the cauline apophyses, each emerging through a simple hole in the perisarc, and in the absence of ‘mamelons’. The hydrotheca is low, slightly higher than wide (around 200 µm in diameter at the aperture), although the length of the abcauline wall increases along hydrocladium [e.g., from 250 µm at first hydrotheca to 300 µm at the seventh one in station 43; slightly longer than previously reported (up to 240 µm in Peña Cantero and González Molinero 2018].

According to Peña Cantero and González Molinero (2018), colonies of O. occulta seem to form tufts of unbranched stems, which has been confirmed here. Only in station 45, one stem gives rise, on the same side and perpendicular to the plane formed by the cauline apophyses, to two secondary stems originating directly from stem internodes, far from the apophyses. These secondary stems start with a short intermediate internode followed by typical internodes with apophyses.

Even when the typical condition seems to be having bifurcated hydrocladia, it is also quite common to find much branched hydrocladia, usually one tertiary hydrocladium on each secondary one, but even a much greater number has been observed; for example, in station 30, up to five third-order hydrocladia on a single secondary hydrocladium. Peña Cantero and González Molinero (2018) reported up to six third-order hydrocladia, three on each secondary one (in one case four).

A feature that seems to be characteristic of the species is the frequent presence of hydrocladial unforked internodes between the branched ones. Peña Cantero and González Molinero (2018) found it on two occasions, but here it has been observed in all the samples studied.

Gonothecae of O. occulta were unknown (Peña Cantero and González Molinero 2018), but putative female gonothecae are present in the material from station Jacobsen. They are club-shaped, with a large circular distal aperture and a diaphragm at their basal third. They are inserted on the elevated part of the internode under the mesial inferior nematophore.

There is some uncertainty about the assignation of the material from station 20b because the hydrothecae are more elongated (the length of the abcauline wall ranges from 300 µm at 1st hydrotheca to 360 µm at the 8th one). It reminds O. incognita in the shape of the hydrotheca and in the bifurcated hydrocladia, although a third-order hydrocladium was once observed on a secondary one. However, O. incognita has more robust hydrothecae and internodes, and the larger nematocysts are bigger (13.7–16.5 × 3.5–4.9 µm). In the material from station 20b, they are 12.5–13 × 3–4 µm, which is in agreement with the size reported for O. occulta by Peña Cantero and González Molinero (2018), and also measured here in material undoubtedly belonging to this species (e.g. 12.5–14 × 4 µm in the material from station 30).

Oswaldella occulta was so far only known off Cape Adare, in the Ross Sea, at depths from 82 to 348 m (cf. Peña Cantero and González Molinero 2018). Present material, which represents the second record for the species, was collected at depths between 19 and 89 m, epibiotic on O. terranovae, and basibiont of H. interpolatum and S. glacialis; mature gonothecae in August (immature in January).

Discussion

Biodiversity

As indicated above, 35 species were found in the collection of hydroids gathered during the REVOLTA project campaigns in the area of Pointe Géologie Archipelago (Table 2). 15 species represent new records for the zone (Table 2), thus raising the number of known species to 45 (see Online Resource), although the presence in the area of some of the previously reported species is quite uncertain. Naumov and Stepanjants (1972) reported several species (i.e. Eudendrium tottoni, Filellum serpens, Hydractinia angusta, Oswaldella billardi, Orthopyxis tincta, Staurotheca dichotoma and Symplectoscyphus plectilis), providing neither descriptions nor figures, so that it is not possible to confirm the identifications. Some of those records even correspond to species likely absent from Antarctic waters (i.e. F. serpens and O. tincta).

The diversity found in this study is similar to that found by Naumov and Stepanjants (1972), who reported 32 species to a depth of 240 m. I found 35 species in a similar depth range, from 19 to 172 m.

In the present study, Anthoatecata, with five species, is much less represented than Leptothecata (30 species). This result is similar to that found by Naumov and Stepanjants (1972), who reported three anthoathecates and 29 leptothecates. These results contrast with those found in Tethys Bay, in the Ross Sea, by Peña Cantero et al. (2013), where anthoathecates represented over 50% of the species (11 out of 20 species studied) in a collection from waters up to a depth of 48 m. This is probably related to the sampling method. Whereas in both the present collection and the one studied by Naumov and Stepanjants (1972) hydroids were obtained by indirect sampling, the material studied by Peña Cantero et al. (2013) was collected by scuba diving, which allows obtaining specimens in better condition and from sheltered micro-habitats that are difficult to sample with indirect sampling gears. Similar results were obtained by Stepanjants (1972) in a study based on scuba diving sampling in shallow waters from the Davis Sea (up to a depth of 50 m). She reported 11 anthoathecates out of 32 species studied. Surprisingly, Galea and Schories (2012) found only three anthoathecates among the 20 species present in a collection from shallow waters (up to a depth of 43 m) also collected by scuba diving.

Substrate

As is often the case in studies of benthic hydrozoans based on indirect sampling methods, information concerning the substrate on which the species live is scarce, especially for species that form large colonies attached directly to the bottom. These colonies usually come on board unattached or basally broken. The most representative example in the present collection is that of O. terranovae, which forms colonies up to 440 mm high and, despite being the most abundant species in the collection, no information on the substrate could be obtained. It is likely that the species grows directly on hard substrate, such as rock, stones or pebbles. Other large species, such as S. nana and Schizotricha unifurcata, much less abundant in the collection, were found attached to stones and pebbles. More information can be obtained about species that form much smaller colonies, which are usually found epibiotically on other organisms. Even when some species were found on a single substrate here (e.g. Obelia bidentata on Symplectoscyphus exochus or Phialella belgicae on Symplectoscyphus naumovi), they are species reported in the literature to grow on different substrates. However, it is worth highlighting the case of C. hicksoni which, despite being abundant in the collection, was only found on O. terranovae. Nevertheless, most species found in the study were observed on various substrates. Eudendrium scotti is the species that grows on a more heterogeneous array of substrates (algae, bryozoans and hydroids). Evidently, as the study was carried out on a collection of hydroids previously sorted from the general collection, the picture is biased. Almost all the information concerning the use of the substrates comes from the hydroids themselves; a plethora of potential substrates is out of reach. The species that use the greatest number of hydroids as basibionts were Campanularia sp. and H. interpolatum (found growing on five different species) and Filellum antarcticum, R. antarcticum, Stegella lobata, S. glacialis and S. naumovi (on three).

As for the basibionts, ten species were found to serve as substrate for other species of hydroids (Table 2). The species that has the highest biodiversity, with thirteen different species of hydroids growing on it, is O. terranovae. This characteristic is not an isolated case for the area of study, because O. terranovae has previously been reported to provide substrate for a great number of hydroids. Peña Cantero (2014b) reported 17 species in a study of benthic hydroids off Queen Mary Coast, remarkably from a single station, and Soto Àngel and Peña Cantero (2019) 23 species from the Weddell Sea, from 17 stations. Oswaldellata terranovae is a real habitat builder. The high biodiversity associated with this species probably has to do with its colony structure, as it forms large stems up to 440 mm high, whose oldest basal parts are polysiphonic and have lost their hydrocladia. These older parts are consequently deprived of nematophores and therefore of protection against epibionts. As a result, O. terranovae can be considered, at least for hydroids, as a hotspot of epibiont biodiversity, with 33 different species of hydrozoans reported so far. Not only does O. terranovae harbour many different species of epibiont hydroids, but it is also very abundant in some Antarctic areas, such as parts of the Ross Sea (Peña Cantero 2017) and, in particular, off Adélie Land (it is present in c. 70% of the stations in this study), which greatly increases its importance in Antarctic benthic communities, providing a suitable substrate for a large diversity of epibitonts.

Other important basibionts in the collection are Antarctoscyphus grandis, S. exochus and S. nana, each with five species. Antarctoscyphus grandis also forms relatively large colonies with a thick stem, whose older parts lose the hydrocladia. Peña Cantero (2017) also found five species on it. Schizotricha nana also has large, branched, polysiphonic stems. However, unlike O. terranovae, the accessory tubes of the polysiphonic part are provided with nematothecae, thus offering some protection against epibionts. Peña Cantero (2017) found four species on it. Symplectoscyphus exochus is a peculiar case, because the bushy colonies of this species consist of re-branching monosiphonic branches and stems; there is no main stem. Surprisingly, the species has a relatively high diversity of epibionts, although the colonies never reach large development.

Hydroid assemblages

In relation to the hydroid assemblages found in the study, it is worth mentioning that the predominant community in the area (assemblage C) is determined mainly by the presence of O. terranovae, a species present in all the stations conforming the group. The species, as stated above, is the most abundant in the study (both in occurrence and biomass) and a true habitat builder, since it represents a suitable substrate for a large number of epibionts, including many species of hydroids. Oswaldella terranovae is so prevalent that most stations belong to this group, and only the accompanying hydrozoan fauna differentiates the two subsets (C1 and C2) recovered in the cluster analysis and supported by the SIMPROF test (Fig. 2). A strong average dissimilarity (67.19) exists between both assemblages (Table 5), mainly as a result of the absence of several species (such as A. elongatus, A. spiralis, E. scotti, S. exochus, S. glacialis and S. nana), or a much lower representation of shared species (e.g. C. hicksoni, Campanularia sp. and H. interpolatum) in C1.

The other three assemblages (A, B and D) are much less represented, each including only three stations. The assemblages show a strong dissimilarity between them, also with C1 and C2, reaching 100% between A and B, and A and C1 (Tables 4 and 5). Assemblages A and B have a relatively large bathymetric range, from 57 to 117 m and from 38 to 172 m, respectively; group D has a narrow and relatively shallow bathymetric range (30 to 43 m). Interestingly, all three assemblages are characterised by species having an extensive bathymetric range (most are eurybathic species, cf. Table 2), so that depth does not seem to be behind the observed grouping. D is the assemblage with the lowest internal similarity, although it has a large dissimilarity with the others. It is also the only assemblage, other than C, in which O. terranovae is also present; in particular it is present in station 27bc which, on the other hand, is the station with the largest species number. The assemblage consists of epibiotic (e.g. H. dendritica, P. belgicae, Lafoeina longitheca) and non-epibiotic species (e.g. A. spiralis, Symplectoscyphus cumberlandicus, S. nana).

Assemblages A and B are the only ones in which O. terranovae is absent, and are characterised by non-epibiotic species. An important difference between the species that characterise these two assemblages has to do with the colony structure. Assemblage B is characterised by A. elongatus, a species forming erect colonies with upwardly directed polyps, which could be indicating a predominantly downward flux of seston, whereas assemblage A is characterised by S. antarctica and S. vanhoeffeni, two species typically forming bushy colonies, which could be indicating the presence of variable hydrodynamic conditions and, therefore, an undefined path for food arrival.

The hydroid assemblages found here fit perfectly into the general benthic communities characterising marine bottoms off Adélie Land. Gutt et al. (2007), in a study based on ROV-transects, found bottoms at depths of 20–110 m characterised by a suspension feeder megabenthic community, with cnidarians, ascidians, and polychaetes being the most dominant animal groups. A rich infauna inhabiting soft sediments and an abundant vagrant fauna occurred only in small patches surrounded by sessile suspension feeders.

Clearly, the predominant hydroid assemblage found in the present study corresponds to the dominant megabenthic community described by Gutt et al. (2007). It is worth mentioning that O. terranovae was found at all 14 stations studied by Gutt et al. (2007), except for the station with the muddiest sediment, situated in front of the ice cliffs of the Astrolabe Glacier. Not only was O. terranovae widely present on the seabed of Adélie Land, but it also characterised one of the groups obtained by Gutt et al. (2007), at depths of between 41 and 90 m. Here, the O. terranovae-dominating assemblage (C) is found in a very similar bathymetric range, from 36 to 133 m.

According to Gutt et al. (2007) the suspension feeder community is shaped by physical parameters such as proximity to glacier, iceberg scouring, light, and current regime, with biological processes, such as competition and recolonization providing heterogenity in biodiversity at spatial scales of tens to hundreds of metres. The combination of both physical and biological parameters is undoubtedly behind the distribution of the hydrozoan fauna of Adélie Land and, consequently, underpins the picture obtained in the present study in which, together with the dominant Oswaldella terranovae’s community, three other assemblages (i.e. A, B and D) were found.

The only previous study addressing Antarctic benthic hydroid assemblages (Peña Cantero and Manjón-Cabeza 2014) dealt with hydroids from the Bellingshausen Sea. However, these hydroids were collected in much deeper waters (from 426 to 2043 m), i.e. on the deepest part of the continental shelf and the continental slope, although the study also included hydroids from the coast of Peter I collected at shallower depths (from 86 to 380 m). These authors found three well-defined assemblages (deep-sea, continental slope and shallowest stations), with depth apparently being the main underlying factor. Bathymetric range is however very narrow in the present study, and depth seems not to play a clear role in the hydroid assemblages found. Other factors, for example those also considered by Peña Cantero and Manjón-Cabeza (2014) (e.g. substrate or species dispersal capabilities), could be behind the assemblages obtained. In relation to the substrate, the important role played by O. terranovae as habitat builder and hotspot of hydrozoan biodiversity should be recalled.

Geographical and bathymetric distribution

The bathymetric range of the species studied, as well as their known whole range, is shown in Table 2. Four out of the six bathymetric groups proposed by Peña Cantero (2004) are represented. By far the dominant group, with 22 species (65%), consists of eurybathic species present from the shallowest waters of the continental shelf to the bottoms beyond the shelf break. The other three groups are evently, but much less represented: five species (15%) are species extending across the entire continental shelf; four (12%) are species present beyond the shelf break and on the continental shelf apart from the shallowest waters (those to a depth of 30 m); finally, three (9%) are shelf species absent from the shallowest bottoms. The other two groups proposed by Peña Cantero (2004) are not represented in the collection. They include species limited to the shallowest waters and species restricted to deep waters beyond the continental shelf break (a bathymetric range not sampled in the study).

Considering the geographical distribution, it is worth mentioning that three species (Filellum magnificum, O. antarctica and Symplectoscyphus hero), so far only known from West Antarctica, were found in the area of study, thus pointing to a circum-Antarctic distribution.

The great originality that characterises the Antarctic benthos is reflected in the high level of endemism at species level in the collection studied. 65% of the species are endemic to Antarctic waters (Table 2). These are species endemic to East Antarctica or circum-Antarctic species (i.e. species distributed in both East and West Antarctica). Although the former is represented by only two species (6%), 20 species (59%) have a circum-Antarctic distribution. This level of endemism is similar to that found in other East Antarctic areas [e.g. 65% off Queen Mary Coast (Peña Cantero 2014b) or 68% in the Ross Sea and neighbouring areas (Peña Cantero 2019)], but lower than in others [e.g. 81% off Low Island, West Antarctica (Peña Cantero 2013)]. An additional important component consists of species commonly found in Antarctic waters but that extend its geographic distribution to sub-Antarctic waters. This component is composed of species with Antarctic-Kerguelen (3 species, 9%), Antarctic-Patagonian (1 species, 3%) and Pan-Antarctic (4 species, 12%) distribution. Thus, most species are restricted to Antarctic or Antarctic/sub-Antarctic waters (30 species, 88%). Only four species are also found outside these waters: Lafoea dumosa and O. bidentata, reported worldwide, Eudendrium generale, found in Australia and Antarctic waters, and F. antarcticum, typically reported in Antarctic waters, but with a record off South Africa.