Species of Schizotricha Allman, 1883 (Cnidaria: Hydrozoa: Halopterididae) from US Antarctic expeditions with the description of two new species

Twelve species of the genus Schizotricha Allman have been studied, two of which are new to science (Schizotricha heteromera sp. nov. and S. trinematotheca sp. nov.). The material studied was collected by several US Antarctic expeditions. Each new species is described and figured, the systematic position amongst allied species discussed and available data concerning autecology and geographical distribution given. The remaining 10 species are considered regarding diagnosis, autecology and geographical distribution. Finally, a general survey of the bathymetrical and biogeographical distribution of the known species of Schizotricha is given.

Remarks. We have here amended the diagnosis of the genus presented by Peñ a Cantero and Vervoort (1999).
Diagnosis. Featherlike, colonial hydroids with branched or unbranched, polysiphonic stem. Hydrocaulus composed of main tube, divided into internodes bearing hydro-and nematothecae (ahydrothecate internodes also present in S. heteromera sp. nov.), and several undivided (accessory) tubes, only provided with nematothecae and accompanying main tube for almost all its length. Hydrothecate cauline internodes with one or more apophyses supporting hydrocladia, one hydrotheca at axil between each apophysis and stem, and several nematothecae associated with each hydrotheca: two flanking hydrothecal apertures and at least one infrathecal nematotheca; suprathecal nematothecae also present in some species. Cauline apophyses alternately arranged. Hydrocladia, except in S. profunda (Nutting, 1900), branched once or several times, divided into internodes (with the exception of S. dichotoma Nutting, 1900), either all hydrothecate or also with ahydrothecate internodes provided with nematothecae. Branching usually taking place at first internode of previous hydrocladia. Forked hydrocladial internodes with one apophysis, supporting lower-order hydrocladium, one hydrotheca at axil of bifurcation and a variable number of nematothecae: two flanking hydrothecal aperture and at least one infrathecal nematotheca (occasionally also with suprathecal nematothecae). Unforked hydrothecate hydrocladial internodes with one or more hydrothecae and several associated nematothecae; two flanking hydrothecal aperture, at least one infrathecal nematotheca and, in certain species, also with suprathecal nematothecae. Cauline and hydrocladial apophyses usually provided with nematothecae. Hydrotheca cup-shaped, with smooth rim and completely adnate adcauline wall. Nematothecae two-chambered and movable. Colonies dioecious, with sexual dimorphism. Gonothecae inserting on hydrocladia between hydrothecal base and upper infrathecal nematotheca and provided with nematothecae. Cnidome composed of microbasic mastigophores, normally in two size classes. The genus presently includes the following species: Schizotricha anderssoni Jäderholm, 1904 Schizotricha crassa Peñ a Vervoort, 2004 Schizotricha dichotoma Nutting, 1900 Schizotricha falcata Peñ a Cantero, 1998 Schizotricha frutescens (Ellis and Solander, 1786) (5Sertularia frutescens Ellis and Solander, 1786) Schizotricha glacialis (Hickson and Gravely, 1907)  Diagnosis. Colonies with branched stems. Main cauline tube divided into simple hydrothecate internodes with two to five infrathecal nematothecae (occasionally also with one or two suprathecal nematothecae). Cauline apophysis with one nematotheca. Hydrocladia up to second order present. Frequently, apophysis supporting secondary hydrocladia followed by an ahydrothecate intermediate internode provided with two nematothecae. Hydrocladia regularly divided into internodes. Unforked hydrocladial internodes with three to six infrathecal nematothecae (occasionally also with one or two suprathecal nematothecae). Hydrotheca deep. Forked hydrocladial internodes with three or four infrathecal nematothecae (occasionally also with one or two suprathecal nematothecae). Hydrocladial apophyses with one nematotheca.
Remarks. Schizotricha anderssoni has much branched stems; indeed, there is continuous branching of the stem. Moreover, this species usually lacks branched hydrocladia. When branched, hydrocladia are either merely bifurcated or they become lower-order stems. Schizotricha anderssoni usually lacks intermediate ahydrothecate internodes in the hydrocladia, except in those becoming lower-order stems.
Ecology and distribution. Schizotricha anderssoni is a shelf species (Peñ a Cantero and Vervoort 1999), having been collected at depths from 75 (Jäderholm 1904) to 800 m (Naumov and Stepan'yants 1962) on stony bottoms (Peñ a Cantero and García Carrascosa 1995). Our material comes from depths between 68 and 346 m. Colonies with gonothecae were found in May (Jäderholm 1904) and in December (Peñ a Cantero and García Carrascosa 1995); our fertile colonies were collected in May, November and December. This species also serves as a substratum for other hydroids (Lafoea sp., Sertularella sp. and Symplectoscyphus sp.).
Schizotricha anderssoni seems to have a circum-Antarctic distribution (Peñ a Cantero and Vervoort 1999). It is known to occur off South Georgia (Jäderholm 1904;Peñ a Cantero and García Carrascosa 1995;present material), in West Antarctica, and in the Ross Sea (Naumov and Stepan'yants 1962)  Diagnosis. Colonies with unbranched stems. Main cauline tube with single hydrothecate internodes with two to five infrathecal nematothecae and occasionally with extra suprathecal nematotheca. Without node between cauline apophyses and hydrocladia; occasionally, however, present and then cauline apophyses with two or three nematothecae. Hydrocladia up to 12th order. Hydrocladia divided into hydrothecate internodes. Unforked hydrocladial internode with one or two infrathecal nematothecae at approximately same level. Hydrotheca deep. Forked hydrocladial internodes with one to three infrathecal nematothecae; hydrocladial apophyses with two or three nematothecae. Remarks. The material of this species was previously described and figured by Peñ a Cantero and Vervoort (2004a). Schizotricha crassa is characterized by the usual absence of a node between cauline apophyses and hydrocladia, the strong development of perisarc, the angle of ca 90u formed by the cauline apophyses, the high degree of hydrocladial branching, the strongly arched structure of the hydrocladia which, due also to the acute angle formed by the cauline apophyses, stand on the same side of the stem, and the characteristic shape of the male gonothecae, which have a relatively long and thin basal chamber.
Sometimes double hydrocladial internodes are present, provided with two hydrothecae, each with the typical set of nematothecae.
Ecology and distribution. Schizotricha crassa was collected at depths from 93 to 238 m. Fertile colonies were found in March, May and June. It is used as a substratum by Sertularella sp. Schizotricha crassa seems to be endemic in West Antarctica, having been collected NE of Joinville Island, in the Antarctic Peninsula area, south of the South Orkney Islands, and off Visokoi and Zavodovski Islands, in the South Sandwich Archipelago (cf. Peñ a Cantero and Vervoort 2004a).
Diagnosis. Colonies with unbranched stems. Main cauline tube divided into single hydrothecate internodes with two or three infrathecal nematothecae at different levels; cauline apophyses with one nematotheca. Hydrodadia strongly branched (hydrocladia up to seventh order reported). Normally, primary hydrocladium giving rise to secondary hydrocladia in its first, fourth and eighth internodes; secondary hydrocladium also giving third-order hydrocladia in its first, fourth and eighth internodes; first tertiary hydrocladium giving rise to fourth-order hydrocladia in its first and fifth internodes; finally, fifth-, sixthand seventh-order hydrocladia forming from the first internodes of previous series. Hydrocladia divided into single hydrothecate internodes. Unforked hydrocladial internodes with a single infrathecal nematotheca. Hydrotheca low. Forked hydrocladial internodes with two or three infrathecal nematothecae at different levels; hydrocladial apophyses with one nematotheca. Gonothecae sickle-shaped.
Ecology and distribution. Schizotricha falcata is a shelf species, probably with a circalittoral distribution (Peñ a Cantero and Vervoort 1999). It had been recorded from depths of 90 (Blanco 1984) to 154 m (Peñ a Cantero 1998); our material was collected between 73 and 150 m. Fertile colonies were found in January (Peñ a Cantero 1998).
Schizotricha falcata seems to be endemic in West Antarctica (Peñ a Cantero and Vervoort 1999), being known only off Low Island (Blanco 1984) and King George Island (Peñ a Cantero 1998), in the South Shetland Islands area, off Astrolabe Island, Antarctic Peninsula, and off Anvers Island, Palmer Archipelago (present material).
Description. Colony composed of a polysiphonic, unbranched stem ca 165 mm high. Stem consisting of a main tube divided into heteromerous internodes, with hydrothecate and ahydrothecate internodes, and several undivided tubes carrying only nematothecae. Main tube usually with an alternate series of hydrothecate and ahydrothecate internodes; the presence of two shorter ahydrothecate internodes between two hydrothecate ones ( Figure 1B) is also common. Each cauline hydrothecate internode ( Figure 1B) with an apophysis supporting firstorder hydrocladium and provided with one nematotheca, one hydrotheca at axil between apophysis and internode, and from six to 12 nematothecae: two flanking hydrothecal aperture, from two to eight below hydrotheca and up to two suprathecal nematothecae. Cauline ahydrothecate internodes provided with two to six nematothecae ( Figure 1B).
Hydrocladia up to 11th order present. Branching taking place at first internode of previous hydrocladium ( Figure 1A). Hydrocladia divided into hydrothecate internodes only, and with up to 21 hydrothecae.
Cauline apophyses, and consequently hydrocladia, well separated due to the presence of ahydrothecate cauline internodes; distance between successive cauline apophyses, however, more or less constant. Consequently, length and number of nematothecae in hydrothecate cauline internodes depending on length and number of ahydrothecate cauline internodes.
Forked hydrocladial internodes ( Figure 1C) provided with one apophysis supporting lower-order hydrocladia and with three, sometimes four nematothecae, one small hydrotheca at axil of branching, and from five to six nematothecae: two flanking hydrothecal aperture and three to four below hydrotheca.
Unforked hydrocladial internodes ( Figure 1D-F) provided with hydrotheca at half its length and four nematothecae: two flanking hydrothecal aperture and two below hydrotheca at approximately same level (occasionally only one); sporadically also with one suprathecal nematotheca.
Hydrotheca cup-shaped, deep and wide ( Figure 1A, D-F). Length increasing along hydrocladia; for example, length of abcauline wall may be 320 mm in first unforked hydrocladial internode and ca 400 mm in 11th. Adcauline wall completely adnate to internode. Abcauline wall slightly convex in lateral view ( Figure 1D).
Gonothecae inserted on hydrocladial internodes at small apophysis between hydrotheca and infrathecal nematothecae ( Figure 1A). Gonotheca fusiform (ca 1300 mm length and 500 mm maximum diameter), provided with a distal, circular aperture and a basal chamber delimited by an inconspicuous circular diaphragm; with two nematothecae ( Figure 1A, G).
Remarks. Schizotricha heteromera sp. nov. is clearly distinguishable from the remaining species of the genus by the heteromerous division of the cauline main tube ( Table I). As shown above, this tube is divided into an alternate series of hydrothecate and ahydrothecate internodes, though the presence of two shorter ahydrothecate internodes between two hydrothecate ones is not uncommon. Schizotricha glacialis, S. frutescens and S. profunda also have a heteromerous division of the cauline main tube, but in these species all the internodes are hydrothecate, the heteromerous condition referring to the presence of internodes bearing either one or two hydrothecae (even three or more in S. frutescens); moreover, this heteromerous division is also found in the hydrocladia.
Schizotricha heteromera sp. nov. is also characterized by the high number of nematothecae present in the cauline hydrothecate internodes, having up to eight infrathecal and up to two suprathecal nematothecae (Table I). In this character, S. heteromera sp. nov. is allied to S. anderssoni and S. multifurcata which also have a high number of infrathecal nematothecae (two to five in S. anderssoni and four to five in S. multifurcata); S. anderssoni may also have up to two suprathecal nematothecae. Nevertheless, apart from the heteromerous division of the stem S. heteromera sp. nov. clearly differs from both species in having unbranched stems and a stronger hydrocladial branching (hydrocladia up to 11th order have been observed). In this last character S. heteromera sp. nov. also differs from the remaining species of the genus except S. crassa, since only S. falcata, S. multifurcata, S. turqueti, and S. unifurcata are close with up to seventh-order hydrocladia reported (Table I).
Schizotricha heteromera sp. nov. is also characterized by a higher number of nematothecae in the hydrocladial apophyses (three to four); only S. vervoorti with two to three nematothecae approaches S. heteromera sp. nov. in this respect, though the two species differ in many features, such as the presence of branched stems in S. vervoorti.
Ecology and distribution. Schizotricha heteromera sp. nov. was collected in May at a depth between 93 and 121 m off Visokoi Island, in the South Sandwich Archipelago (Scotia Sea).
Etymology. The specific name heteromera refers to the heteromerous division of the main cauline tube.  internodes. Unforked hydrocladial internodes with two nematothecae below hydrotheca at different levels. Hydrotheca deep. Forked hydrocladial internodes with two or three infrathecal nematothecae; hydrocladial apophyses with two nematothecae.

Schizotricha jaederholmi
Remarks. This constitutes the second record of the species. From the previous description, the present material differs in the presence of up to seven infrathecal nematothecae in the cauline internodes and in the occasional presence of a single infrathecal nematotheca in the most distal hydrocladial internodes.
Ecology and distribution. Schizotricha jaederholmi is endemic to the South Georgia area, where it was previously reported from depths of 252 to 310 m (Peñ a ; the present material was collected at 261-267 m. Diagnosis. Colonies with branched stems. Main cauline tube with single hydrothecate internodes with four or five infrathecal nematothecae. Hydrocladia up to seventh order recorded. Hydrocladia heteromerously divided into internodes; with ahydrothecate internode, provided with two nematothecae, following cauline and hydrocladial apophyses. Unforked hydrocladial internodes with two infrathecal nematothecae at different levels. Hydrotheca deep. Forked hydrocladial internodes with four or five infrathecal nematothecae; hydrocladial apophyses with one nematotheca.
Remarks. One of the two infrathecal nematothecae of the unforked hydrocladial internodes may be absent, especially in the distalmost internodes. The apophyses of the forked hydrocladial internodes are relatively long and provided with one nematotheca.
Schizotricha multifurcata occurs mainly in the sub-Antarctic Region (Peñ a Cantero and Vervoort 1999), being unknown from High Antarctica. It has been found off the Patagonian coast (El Beshbeeshy 1991), off Bouvet (Stechow 1925) and Heard Islands (Allman 1883), both at the borders of the Antarctic Region, and off Bristol (Peñ a Cantero and García Carrascosa 1995; present material), Thule and Candlemas islands (present material), in the South Sandwich Archipelago. Diagnosis. Colonies with branched stems. Main cauline tube with single hydrothecate internodes with one or two infrathecal nematothecae. Hydrocladia up to second order. Hydrocladia heteromerously divided into internodes; with ahydrothecate internode, provided with one nematotheca, following cauline and hydrocladial apophyses. Unforked hydrocladial internodes with one infrathecal nematotheca. Hydrotheca low. Forked hydrocladial internodes with one infrathecal nematotheca.
Ecology and distribution. Schizotricha nana had previously been considered a shelf species (Peñ a Cantero et al. 1996), being known from depths between 43  and 385 m (Vanhö ffen 1910) on sandy, stony and rocky bottoms (Stepan'yants 1979). Our present material comes from 91 to 1890 m, once occurring on gravel, considerably increasing the bathymetric range which now reaches bathyal depths. Peñ a Cantero et al. (1996) reported fertile colonies in January and February, whereas we observed gonothecae in colonies collected in March, May and June. We found colonies of Billardia sp., Campanularia sp., Clytia sp., Eudendrium sp., Halecium sp., Lafoea sp. and Symplectoscyphus sp. epibiotic on S. nana.
Schizotricha nana has a circum-Antarctic distribution (Peñ a Cantero et al. 1996), being known from both East and West Antarctica. In the former it has been recorded at Gauss Station (65u219S-86u069E) (Vanhö ffen 1910), in the Davis Sea (Briggs 1938; and off Adélie Coast (Naumov and Stepan'yants 1972). In West Antarctica it has been reported from the eastern part of the Weddell Sea (Peñ a Cantero et al. 1996)  Diagnosis. Colonies with unbranched stems. Main cauline tube with single hydrothecate internodes with two to five infrathecal nematothecae. Hydrocladia up to fourth order. Hydrocladia heteromerously divided into internodes; with ahydrothecate internode, provided with one nematotheca, following cauline and hydrocladial apophyses. Unforked hydrocladial internodes with one infrathecal nematotheca. Hydrotheca deep. Forked hydrocladial internodes with two infrathecal nematothecae.
Remarks. The material of this species was previously described and figured by Peñ a Cantero and Vervoort (2004a). The identity of material from Sta. 000AG is slightly doubtful since it differs from S. southgeorgiae in a few characters. Primarily, the occasional presence of one suprathecal nematotheca in the cauline internodes, usually when the following internode is shorter and provided with only two infrathecal nematothecae. Secondly, hydrocladia were unbranched, but note that this material consists of a single distal stem fragment. Thirdly, two infrathecal nematothecae occurred at different levels in the first hydrocladial internodes. Those internodes could be considered homologous to the first forked internodes of S. southgeorgiae that have two infrathecal nematothecae. In the remaining hydrocladial internodes with two infrathecal nematothecae, with a single exception, there is always an indication of reconstruction. Diagnosis. Colony with unbranched stem. Main cauline tube with single hydrothecate internodes with four to six infrathecal nematothecae; cauline apophysis with one nematotheca. Hydrocladia up to third order. Hydrocladia heteromerously divided into internodes; with single and double hydrothecate internodes and with ahydrothecate internode, provided with two to four nematothecae, following cauline and hydrocladial apophyses. Single unforked hydrocladial internodes with two infrathecal nematothecae at different levels and one or two suprathecal nematothecae; double internodes with two infrathecal nematothecae at different levels under first hydrotheca, three between both hydrothecae, and one suprathecal nematotheca above second hydrotheca. Hydrotheca deep. Forked hydrocladial internodes with five to seven infrathecal nematothecae; hydrocladial apophyses with one nematotheca. Description. Colony composed of polysiphonic, unbranched stem 85 mm high. Stem consisting of a main tube divided into internodes provided with hydrothecae and nematothecae, and several undivided accessory tubes carrying only nematothecae. Each stem internode provided with a long apophysis with one nematotheca, a small hydrotheca at axil between apophysis and internode, and from six to eight nematothecae: two flanking hydrothecal aperture and four to six infrathecal nematothecae.
Cauline apophyses alternately arranged in two planes, making an obtuse angle and supporting hydrocladia. A distinct node present between cauline apophyses and hydrocladia ( Figure 2B).
Hydrocladia divided into heteromerous internodes, with single and double hydrothecate internodes. Up to third-order hydrocladia present (Figure 2A). Branching taking place at first hydrothecate internode of successive hydrocladia. First hydrocladial internode deprived of hydrotheca, but provided with two or three nematothecae (Figure 2A). Hydrocladia long, with up to 29 hydrothecae.
Forked hydrocladial internodes ( Figure 2B) provided with a short apophysis with one nematotheca, a small axillary hydrotheca, and seven to nine nematothecae: two flanking hydrothecal aperture and five to seven infrathecal nematothecae.
Apophyses of forked hydrocladial internodes followed by an intermediate ahydrothecate internode provided with three or four nematothecae ( Figure 2B).
Single unforked hydrocladial internodes ( Figure 2C-E) with hydrotheca at half their length, and five to six nematothecae: two flanking hydrothecal aperture, two below hydrotheca at different levels, and one or two suprathecal nematothecae.
Double unforked hydrocladial internodes ( Figure 2F) with two hydrothecae and 10 nematothecae, two flanking each hydrothecal aperture, two or occasionally three nematothecae when a gonotheca is present at different levels under first hydrotheca, three nematothecae at different levels between both hydrothecae, and one suprathecal nematotheca distal to second hydrotheca.
Gonothecae inserted on small apophyses between infrathecal nematothecae in hydrocladial internodes (Figure 2A). Usually one gonotheca per internode; sometimes two. Gonotheca fusiform (ca 1000 mm length and 320 mm maximum diameter), provided with a terminal, circular aperture and a basal chamber delimited by a circular diaphragm; provided with two nematothecae ( Figure 2G).
Remarks. Schizotricha trinematotheca sp. nov. is allied to S. anderssoni, S. multifurcata, S. nana, S. unifurcata and S. southgeorgiae in having intermediate ahydrothecate internodes following the cauline and hydrocladial apophyses. With S. multifurcata it also shares the presence of one nematotheca on the hydrocladial apophyses. Schizotricha trinematotheca sp. nov. differs, however, from all those species in the number of nematothecae present in the intermediate internodes, there being a single nematotheca in S. nana and S. southgeorgiae and two nematothecae in S. anderssoni, S. multifurcata and S. unifurcata, whereas in S. trinematotheca sp. nov. there are two to four. Schizotricha trinematotheca sp. nov. also differs from all those species, with the exception of S. southgeorgiae, in having unbranched stems. From S. nana it also differs in having deep instead of low hydrothecae. Schizotricha trinematotheca sp. nov. differs from all of those species in having single and double hydrothecate hydrocladial internodes. In this character it approaches S. glacialis. Nevertheless, this last species differs from S. trinematotheca sp. nov. in the absence of ahydrothecate intermediate internodes and in the presence of a single infrathecal nematotheca in the unforked hydrocladial internodes. In having suprathecal nematothecae S. trinematotheca sp. nov. also differs from all the above-mentioned species except S. anderssoni.
Schizotricha dichotoma, a non-Antarctic species of the genus, may also have intermediate internodes, but differs in having a completely different hydrocladial structure, lacking internodes, and much larger hydrothecae.
Schizotricha frutescens and S. profunda, two other non-Antarctic species of the genus, may also have more than one hydrotheca per internode, but they differ mainly in having branched stems, in lacking ahydrothecate intermediate internodes, and in having a single infrathecal nematotheca in the unforked hydrocladial internodes. Moreover, S. frutescens lacks a node between the cauline apophyses and the hydrocladia, and S. profunda has unbranched hydrocladia.
Ecology and distribution. Schizotricha trinematotheca sp. nov. was found in February at a depth between 150 and 157 m off Buckle Island, in the Balleny Islands (Victoria Land).

Schizotricha turqueti
Remarks. Occasionally in this species it is possible to find stems with false branching, due to previous fracture of the original stem. This is corroborated by the fact that the main stem is bifurcated, giving rise to two equally developed secondary stems; this does not constitute the branching pattern found in the typically branched species of Schizotricha.
The unforked hydrocladial internodes are typically provided with a single infrathecal nematotheca, though in the material from Sta. 721/704 there is occasionally a second infrathecal nematotheca in the first and sometimes in the second internode following the forked hydrocladial internodes.
The material from Sta. 32/2020, Sta. 721/775 and Sta. 731/1944 resembles Schizotricha glacialis (Hickson and Gravely, 1907) in the frequent presence of double internodes. However, in the remaining features that material is closer to S. turqueti since up to sixthorder hydrocladia are present, whereas in S. glacialis only secondary hydrocladia have been reported. Moreover, only one nematotheca has been reported on the hydrocladial apophyses in S. glacialis, whereas one or two are present in S. turqueti, as also occurs in the material under discussion. Therefore, we conclude that double internodes sometimes occur in S. turqueti, so that this feature is not restricted to S. glacialis. In fact, double internodes also occur in S. crassa and S. trinematotheca sp. nov. For the moment, however, S. glacialis and S. turqueti are kept separate because in S. glacialis the hydrocladia are either unforked or have but a single ramification, even in fertile material (cf. Hickson and Gravely 1907;. Ecology and distribution. Schizotricha turqueti is a eurybathic species. It has been recorded from the intertidal level (Billard 1906) to a depth of 330 m (Peñ a Cantero et al. 1996) on bottoms of gravel and pebbles with mud (Peñ a Cantero and García Carrascosa 1995) and on muddy bottoms (Peñ a Cantero et al. 1996); our material was collected at depths from 40 to 1890 m, considerably increasing its bathymetric range, sometimes on pebbles. Fertile colonies have been reported previously only in January (Peñ a Cantero and García Carrascosa 1995; Peñ a Cantero et al. 1996), whereas we observed gonothecae in colonies collected in January, February, March, May, and December. We found colonies of Antarctoscyphus sp., Clytia sp., Campanularia sp., and Lafoea sp. using S. turqueti as a substratum.
Previous to the present study, Schizotricha turqueti seemed to be endemic to West Antarctica, having been reported from Booth-Wandel Island by Billard (1906), off Elephant Island (South Shetland Islands) by Peñ a Cantero and García Carrascosa (1995) and Peñ a Cantero et al. (1996), and from the southern part of the Weddell Sea by Peñ a peculiarities, being considered as a district of the Antarctic region by several authors. It seems to be closer to the Patagonian area in its shallow-water fauna, but closer to the Antarctic region as far as the deep-water fauna is considered. Table II shows the bathymetrical distribution of the known species of Schizotricha. Most of the species have a wide bathymetric range. However, of the 13 known species inhabiting Antarctic waters, only S. turqueti must be considered a eurybathic species, extending from the shallowest levels of the continental shelf to bathyal depths. Most species are shelf species, i.e. species inhabiting bottoms of the continental shelf which in the Antarctic has the shelf-break two to four times deeper than in other oceanic regions. These shelf species are S. crassa, S. falcata, S. heteromera sp. nov., S. jaederholmi, and S. trinematotheca sp. nov. They are absent from the shallowest levels of the continental shelf, impacted by different manifestations of ice. Another group of species is that constituted by species extending