(FIGS 3–5; TABLE 1)
Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: BE47D4D3-33D1-47E9-84A6-CE5AF0C96A51
Material: Holotype AMNH _ IZC 00331556 (1 specimen), ANT XV/3 Cruise, EASIZ II Program, R / V ‘ Polarstern’, Sta. PS 48/197, Kapp Norvegia, Antarctica, Southern Ocean, 71°17’S 12°36.30’W, 416 m, 15 February 1998, Agassiz Trawl, collected by P. López-González. Paratypes AMNH 4775 (five specimens), same collection data as holotype. AMNH 4662 (one specimen), ANT XIX/5 Cruise, LAMPOS Program, R / V ‘ Polarstern’, Sta. PS 61/150, Burdwood / Namuncurá Bank, Scotia Sea, Antarctica, Southern Ocean, 54°30.22’S 56°08.20’W, 286.3–290.3 m, 6 April 2002, Agassiz Trawl, collected by E. Rodríguez. AMNH 4663 (ten specimens), ANT XIX/5 Cruise, LAMPOS Program, R / V ‘ Polarstern’, Sta. PS 61/153, Burdwood / Namuncurá Bank, Scotia Sea, Antarctica, Southern Ocean, 54°31.22’S 56°08.93’W, 277–296 m, 6 April 2002, Bottom Trawl, collected by E. Rodríguez. AMNH _ IZC 00331550 (one specimen), ANT XXIII/8 Cruise, EBA Program, R / V ‘ Polarstern’, Sta. PS69/662-1, South Shetland Islands, Antarctica, Southern Ocean, 61°35.91’S 57°17.04’W / 61°35.41’S, 57°20.60’W, 425.4–431.8 m, 30 December 2006, Bottom Trawl, collected by P. López-González. AMNH _ IZC 00361336 (one specimen), ANT XXIX/9 Cruise, FOS Program, R / V ‘ Polarstern’, Sta. PS 82/283-2, Weddell Sea, Antarctica, Southern Ocean, 74°59.61’S 29°22.89’W, 406.7–411 m, 7 February 2014, Agassiz Trawl, collected by R. Zapata. AMNH _ IZC 00361337 (one specimen), ANT XXIX/9 Cruise, FOS Program, R / V ‘ Polarstern’, Sta. PS 82/283-2, Weddell Sea, Antarctica, Southern Ocean, 74°59.46’S 29°22.75’W, 406.7–411 m, 7 February 2014, Agassiz Trawl, collected by R. Zapata. NHMUK (one specimen), JCR15005, event #61, NW of Coronation Island, South Orkney Islands, Scotia Sea, Antarctica, 60°21.3180’S 46°41.0820’W, 516.97 m depth, 9 March 2016, Agassiz trawl, collected by E. Rodríguez.
Additional material examined for comparison: Stomphia selaginella (Stephenson, 1918): AMNH 4326 (20 specimens); locality: ANT XV/3 Cruise, EASIZ II Program, R / V ‘ Polarstern’, Sta. PS 48/220, Southern Ocean, Antarctica, Weddell Sea, Austasen, 70°50.40’S 10°35.40’W, collected on 19 February 1998 by P. López- González (236 m). Actinostola crassicornis (Hertwig, 1882): AMNH 4698 (three specimens); locality: ANT XIX/5 Cruise, LAMPOS Program, R / V ‘ Polastern’, Sta. 61/153, Drake Passage, Antarctica, Burdwood / Namuncurá Bank, 54º31.22’S 56º08.93’W, collected on 6 April 2002 by E. Rodríguez (277 m). Actinostola georgiana Carlgren, 1927: AMNH 4803 (one specimen); locality: ANT XXI/2 Cruise, BENDEX Program, Sta. PS 65/019-1, Southern Ocean, Bouvet Island, 54°30.09’S 03°14.13’W, collected on 24 November 2003 by E. Rodríguez (247 m).
Diagnosis: Species with mesogleal marginal sphincter and tentacular longitudinal musculature, younger mesenteries unequally developed, no acontia and welldeveloped, thick chitinous cuticle on column.
Etymology: The species name is Latinized Greek from µάρµαρο, marble. It is named in honour of Pablo López- González (Universidad de Sevilla, Spain). He was the first to collect this species and we thank him for his contributions to the diversity of Anthozoa in the Southern Ocean and his mentoring of systematists working on this group.
External anatomy (Fig. 3): Some specimens are flat with triangular body, with elevated central column (Fig. 3A, B) and broad, flat pedal disc (Fig. 3B, C), others more elongate (Fig. 3C). Pedal disc broader than column, irregular in shape (Fig. 2B) with mesenterial insertions visible on limbus (Fig. 3C); 9–21 mm in diameter in preserved specimens. Column cylindrical, broader proximally, becoming less so distally due to contraction of scapulus into scapus in most specimens (Fig. 3B, D); in less contracted specimens, column visibly divided into short, smooth scapulus and long scapus (Fig. 3E); column diameter 5–16 mm at midcolumn and 7–22 mm length in preserved specimens. Scapus with small tubercles in longitudinal rows throughout scapus (Fig. 3F); column epidermis may be absent in certain regions (Fig. 3F). Thick, golden cuticle on column (Fig. 3A, F); may form a thick ‘crust’ easily detached from animal (Fig. 3G); cuticle completely absent from column in some specimens (Fig. 3C, E). Margin of column tentaculate (Fig. 3C, E). Oral disc small, circular in contracted specimens; diameter 1–3 mm in preserved specimens. Tentacles about 50, slender (Fig. 3C, E); putatively arranged in four cycles; up to 5 mm in preserved specimens.
Internal anatomy, microanatomy (Fig. 4): Aboral end flat, not physa-like (Fig. 4A). Overall body wall thickness varies along column (Fig. 4A–C): scapus epidermis 85–271 µm, mesoglea 92–398 µm and gastrodermis 40–190 µm (Fig. 4D); scapulus epidermis 101–169 µm, mesoglea 79–248 µm and gastrodermis 91–158 µm (Fig. 4E). Thick, brown, non-stratified cuticle on scapus (Fig. 4B, C, E), absent from scapulus (Fig. 4D); cuticle also on pedal disc (Fig. 4A). Longitudinal circular endodermal musculature of column strong, with high muscle processes in distal column (Fig. 4F), also observed in dissections (Fig. 4B, arrow). Mesogleal marginal sphincter musculature (Fig. 4G). Longitudinal musculature of tentacles mesogleal (Fig. 4H). Actinopharynx short, strongly longitudinally sulcate (Fig. 4A); siphonoglyphs not differentiated (Fig. 4I).
Two cycles of mesenteries distally at scapulus-level (Fig. 4J): six pairs of perfect mesenteries of first cycle and four pairs of second cycle with a single mesentery of a pair in the dorso- and ventral-lateral exoceles (but seen in scapulus: Fig. 4K). Three cycles of mesenteries at distal column: six pairs of perfect mesenteries of first cycle, two cycles of imperfect mesenteries (6 + 6 + 12: Fig. 4K, L). More mesenteries proximally than distally (6 + 6 + 12 + 24: Fig. 4L, N). Retractors of mesenteries of first cycle strong, diffuse (Fig. 4K, N, O); well developed, pinnate, with well-developed central mesogleal lamella proximally (Fig. 4P). Retractors of second and third cycles weaker than in first cycle, diffuse, occupying entire mesentery length (Fig. 4K, N). Mesenteries of fourth cycle small, without well-developed musculature (Fig. 4P). Parietobasilar musculature well developed, strong in mesenteries of first to third cycles both distally (Fig. 4O) and proximally (Fig. 4Q); absent in mesenteries of fourth (Fig. 4N). Basilar musculature present (not shown). Gametogenic tissue in third cycle mesenteries (Fig. 4L); specimens collected in December sterile and those in February females. Species inferred gonochoric.
Cnidom: spirocysts, basitrichs, b -mastigophores and p -mastigophores A. See Figure 5 and Table 1 for size and distribution.
Distribution and natural history: Multiple individuals of Chitinactis marmara were collected around the South Shetland Islands in the Antarctic Peninsula, the south-west of the South Orkney Islands and east of Burdwood/Numuncurá Bank in the Scotia Arc, from 277–517 m depth. Specimens of C. marmara were also collected from the eastern Weddell Sea (at Kapp Norvegia) between 400 and 416 m depth, some of them collected together with multiple individuals of Aulactinia sulcata (Clubb, 1902). We consider C. marmara a shelf endemic in Antarctica and sub- Antarctica. The new record of A. sulcata extends the circumpolar distribution of the species found in the Bransfield Strait, South Shetland Islands, Elephant Islands and the Drake Passage in the Antarctic Peninsula and McMurdo Sound (Victoria Island) in the Ross Sea (Rodríguez & López-González, 2013), to include the south-eastern Weddell Sea.
Remarks: Chitinactis is a monotypic genus easily differentiated from other actinostolids based on the following combination of characters: mesogleal tubercles and distinct columnar cuticle, mesogleal longitudinal tentacle musculature and unequal development of younger mesenteries. Although the large and common b -mastigophores found in C. marmara (Fig. 5G) strongly resemble those in the tentacles of some actinostolids (e.g. Hormosoma, Stomphia and Actinostola), they are always only found in the filaments. To date, no specimens having the attributes described for C. marmara have been reported in the literature for the Southern Ocean or anywhere else.