Pachycerianthus antarcticus, Stampar & Mills & Keable, 2020

Stampar, Sérgio N., Mills, V. Sadie & Keable, Stephen J., 2020, Ceriantharia (Cnidaria) from Australia, New Zealand and Antarctica with Descriptions of Four New Species, Records of the Australian Museum 72 (3), pp. 81-100 : 85-87

Pachycerianthus antarcticus sp. nov.

Figs 5–7 , Table 2

Holotype. NIWA Invertebrate Collection ( NIC) NIWA 36894 Ross Sea, Antarctica, RV Tangaroa Station TAN0802/100, 76.2020°S 176.2480°E (18/ii/2008), depth 447–451 m. Paratype. NIWA 103267 Off Young Island , Antarctica , RV Tangaroa Station TAN0402/252, 66.3647°S 162.5762°E (05/iii/2004), depth 942 m (1 specimen) .

Diagnosis. Marginal tentacles in two pseudocyles and labial tentacles in four pseudocycles. Four mesenteries attached to siphonoglyph, M and m or B and b with almost the same length and absence of directive labial tentacle.

Variation. Short (up to 36 mm long) and broad (10 mm width) cerianthid; 20–32 white marginal tentacles (practically absent in both specimens, only the base is present) ( Fig. 4 ), arrangement 1212.1212.1212; 28–36 white tentacles (up to 4 mm long in preserved specimens), arrangement 2134.3142.3142, unpaired labial tentacle absent; pleated stomodeum extending over 1/5 of total body length, hyposulcus less than 1 mm long, hemisulci absent; siphonoglyph narrow, connected only to directives and P2 (4 mesenteries); free parts of sterile directive mesenteries very short, without mesenterial filaments. Second protomesenteries (P2) short, sterile, bearing ciliated tracts without craspedonemes, followed by a long cnido-glandular tract (c. 80% of the mesentery). Third protomesenteries sterile, longer than P2, with craspedonemes. M and m-metamesenteries long, almost equal length (M longer than m), fertile; M1 and M2 reach more than 1/2 of gastral cavity with a small bundle of craspedonemes; B and b-mesenteries, short, sterile, almost equal length; see Fig. 6 for schematic arrangement of mesenteries. The cnidome of the species ( Fig. 7 ) is composed of spirocysts, holotrics, atrichs, microbasic b-mastigophores (four types) and ptychocysts distributed as shown in Table 2.

Holotype description ( NIWA 36894). Small polyp, 3.6 cm long, 11 mm diameter just below marginal tentacles and 4 mm near aboral end. Marginal tentacles 20, arranged in two pseudocycles. The space between cycles of marginal and labial tentacles white. Labial tentacles 28, about 3 mm long, white, directive labial absent, arrangement of marginal tentacles 1212.1212.1212… and labial tentacles 2134.3142.3142.... Oral disk 11 mm wide, stomodeum 6 mm long, white, siphonoglyph narrow and elongate with 4 mesenteries attached, hyposulcus 2 mm long with short hemisulci 1 mm long. Free parts of directive mesenteries very short, without mesenterial filaments. Second protomesenteries short, sterile, bearing small ciliated tracts with bundles of craspedonemes. Third protomesenteries sterile, short, with craspedonemes. M and m-metamesenteries long and almost equal length, fertile; M1 and M2 reaching almost aboral pore (M1=M2), with bundles of craspedonemes; B and b-mesenteries sterile, B two times longer than b.

Remarks. Pachycerianthus antarcticus is the first species of Ceriantharia to be recorded for the Antarctic region. In contrast to the pattern of species occurrence in the Arctic, records in the southern hemisphere have previously been restricted to semitropical or temperate regions (e.g., Molodtsova et al., 2011; Stampar et al., 2014).

Some aspects related to the species biology are important to highlight. The first is the presence of oocytes that appear to be fertilized in the gastrovascular cavity. Internal fertilization is not unknown in Ceriantharia ( Nyholm, 1943) , but appears to be rare and there is no data on the fertilization process. Presently it is unclear if self-fertilization or oral sperm capture occurs. Another aspect concerns the size of the ptychocyst capsules and the tube construction. Based on the small pieces of tube close to the specimen body, it is possible to observe a coarse grain size in the sediment used, so the construction of the tube must require longer ptychocyst filaments to be able to use this sediment as part of the tube ( Stampar et al., 2015). Based on current data, no further conclusions are possible, but a deeper analysis of the relationship between ptychocyst capsule size and sediment particle size would be useful to examine in future studies because the ecological role of these organisms can be quite important in modifying benthic environments (e.g., Shepard et al., 1986).

Distribution. Southern Ocean, Antarctica.

Etymology. The specific name “antarcticus” (derived from the ancient Greek ἀνταρκτικóς opposite to the Arctic) refers to the geographical occurrence of the specimens.

Figures

Figure 4. Cnidome of Pachycerianthus fiordlandensis sp. nov. based on two specimens (NIWA 139391; 139400).

Figure 5. Specimens of Pachycerianthus antarcticus sp. nov. (A) Whole specimen with tube, NIWA 36894, holotype (photo: Stefano Schiaparelli, MNA Genoa, IPY-CAML); (B) detail of insertion area of marginal tentacles (arrows); (C) detail of mesenteries full of eggs; and (D) eggs (fertilized?).

Figure 6. Graphical representation of the arrangement of mesenteries of Pachycerianthus antarcticus sp. nov.Abbreviations: M.C., multiplication chamber; dm, directives; T.P., terminal pore; S, siphonoglyph; B, betamesenteries (convoluted mesentery); M, metamesenteries (double filament); P, protomesenteries.

Figure 7. Cnidome of Pachycerianthus antarcticus sp. nov. based on the holotype.