Sicyonis erythrocephala ( Pax, 1922 )

Gusmão, Luciana C. & Rodríguez, Estefanía, 2021, Deep-Sea Anemones (Cnidaria: Anthozoa: Actiniaria) From The South Atlantic, Bulletin of the American Museum of Natural History 2021 (444), pp. 1-73 : 13-17

Sicyonis erythrocephala ( Pax, 1922) Figures 5–6, table 2

MATERIAL: MNRJ 8167 (4 specimens) ; locality: Mid-Atlantic Ridge, Ak. Ioffe Cruise 29, Superestação 7, Sta. 201, series #1030, 29°27.12′ S 10°08.72′ W, collected on 20 November 2009 by MAR-ECO #38295, Shirshov St. 2184–5 (4120 m) .

MATERIAL EXAMINED: Stomphia selaginella ( Stephenson, 1918a) : AMNH 4326 (20 specimens) ; locality: ANT XV/3 Cruise , EASIZ II Program , RV Polarstern, Sta. PS 48/220, Southern Ocean , Antarctica, Weddell Sea, Austassen, 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 (3 specimens) ; locality: ANT XIX/5 Cruise , LAM- POS Program , RV Polastern , Sta. 61/153, Burdwood / Namuncurá Bank, 54°31.22′ S 56°08.93′ W, collected on 6 April 2002 by E. Rodríguez (277 m). Actinostola georgiana Carlgren, 1927 : AMNH 4803 (1 specimen) ; locality: ANT XXI/2 Cruise , BENDEX Program , RV Polarstern, 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) .

EXTERNAL ANATOMY (fig. 5): Pedal disc small, circular, flat, 25–40 mm in diameter in preserved specimens (fig. 5A). Column firm, cylindrical, tall, smooth, not divisible into regions (fig. 5A); margin tentaculate (fig. 5B). Column white/light beige (fig. 5A); 35–50 mm in diameter and 10–49 mm in length in preserved specimens. Oral disc circular, as wide or slightly wider than column, with central mouth, same color as column (fig. 5B); 30–48 mm in diameter in preserved specimens. Tentacles approximately 132, smooth, short, pointed, with no markings, in 2–3 cycles on outer half of oral disc (fig. 5B); all tentacles translucent beige in preserved specimens (fig. 5B). Inner tentacles slightly longer than outer ones (fig. 5B); longest tentacle up to 18 mm in preserved specimens.

INTERNAL ANATOMY AND HISTOLOGY (fig. 5): Body-wall thickness uniform throughout column with thin gastrodermis and thick mesoglea (fig. 5C); epidermis of column mostly absent (fig. 5C). Marginal mesogleal sphincter musculature weak, diffuse, with fibers mostly close to gastrodermis and narrow proximally (fig. 5C); muscle fibers with reticular arrangement in mesoglea (fig. 5D). Longitudinal musculature of tentacles mesogleal (fig. 5E).

Mesenteries irregularly arranged in five cycles, 68 pairs at midcolumn; more mesenteries proximally than distally. Approximately 16 mesenteries of first and second cycles, perfect, including two pairs of directives, each associated with one siphonoglyph (fig. 5F); third to fifth cycles imperfect, with filaments (fig. 5F, G). Fifth cycle of mesenteries imperfect, fertile (fig. 5F). Mesenteries in pairs of third to fifth cycle unequally developed (fig. 5G). Anomalous pair of fourth cycle fused (fig. 5J). Oocytes 1–5 per mesentery in various stages of development, including pre- and early vitellogenic oocytes (fig. 5L, M), with trophonema (arrow: fig. 5M). All specimens collected in November female; major axis of oocytes up to 0.9 mm. Species inferred gonochoric. Retractors of all mesenteries weak, diffuse (fig. 5G–I); parietobasilar of larger mesenteries well developed with free mesogleal flap (fig. 5I); rest of mesenteries with weak to indistinct parietobasilar musculature (fig. 5K).

CNIDOM (fig. 6): Spirocysts, basitrichs, p -mastigophores A. See figure 6 and table 2 for size and distribution.

DISTRIBUTION AND NATURAL HISTORY: Sicyonis erythrocephala was originally recorded from the Bellingshausen Sea in the Antarctic Peninsula ( Pax, 1922; Carlgren, 1927). After Fautin (1984) synonymized S. aurora Carlgren and Stephenson, 1929 , and S. antarctica Carlgren, 1939 , with S. erythrocephala , the species is also known from Tasmania ( Carlgren and Stephenson, 1929), off Coats Land in the Weddell Sea ( Carlgren, 1939) and all around Antarctica up to 42° S ( Fautin, 1984). Thus, S. erythrocephala is mostly a widespread Antarctic species distributed in three districts of the Southern Ocean (i.e., Atlantic, Pacific, and Indian sensu Rodríguez et al., 2007). Our material significantly extends the geographic range of the species further north to the SMAR (~ 29° S) and is the deepest record for the species, extending the bathymetric range of the species from 3879 m to 4120 m.

REMARKS: Our material is identified as a Sicyonis based on a combination of a mesogleal longitudinal musculature in the tentacles (absent in Tealidium Hertwig, 1882 ; Anthosactis Danielssen, 1890 ; Paranthus Andres, 1883 ; Bathydactylus Carlgren, 1928a ; Antiparactis Verrill, 1899 ; Pseudoparactis Stephenson, 1920 ; Antholoba Hertwig, 1882 ), unequal development of mesenteries in a pair (absent in Pycnanthus McMurrich, 1893 ; Hormosoma Stephenson, 1918a ; Cnidanthus Carlgren, 1927 ), absence of b -mastigophores in tentacles (present in Actinostola Verrill, 1883 ; Stomphia Gosse, 1859 ), and number and cycles of mesenteries and pseudotentacles present in Ophiodiscus Hertwig, 1882 ( Carlgren, 1949; Fautin, 1984; Rodríguez et al., 2008; Eash-Loucks and Fautin, 2012). It can be further differentiated from Parasicyonis Carlgren, 1921 , by the presence of filaments in fertile mesenteries (present in Parasicyonis ; absent in S. erythrocephala ) and number of perfect mesenteries (some mesenteries of fourth cycle perfect in Parasicyonis ; no mesenteries of fourth cycle perfect in S. erythrocephala ) ( Carlgren, 1921, 1949; Rodríguez et al., 2008; Eash-Loucks and Fautin, 2012). From Synsicyonis Carlgren, 1921 , our specimens can be differentiated by the number of cycles of mesenteries (four in Synsicyonis ; up to six in Sicyonis and five in S. erythrocephala ) and the fertility of the mesenteries (third cycle of mesenteries fertile in Synsicyonis ; only fourth cycle of mesenteries fertile in S. erythrocephala ) ( Carlgren, 1921, 1949; Rodríguez et al., 2008).

Sicyonis is a deep-sea genus of poorly known species whose identification is difficult due to unknown variability in diagnostic morphological characters ( Molodtsova et al., 2008). Despite these difficulties, our specimens agree with the description of the external and internal anatomy given by Carlgren (1927) for S. erythrocephala . Except for the size of the nematocysts in the tentacles and actinopharynx, which are closer to the measurements given by Carlgren (1939) than those from Carlgren (1927), our specimens show only slight differences in the number of mesenteries described for S. erythrocephala : they have 68 pairs whereas Carlgren recorded 70 and 88 pairs ( Carlgren 1927, 1939, respectively). The differences in number of mesenteries could be due to the different size of specimens or the difficulty in counting mesenteries in general, but especially in badly preserved deep-sea specimens. It is important to note that S. erythrocephala falls into a group within Actinostolidae that, in general, does not follow the Actinostola rule, even indistinctly ( Sanamyan et al., 2015). This is seen in our specimens of S. erythrocephala in which small differences within a pair can be observed only in the histological sections (fig. 5G), but not in the dissections. Sicyonis erythrocephala can be differentiated from five of the species of the genus by features of the tentacles: development of the basal aboral thickening of the tentacles (present in S. erythrocephala ; absent in of S. variabilis ), basal aboral thickening projecting as large swollen bulb (absent in S. erythrocephala ; present in S. tuberculata ), number of tentacles (132 in S. erythrocephala ; 144 in S. gossei ; 200 in S. tubulifera ), morphology of tentacles (wartlike in S. crassa ; filiform in S. erythrocephala ), and distribution of tentacles in oral disc (marginal in S. heliodiscus ; outer half of oral disc in S. erythrocephala ). Sicyonis erythrocephala can be further distinguished from the other species of the genus by the marginal sphincter musculature (well developed in S. hemisphaerica and S. sumatriensis ; weak in S. erythrocephala ) and its nature (alveolar proximally and distally in S. carey ; reticular throughout in S. erythrocephala )