Hormathia pectinata ( Hertwig, 1882 )

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 : 41-43

Hormathia pectinata ( Hertwig, 1882)

Figures 17–18, table 8

MATERIAL: MNRJ 6759 (3 specimens) ; locality: Projeto Programa Obs, Bordo da Frota Arrendada Obs Doação de Carlos M.L. Silva, CF/ V Kimpo Maru, southwestern Atlantic , Brazil, off the coast of Rio Grande do Sul ( RS), 33°46.1′ S 52°00.03′ W, collected on 27 July 2002 (526 m) .

MATERIAL EXAMINED: Hormathia armata: AMNH 4005 (1 specimen) ; locality: ANT XIX/3

Cruise, ANDEEP I Program, RV Polarstern, Sta. PS 61/106–1, 61°38.02′ S 57°32.07′ W, Southern Ocean , Drake Passage, Antarctica, collected on 14 February 2002 (424–427 m). Hormathia lacunifera: AMNH 4106 (2 specimens) ; locality: ANT XIX/3 Cruise, ANDEEP I Program, RV Polarstern, Sta. PS 61/049–1, 61°11.57′ S 54°42.99′ W, Southern Ocean , Antarctica, South Shetland Islands, Elephant Islands (272–306 m) .

EXTERNAL ANATOMY (fig. 17): Body short and broad in preserved specimens, up to 52 mm in length and 58 mm in diameter (fig. 17A). Pedal disc flat, circular, adherent, 23 mm in diameter in preserved specimens (fig. 17A). Column cylindrical with deciduous cuticle (fig. 17A), often only between large tubercles (fig. 17A); tubercles smooth, distributed in entire column, regularly arranged (fig. 17A, H). Column divisible into scapus and scapulus; short capitulum not distinct, retracted into scapus in contracted preserved specimens (fig. 17A, B). Margin of capitulum tentaculate (fig. 17E). Column white in preserved specimens with brown cuticle (fig. 17A); 18 mm in length and 39 mm in diameter in preserved specimens. Oral disc circular, small, narrower than column in preserved specimens (fig. 17A). Oral disc contracted in all specimens, 10 mm in diameter in preserved specimens. Tentacles 92–96, short, thick, pointed, displaced to margin of oral disc.

INTERNAL ANATOMY AND HISTOLOGY (fig. 17): Body with wall thickness fairly uniform along column: epidermis and gastrodermis thinner than thick mesoglea (fig. 17E); limit between scapus and scapulus gradual (fig. 17E). Marginal sphincter musculature mesogleal, alveolar, long, narrow (fig. 17B, E); fibers in most of mesoglea, close to gastrodermis distally and scarce and narrow proximally (fig. 17B, E). Longitudinal musculature of tentacles ectodermal (fig. 17F); tentacles without mesogleal aboral thickenings (fig. 17G). Actinopharynx up to 20 mm in length, approximately one half of column’s length; longitudinally sulcated throughout; with thick and highly glandular epidermis (not shown). Specimens with two differentiated siphonoglyphs (fig. 17H) with thin gastrodermis and epidermis and thick mesoglea.

Mesenteries hexamerously arranged in four cycles (6+6+12+24 = 48 pairs) on most of body length (fig. 17H, I): at actinopharynx-level first cycle perfect, including two pairs of directives, each associated with one siphonoglyph; remaining pairs of second to fourth cycles imperfect. All mesenteries with filaments; those of first to third cycles with acontia. All examined specimens sterile. Retractors of first to third cycles well developed, diffuse (fig. 17I), restricted to distal part of mesentery (fig. 17J, K); those of fourth cycle very weak (fig. 17L). Parietobasilar musculature weak in all mesenteries, (fig. 17I–L), stronger in mesenteries of fourth cycle (fig. 17K, L). Basilar musculature of mesenteries strong (not shown).

CNIDOM (fig. 18): Spirocysts, basitrichs, p -mastigophores B1. See figure 18 and table 8 for size and distribution.

DISTRIBUTION AND NATURAL HISTORY: Hormathia pectinata has been recorded from Chile and Argentina ( Hertwig, 1882; McMurrich, 1893; Carlgren, 1959; Riemann-Zürneck, 1973) going as far north on the eastern coast of South America as the southern edge of the La Plata estuary off the coast of Mar del Plata. This species has a discontinuous distribution in the SWA ( Riemann-Zürneck, 1986). Specimens of H. pec- tinata from this study were collected at a single site off the coast of Rio Grande do Sul in southern Brazil at 526 m. This new record extends the geographic range of H. pectinata north of La Plata River including the southern Brazilian coast, and fall within the shallower range of the species bathymetric distribution in bathyal (405– 1220 m) rather than abyssal waters (2812–2925 m) ( Rodríguez and López-González, 2001). Hormathia pectinata shows an overlapping geographic and bathymetric distribution only in its sub-Antarctic range with H. lacunifera , which is a circumpolar Southern Ocean species similarly distributed from continental shelf and bathyal depths. Hormathia spinosa ( Hertwig, 1882) , on the other hand, is found in sub-Antarctic region and Japan ( Hertwig, 1882) whereas H. armata is known only from the Antarctic Peninsula and Weddell Sea at shallower continental waters ( Rodríguez and López-González, 2013).

REMARKS: The genus Hormathia is one of the most distinctive genera of Hormathiidae and is defined by characters that have all been found in our specimens: 6 pairs of perfect and sterile mesenteries, same number of mesenteries distally and proximally, column divisible into scapus and scapulus; scapus with tubercles and cuticle; no cinclides; 96 tentacles without aboral thickenings, and absence of cuplike pedal disc. Six species of Hormathia have been recorded in the Southern Hemisphere ( Dunn, 1983; Rodríguez and López-González, 2013; Fautin, 2016) though only four might be valid ( H. pectinata , H. lacunifera , H. spinosa , H. armata: Rodríguez and López-González, 2013 ). The anatomy and microanatomy of the individuals of H. pectinata examined in this study agrees with the description of Riemann-Zürneck (1973), including the number and cycles of tentacles and mesenteries. We observed the same thickening of the mesoglea in the retractors of mesenteries observed by Riemann-Zürneck (1973) (see fig. 17I–L). Unfortunately, our specimens were sterile, so we cannot comment on the fertility of the species. Though the comparative work of Riemann-Zürneck (1973) found that the taxonomic significance of the cnidon within the genus Hormathia is low, the size of basitrichs in the acontia is regarded as a distinctive character that differentiates species within the genus. Though the measurement of basitrichs in the acontia of our specimens is on the higher side of the range, it clearly identifies our specimens as H. pectinata and differentiates them from the remaining species of the genus in the southern Atlantic. Similar longer basitrichs in the acontia of specimens of H. pectinata were also found by Riemann- Zürneck (1973) in material from the coast of Argentina. The small size of individuals of H. pectinata found in Chile (type locality) might explain the particularly short basitrichs of the acontia in this material. Here, we confirm the presence of p -mastigophores B 1 in relative abundance in the acontia of H. pectinata (fig. 18Q), which had been previously found by Riemann- Zürneck (1973) in H. pectinata from Argentina. This highly distinctive character further corroborates the identification of our specimens as H. pectinata and differentiates it from other species in the genus, and could explain the lack of a sister relationship between this species and other Hormathia spp. in all phylogenetic analyses to date (e.g., Gusmão and Daly, 2010; Rodríguez et al., 2012, 2014; Gusmão et al., 2019a, 2019b). It might also be possible that p -mastigophores have been overlooked in the acontia of other hormathiids and that further examination will reveal their presence in other genera or species in the family. The cnidom of our specimens agree generally with the one given by Riemann- Zürneck (1973) for H. pectinata , except that we found p -mastigophores B 1 in the column and basitrichs in a larger category in the filaments, a difference that has been used to differentiate H. pectinata from H. armata and H. spinosa (E.R., personal obs.). It is possible that these large basitrichs in the filaments are the result of contamination from acontia as they overlap in size. In addition, although the p -mastigophores B1