Actinernus mercedae, Gusmão & Rodríguez, 2021

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 : 6-7

Actinernus mercedae , sp. nov., Gusmão, López-González, Rodríguez

Figures 2–3, table 1

MATERIAL: Holotype AMNH 4028 (1 specimen); locality: ANT XIX/3 , ANDEEP I Program, RV Polarstern , Sta. PS 61/114–10, Southern Ocean, Drake Passage, Antarctica, 61°43.70′ S

60°42′ W, collected on 19 February 2002 (2853– 2856 m). Paratypes AMNH 4078 (3 specimens, same collection data as holotype). Paratype AMNH 4035 (1 specimen, same collection data as holotype). MNRJ 8183 (3 specimens) ; locality: Ak. Ioffe Cruise 29, Superestação 10, Sta. 202, series #1044, South Mid-Atlantic Ridge , 33°40.07′ S 23°05.57′ W, collected on 23 November 2009 by MAR-ECO #38216 (4740 m) .

MATERIAL EXAMINED: Actinernus michaelsarsi: USNM 88367/386969 (1 specimen; holotype); locality: United States, North Atlantic Ocean, Blake Plateau, 350 mile ESE of Charleston, South Carolina at the Wreck Site of S.S. Central America, 31°30′ N 77°00′ W, collected Sept 1989 by Columbus America Discovery Group; Herdendorf, C. E.; Evans, R.; by Arctic Discoverer R / V and Nemo DSR / V (2200 m). Porponia antarctica Carlgren, 1914 : NMSZ 1921.143.1756 (2 specimens; syntypes); locality: Scotia Collection, off Coats Land, 71°22′ S 16°34′ W, collected on 16 March 1904 (2578 m). NMSZ 1921.143.1756 (1 specimen; syntype, same collection data as other syntypes). Porponia elongata Hertwig, 1882 : NHM 1889.11.25.29 (2 specimens; syntypes); locality: Challenger Collection, Sta. 160, collected on 13 March 1874 (4755 m). Porponia robusta Hertwig, 1882 : NHM 1889.11.25.30 (1 specimen; holotype); locality: Challenger Collection, Sta. 237, 34°37′ N 140°32′ E (3429 m). Isactinernus quadrilobatus Carlgren, 1918 : UUZM [Typsamlingen nr 102 a ( Anthozoa)]. Syntype. “Dr Sixten Bocks Japan Exp. 1914, KIUSCHIU, Goto Islands, Osesaki, 23 miles NV t. V 1/2 V – 122°10′ Ost, 165 m, datum: 14/5, formol, Type” (1 specimen). UUZM [Typsamlingen nr 102 b ( Anthozoa)]. Syntype “Dr Sixten Bocks Japan exp. 1914, KIUSCHIU, Goto Islands, Osesaki, c.a 170 m, datum: 14/5, formol, Isactinernus 4-lobatus orig: determ. Carlgren” (1 specimen). Synactinernus flavus Carlgren, 1918 : UUZM. [Typsamlingen nr 232 ( Anthozoa)]. Holotype. “Dr Sixten Bocks Japan Exp. 1914, KIUSCHIU, Goto Islands, 28 miles N, 1/2 Ost frain Shirase fyr, 128°50′ O.L. 33°41′ N Br, 110 m, datum: 17/5; botten: sand, formol. Synactinernus flavus orig: determ. Carlgren.”

EXTERNAL ANATOMY (fig. 2): Body short and broad in preserved specimens, with an inverted triangle shape (fig. 2A, B). Pedal disc well developed, small, flat or slightly curved, rectangular; up to 8 mm in diameter in preserved specimens (fig. 2B). Column smooth, not divisible into regions, widened distally, rectangular (flattened in one axis) (fig. 2A, B); with cartilaginous texture (fig. 2B). Column up to 31 mm in diameter and 39 mm in length in preserved specimens. Oral disc wider than column in preserved specimens (fig. 2B), forming at least four lobes (fig. 2C) in preserved specimens and eight in live ones (fig. 2A). Oral disc up to 44 mm in diameter in preserved specimens; apparently nonretractable. Margin of column tentaculate (fig. 2A, B, D). Tentacles 59–62, long, very slender, pointed, of “filamentous” aspect (fig. 2A–C), with prominent mesogleal basal aboral thickenings of triangular shape in preserved specimens (fig. 2D, E); longest tentacle up to 15 mm.

INTERNAL ANATOMY AND HISTOLOGY (fig 2): Marginal sphincter musculature absent (fig. 2F). Base of tentacles with mesogleal aboral thickening (fig. 2F, G); longitudinal tentacle musculature ectodermal (fig. 2H). Actinopharynx short, approximately one third of column’s length, longitudinally sulcated throughout. Two sectioned specimens with up to 46 mesenteries in four cycles distally (6+4+8+5 pairs) (fig. 2J); mesenteries after first 6 pairs on lateral endoceles of first cycle with retractor muscles as in directives; after second cycle (6+4 = 10 pairs), mesentery formation bilateral in middle zone of lateral endoceles. All mesenteries perfect, including two pairs of directives, each associated with one siphonoglyph (fig. 2I–J), except 4–8 imperfect mesenteries (depending on specimen) of third and fourth cycles. In third and fourth cycle, mesenteries within same pair unequally developed (fig. 2K). More mesenteries distally than proximally. Thirty-five mesenteries proximally in three cycles; all mesenteries with filaments and gametogenic tissue. All specimens collected in November sterile; those collected in February with weakly developed spermatic cysts in all mesenteries. Retractors of all mesenteries weak, short, diffuse (fig. 2L), difficult to discern; parietobasilar musculature very weak in all mesenteries (fig. 2M). Basilar musculature of mesenteries absent (fig. 2N).

CNIDOM (fig. 3): Spirocysts, basitrichs, b -mastigophores, p -mastigophores A, and holotrichs. See figure 3 and table 1 for size and distribution.

DISTRIBUTION AND NATURAL HISTORY: Specimens of Actinernus mercedae , sp. nov., were collected in the Southern Ocean in the Drake Passage (61°43.70′ S, 60°42.62′ W) between 2852–2856 m, and in the South Atlantic, east of the Mid-Atlantic Ridge at 4740 m depth.

ETYMOLOGY: This species is named after Mercedes Conradi (Universidad de Sevilla, Spain), who kindly collected, photographed, and preserved specimens of the new species for the first time.

REMARKS: After the revision of the type material and newly collected material, Rodríguez and López-González (2013) considered five valid species for Actinernus : A. antarcticus , A. elongatus , A. michaelsarsi , A. nobilis , A. robustus , and two new undescribed species from Antarctica (E.R., personal obs.). Uchida (2007) considered the same five valid species of Actinernus . Contra Fautin (2016), Rodríguez and López-González (2013) formalized this point of view on the membership of Actinernus , rejecting the synonymy of A. antarcticus and A. elongatus by Dunn (1983) and considered A. antarcticus valid. Here we describe Actinernus mercedae , which is placed within the genus due to its lobed column that is more developed distally, absence of marginal sphincter musculature, distinct mesogleal thickenings on aboral side of tentacles, mesenterial arrangement following the Endocoelantheae plan (see Carlgren, 1949), two siphonoglyphs and weak retractor and parietobasilar musculatures. Actinernus mercedae resembles other two genera within the family ( Isactinernus Carlgren, 1918 , and Synactinernus Carlgren, 1918 ) because it has a lobed distal column ( Carlgren, 1949; Uchida, 2007). However, it can be easily distinguished from those genera because of the bilateral arrangement of the younger mesenteries in Actinernus (cyclic in the other two genera, Carlgren, 1918; Uchida, 2007; Izumi et al., 2019) and the distinct mesogleal aboral thickenings in the base of the tentacles in Actinernus (not distinct in the other two genera) ( Izumi et al., 2019); furthermore, the lobes in species of Actinernus are not as marked as those in the other two genera (see Izumi et al., 2019). Actinernus mercedae is the third species of the genus described for the Southern Ocean: Actinernus antarcticus and A. mercedae are present in the Atlantic portion of Antarctica whereas A. elongatus is known from the sub-Antarctic Indian Ocean. Because the internal anatomy of members of Actinernus is often hard to observe due to preservation issues, the only characters to show consistent anatomical differences between the valid species of Actinernus in the Southern Ocean are the development and shape of the basal aboral thickenings of the tentacles ( Rodríguez and López-González, 2013). However, the number of mesenteries and their arrangement and their perfect or imperfect nature also seem to differ among the species. Actinernus mercedae is easily differentiated from all other species of the genus in the Southern Ocean by the more or less defined oral disc lobes with very conspicuous triangular mesogleal aboral thickenings in tentacles (fig. 2A). Though Dunn (1983) hypothesized that the lobes in the oral disc of Actinernus spp. are artefacts of the contraction of tentacles ( Fautin and den Hartog, 2003), specimens of A. mercedae display lobes in the oral disc even in extended state (fig. 2A). Actinernus mercedae can be differentiated from A. elongatus by the rounded mesogleal aboral thickenings in tentacles of this species, and by the gradual decrease in diameter between the basal aboral thickenings and the tentacles as well as for the morphology of the tentacles themselves in A. antarcticus (i.e., filiform tentacles in A. antarcticus vs. filamentous tentacles in A. mercedae ) (see fig. 4). In addition, the number of mesenteries differs between A. antarcticus and A. mercedae (i.e., up to 68 mesenteries in A. antarcticus vs. 46 in A. mercedae ; see Carlgren, 1914, 1918, for A. antarcticus ). Given our current lack of understanding about the distribution of cnidae in species of Actinernus , we should not use this character for specific identification (Rodríguez and López- González, 2013). However, we acknowledge that the cnidae of our specimens differ from that of A. antarcticus in the presence of spirocysts and two categories of p -mastigophores A in the column; presence of p -mastigophores A and lack of b -mastigophores and small basitrichs in tentacles; lack of spirocysts and smaller basitrichs and presence of two kinds of p -mastigophores in the filament. Geographic distribution also varies among valid species of Actinernus in the Southern Ocean: A. antarcticus is known from Antarctic waters (Kapp Norvegia and Antarctic Peninsula) whereas A. elongatus is known from sub-Antarctic waters of the Indian Ocean. Actinernus mercedae is known from its type locality in the Drake Passage and in the South Atlantic outside the sub-Antarctic region (~ 33° S) at depths similar to records of A. elongatus (i.e., 4750 m) ( Rodríguez and López-González, 2013). Of the three species of Actinernus distributed in the northern hemisphere, Actinernus mercedae resembles A. robustus from Japan in the shape of the tentacular mesogleal aboral thickenings and lobed column (see Hertwig, 1882; Carlgren, 1918; Uchida, 2007). The circumscription of characters in A. robustus is not very clear since the single specimen examined and described by Carlgren (1918) besides the holotype (that of Hertwig, 1882) differs relative to the holotype (i.e., large specimen: up to 75 mm in column height and up to 190 tentacles, eight lobes of similar size, companions in younger developing pairs of mesenteries of similar size, depth 150–600 m) to the holotype and the additional material examined by Uchida (2007), i.e., up to 40 mm height and up to 90 tentacles, eight lobes divided in four bigger and four smaller ones, companions in newly developed pairs of unequal size, depth 1115–3429 m.These differences led Uchida (2007) to hypothesize that there are probably two species in Japan. In addition, Uchida’s interpretation of the formation of the mesenteries in Actinernus differs slightly from that of Carlgren (1918, 1949). Although both authors agree in that after the second cycle mesenteries appear bilaterally in each of the lateral endoceles, for Uchida (2007) new pairs of mesenteries appear in the middle zone of each of the eight lateral endoceles and are of unequal size, whereas for Carlgren new mesenteries in a pair usually grow from the outer side of the octant inward and are of similar size ( Carlgren, 1918, fig. 13). Corroborating the formation and arrangement of the younger pairs of mesenteries is hard in these species because of the weak and diffuse nature of the retractors and its preservation in the rather few specimens available. In the examined specimens of A. mercedae (AMNH 4078, MNRJ 8183) unequally developed companions corresponding to the third and fourth cycle of mesenteries are present in the same octant at the actinopharynx level, whereas in A. robustus mesenteries companions of a pair are of unequal size from the fourth cycle on in the same octant at a similar level ( Uchida, 2007). There are also differences in the cnidae of both species (see