Cetoconcha gigas Pacheco & Teso & Pastorino 2024, sp. nov.

Cetoconcha gigas sp. nov. (Fig. 9A–G) urn:lsid:zoobank.org:act: 16332040-0DB3-48EF-8B00- 111A3DC37417 Pholadomya cf. adelaidis Hedley 1916 — Mühlenhard-Siegel 1989: 167, fig. 44. Poromya spinulosa (non Thiele 1912)— Engl 2012: 64, pl. 13, fig. 4c. Type material: Holotype: USNM 904483, one individual. Paratype: USNM 1283153, one individual. Type locality: Zavodovski Island, SW of island, South Sandwich Islands, Scotia Sea (56°19.8ʹ S 27°38.4 ʹ W), 575 m. Etymology: The name gigas in Latin means giant and refers to the large size of the species within the genus. Distribution New records: South Sandwich Island and off West Antarctic Peninsula. Literature records: Off Elephant Island as Pholadomya cf. adelaidis (Mühlenkardt-Siegel 1989) and Weddell Sea as Poromya spinulosa (Engl 2012). Bathymetry: 278–575 m. Material examined: 65°39 ʹ 36″S, 68°04 ʹ 48″W, 278 m (USNM 1283153) one individual (paratypes). Diagnosis Shell elongated, ≤ 61 mm long; ornamented with ≤90 weak, closely spaced radial lines. Hinge of right valve with obsolete cardinal tooth; less valve with a longitudinal ridge near end of posterodorsal margin. Siphons surrounded by 19 siphonal tentacles. Description Shell large in size (≤ 61 mm L), thin, ovate, elongated, inequilateral. Surface with ~90 weak, closely spaced radial lines interspaced by groups of four to eight cylindrical granules, usually covered by debris, and commarginal growth lines. Periostracum brownish. Umbo subcentral (displaced anteriorly) and prosogyrus. Antero- and posterodorsal margin gently descendent. Anterior margin rounded, posterior rounded to acute. Ventral margin widely curved. Right cardinal tooth weak, obsolete, fitting in a small socket in less valve. Less valve with elongated longitudinal ridge near end of posterodorsal margin. Resilifer opisthodetic. Interior iridescent, with numerous closely spaced radial lines not corresponding to external lines. Adductor muscle scars well marked, anterior larger than posterior. Longitudinal scar marking insertion of septum laterally to shell. Ring of 19 siphonal tentacles covered by papillae surrounding siphons. Remarks This is the only species of Poromyoidea with a ring of 19 siphonal tentacles (Fig. 9G), deviating from the usual 13–15 tentacles observed in other species (Allen and Morgan 1981, Krylova 2001, Leal 2008). Cetoconcha gigas is also characterized by its large size. Lots USNM 904483 and USNM 1283153, initially identified as P. adelaidis and now assigned as the holotype and paratype of C. gigas, respectively, measure 48 and 25 mm, respectively. In contrast, Cetoconcha spinulosa typically ranges between 10 and 21 mm. These lots also differ from Cetoconcha spinulosa in contour, the number of radial lines, and the development of the cardinal tooth (Fig. 9D). Cetoconcha sarsii and Cetoconcha alephtinae (Krylova, 1991) also have an elongated shape; however, according to their illustrations, both have a contour more rectangular and the umbo less prominent than in Cetoconcha gigas. In addition, all these species have ≤15 siphonal tentacles. The specimens figured by Engl (2012) and Mühlenhard-Siegel (1989) identified as Poromya spinulosa and Pholadomya cf. adelaidis, respectively, showed large sizes (46 and 61 mm long, respectively). Engl’sspecimen (2012: pl.13, 4c) isidenticalto USNM 904483, and the photograph of the hinge provided by Mühlenhard-Siegel (1989) showed an obsolete cardinal tooth. In addition, Mühlenhard-Siegel (1989) mentioned that the shell had more radial lines than usual. These specimens correspond to Cetoconcha gigas. Verticordioidea Stoliczka 1870 Lyonsiellidae Dall, 1895