Lyonsiella tentaculata Pacheco & Teso & Pastorino 2024, sp. nov.

Lyonsiella tentaculata sp. nov. (Figs 11A, B, D, F, G, 12) urn:lsid:zoobank.org:act:1443D27C-D033-4721-B12CE1B7BA98F3E8 Type material: Holotype: MACN-In 44327, one individual. Paratypes: MACN-In 44326, two valves. Type locality: Mar del Plata submarine Canyon (38°01 ʹ 26″S, 53° 51 ʹ 01″W), 2212 m. Etymology: The name tentaculata, from the Latin tentaculum, refers to the numerous siphonal tentacles in comparison to other species of the genus. Distribution New records: Mar del Plata Submarine Canyon (1395–2212 m) and Burdwood Bank (647 m). Bathymetry: 647–2212 m. Material examined: MACN-In 44328 (37°49 ʹ 40″S, 54°07 ʹ 56″W, 1395 m) one individual; MACN-In 44326 (54°36 ʹ 14″S, 62°50 ʹ 55″W, 647 m) two valves (paratypes). Diagnosis Shell ≤ 14 mm long, ornamented by ≤50 radial lines spaced by radially oriented groups of pyramidal spines. Siphons surrounded by 25 siphonal tentacles, 12 associated with inhalant siphon and 13 with exhalant. Description Shell medium in size (≤ 14 mm L, 11 mm H, 4.6 mm W), fragile, somewhat translucid, oval-trapezoidal, inequilateral. Inequivalve; left valve more rectangular, right valve slightly larger and with dorsal and ventral margins projecting beyond the left valve. Surface covered by ≤50 radial lines spaced by radially oriented groups of four to eight pyramidal spines, usually covered by debris. Umbo prosogyrus, displaced anteriorly. Anterodorsal margin diagonally inclined, lunule weak, anterior to resilium. Anterior margin rounded, slightly extended beyond umbo. Posterodorsal margin somewhat straight, almost parallel to ventral margin; both margins connect with nearly vertical posterior margin through rounded corners. Hinge edentulous, with elongated lithodesma. Interior iridescent, with radial lines corresponding to external striae. Adductor muscle scars slightly marked, anterior more than posterior. Palial line noticeable. Ctenidium composed of a reduced outer demibranch and an inner demibranch with ~50 and ~60 branchial filaments, respectively. Inhalant siphon with 12 inner arborescent tentacles on the edge of the aperture and 10 outer small tentacles surrounding it, plus two small dorsal tentacles. Exhalant siphon surrounded by 13 outer small tentacles (seven larger intercalated by six smaller). Byssus present. Remarks Lyonsiella abyssicola is a comparable species; however, it has 16 tentacles surrounding both siphons, whereas L. tentaculata has 25. The number of tentacles is the same both in the largest individual (MACN-In 44327, 14 mm L) and in the smallest individual (MACN-In 44328, 7 mm L), with a similar size to L. abyssicola, hence it can be discarded as a variation attributable to size. Additionally, the new species reaches a larger size and exhibits more radial lines than L. abyssicola, (≤ 14 mm and 50 lines vs. 5 mm and 20 lines) (Fig. 11A, B). A clear character visible by SEM is the shape of the microscopic spines. Lyonsiella tentaculata has pyramidal and acuminated spines, whereas L abyssicola, Lyonsiella frieli Allen & Turner, 1974 and Allogramma formosa (Jeffreys, 1882), have thinner spines with stellated termination (Oliveira and Absalão 2010a) (Fig. 12C, D). Other comparable Lyonsiella species include Lyonsiella subquadrata (Jeffreys, 1882), distinguished by a wider, more prolonged, and ovate anterior margin (Fig. 11E); Lyonsiella perplexa Allen & Turner, 1974, with an angular anterior margin and a lower number of tentacles (~15); Lyonsiella fragilis, with a much more horizontal anterodorsal margin, a more prolonged anterior margin, and fewer radial lines (8–18); and Lyonsiella pipoca Oliveira & Absalão, 2010, with short and wider spines oriented irregularly, not radially, and with a few isolated delicate ‘projections’.