Cardiomya Ƒagilissima (E. A. Smith, 1885 )

Pacheco, Leonel I., Teso, Valeria & Pastorino, Guido, 2024, Use of traditional tools and micro-computed tomography for the taxonomy of carnivorous bivalves from the deep waters of Southwestern Atlantic, Zoological Journal of the Linnean Society 202, pp. 1-23 : 6-8

Cardiomya Ƒagilissima (E. A. Smith, 1885)

( Fig. 3)

Neaera fragilissima EA Smith 1885: 53 pl. 9, fig. 1–1b.

Cuspidaria fragilissima — Pelseneer 1888a: 23, pl. 4, fig. 1; 1903: 28.

Myonera fragilissima — Dall and Smith 1886 in Dall 1886: 302; Soot-Ryen 1951: 23; Powell 1960: 185; Branch 1991: 51; Aldea and Troncoso 2008: 108, fig. 117; Aldea and Troncoso 2010: 213, fig. 255; Engl 2012: 67, pl. 14, fig. 10a, b.

Miomera fragilissima (in error pro Myonera )— Carcelles 1953: 225.

Cardiomya fragilissima —Allen

Pouthiers and Bernard 1995: 151. and Morgan 1981: 422;

Type material: [ Neaera fragilissima ] 1887.2.9.2431, two fragmented valves.

syntype:

NHMUK

Type locality: Off Prince Edward Island, 548 m.

Distribution

New records: Mar del Plata Submarine Canyon (998–1395 m) and Malvinas / Falkland Islands (646–845 m), Argentina.

Literature records: Antarctica: Peter I to Marion Islands (EA Smith 1885, Pelseneer 1903, Branch et al. 1991, Aldea and Troncoso 2008).

Bathymetry: 548–1873 m.

Material examined: 37°49 ʹ 39″S, 54°07 ʹ 56″W, 1395 m (MACN-In 44323) three individuals; 37°53 ʹ 50″S, 54°30 ʹ 27″W, 998 m (MACN-In 44324) one individual; 38°00″58″S, 54°30 ʹ 19″W, 1006 m (MACN-In 44325) three individuals and two valves; 51°59 ʹ 31″S, 56°37 ʹ 58″W, 645–845 m (USNM 898540) two valves.

Description

Shell large-sized (L, 19.7 ± 3.66 mm; H, 13.4 ± 2.8 mm; W, 5.1 ± 1.2 mm; N = 8), elongated, rectangular, fragile, and thin. Surface with ~50 closely spaced, weak radial riblets, except dorsal to the rostral depression. Riblets almost equally prominent, slightly more defined on the posterior side of the disc, and weak and inconspicuous on the anterior side. Growth lines marked, irregularly crossing radial riblets, providing a subtle cancellated appearance. Umbo central, blunt. Rostrum short (approximately one-third of the shell length), tube shaped, with four or five radial lines on dorsal surface. Posterodorsal margin initially straight, then sharply curved at beginning of rostrum. Anterodorsal margin long, gently descending, followed by a somewhat truncated anterior margin and a widely rounded and crenulated ventral margin. Posteroventral margin highly variable, from unobtrusive to gently marked. Hinge of less valve edentulous, with a slight thickening in the margin immediately posterior to resilifer. Right posterior lateral tooth somewhat prominent and elongated. Condrophore wide, slightly sunken in contact with tooth. Anterior adductor muscle scar oval, posterior triangular. Posterior adductor, septal, and pedal muscles scars well marked, limited posteriorly by a subtle ridge from the rest of the disc located interior to posteriormost defined radial riblet. Palial line not visible. Siphonal sinus shallow.

Remarks

EA Smith (1885) classified several Neaera species into different sections. Neaera fragilissima was included in section M, which encompassed species with edentulous valves, radiating sculpture, and a short, anteriorly inclined resilifer. Subsequently, Dall and E. A. Smith ( Dall 1886) described the genus Myonera as possessing an edentulous hinge and either concentric or radial ornamentation. Ŋey included within this genus all the species from sections L and M of EA Smith (1885), thus incorporating N. fragilissima . However, in the original description of Cardiomya fragilissima, EA Smith (1885: 54) mentioned a ‘very insignificant ridge’, … ‘which might be regarded as an incipient lateral tooth’. Upon closer examination of a photograph of the reconstructed syntype, it is unequivocally identified as a tooth ( Fig. 3A). Moreover, it is crucial to note that, although the radial ornamentation of Myonera typically refers to the limited ridges found in certain species, like Myonera paucistriata ( Dall, 1886) , Cardiomya fragilissima , akin to all Cardiomya species, exhibits numerous radial riblets.Consequently, we align with the proposals of Allen and Morgan (1981) and Poutiers and Bernard (1995) in designating Cardiomya as the most appropriate genus for this species.

Ŋe main variation observed among all the specimens studied pertains to the posteroventral margin, whereas the remaining contour, hinge structure, and ornamentation remain consistent. Ŋe specimen shown in the paper by Aldea and Troncoso (2008) exhibits a shorter anterior region and a shoulder, differing significantly from the syntype and our samples. Ŋis discrepancy is likely to be aưributable to the size of the specimen (2.9 mm × 1.8 mm), suggesting a juvenile. When examining the soss parts, there is agreement with Pelseneer’s (1888b) description, although most of the information provided by the author consists of general anatomical features common among Cardiomya species ( Allen and Morgan 1981, Morton 2015, Machado et al. 2016).

Cardiomya abyssicola Verril & Bush, 1898 and Cardiomya striata (Jeffrey, 1876) share with Cardiomya fragilissima a large number of weak riblets of similar prominence, rectangular contours, and an elongated anterior margin. However, only Cardiomya fragilissima lacks riblets on the region of the rostrum dorsal to the rostral depression and has a much stronger contrast between the defined posterior riblets and the weak anterior ones, which show a gradient from riblets to striae and from striae to lines as they approach the anteriormost region, sometimes becoming nearly imperceptible ( Fig. 3A–D).