Cyclocardia thouarsii (d’Orbigny, 1845), d'Orbigny, 1845

Güller, Marina & Zelaya, Diego G., 2013, The Families Carditidae and Condylocardiidae in the Magellan and Perú – Chile provinces (Bivalvia: Carditoidea), Zootaxa 3682 (2), pp. 201-239 : 207-209

Cyclocardia thouarsii (d’Orbigny, 1845)

Figs. 3 A– O, 9B, Appendix 3

Cardita thouarsii d’Orbigny, 1845: 579 —d’Orbygny, 1847?: pl. 84, figs. 1–3. Actinobolus thouarsii D’Orbigny, 1846 — Tryon, 1872: 255.

Cardita Thouarsi [sic] [recte thouarsii ] d’Orbigny—Rochebrune & Mabille, 1889: H111. Cardita congelascens Melvill & Standen, 1912: 362 , pl. 1, fig. 23.

Cyclocardia thouarsi [sic] [recte thouarsii ] Orbigny, 1846— Soot-Ryen, 1959: 39. Cyclocardia thouarsi [sic] [recte thouarsii ] (Orbigny, 1846)— Dell, 1964: 189, pl. 2, fig. 9.

Type localities: Îles Malouines [ Malvinas / Falkland Islands] ( Cardita thouarsii ); Burdwood Bank, 56 fathoms [102 m] ( Cardita congelascens ).

Material examined: Holotype of Cardita thouarsii ( NHMUK 1854.12.4.760), 1 syntype of Cardita congelascens ( RSM 1921.143.711), and 19 lots (Appendix 3, Table 1 ).

Other published records: Baie Orange [55º31’S 70º26’W] ( Rochebrune & Mabille, 1889) (Appendix 3, Table 2 ).

Known distribution: Cape Horn [55°S] to Malvinas / Falkland Islands [50°S]. Living specimens: 198– 903 m.

Description: Shell large (maximum observed L = 21.3 mm), transversely ovate to rectangular (H/L = 0.89 ± 0.06, n = 6), inequilateral, somewhat inflated (W/H = 0.66 ± 0.02, n = 6), solid ( Figs. 3 A–K). Anterior end produced, lower than posterior end. Antero-dorsal margin short, markedly concave, forming a prominent angle at the junction with anterior margin. Anterior margin short, curved, continuous with ventral margin, which is slightly expanded posteriorly. Posterior margin larger than the anterior one, forming a wide, almost vertical curve. Posterodorsal margin long, slightly sloping ( Figs. 3 A–H, J, K). Lunule short, wide and moderately deep; escutcheon elongated, extremely narrow ( Fig. 3 I). Beaks inflated, anteriorly displaced, anteriorly directed ( Figs. 3 A–K). Prodissoconch small (about 225 µm in length); P-1 smooth, P-2 with low commarginal lines ( Fig. 3 O). Shell surface sculptured with 16–22 moderately elevated, radial ribs, closely and finely scaled; interspaces as wide as or narrower than radial ribs, with fine commarginal lamellae ( Figs. 3 A–C, E–G). Periostracum thick, pale buff, forming hair-like projections ( Figs. 3 A, B, E, L). Inner margin crenulated, coincident with outer shell sculpture. Pallial line continuous ( Figs. 3 H, J, K).

Hinge plate relatively narrow, anterior and posterior halves nearly equal in height ( Figs. 3 M, N). Right valve ( Fig. 3 N): anterior cardinal tooth (CA3) hooked, dorsally fused with shell margin, anterior part (CA3a), short, narrow, ventrally directed; posterior part (CA3b) large, with widely posteriorly projected triangular base. Posterior cardinal tooth (CP5b) long, straight, moderately solid, parallel to nymph. Anterior lateral tooth (LAI) short, narrow.

Posterior lateral tooth (LPI) elongated, narrow, close to dorsal margin. Left valve ( Fig. 3 M): two diverging cardinal teeth; the anterior (CA2a) small, triangular, ventrally directed, with subcentral cusp, one third the size of the posterior one (CP4b). Posterior cardinal nearly straight, distally widened. Anterior lateral (LAII) and posterior lateral (LPII) small. Ligament external, located on a nymph which extends for two thirds the length of posterodorsal margin ( Figs. 3 I, M, N).

Anatomy (Fig. 9B): Mantle margin with a large inhalant-pedal aperture, extending for about 4/5 of mantle margin length, separated from a relatively large posterior exhalant opening by a short suture. Transverse section of anterior and posterior adductor muscles large, the former reniform, the latter ovate. Inner and outer demibranchs posteriorly fused with each other and with mantle margin. Outer demibranch two thirds the size of inner one, showing 202 filaments in a 18 mm long specimen; ascending and descending lamellae equally developed. Inner demibranch with 214 filaments; ascending lamella as high as descending lamella. Labial palps with ill defined sorting ridges. Foot with posterior byssal gland.

Remarks: Dell (1964) suggested that Cyclocardia thouarsii and C. velutina could correspond to variations of a single species. From the present study it is clear that they are two different species: Cyclocardia thouarsii is more laterally compressed, has a more elongated shell outline, it is sculptured with lower radial ribs, the beaks are more anteriorly placed, and the hinge plate is narrower, with more delicate teeth than C. velutina .

Cyclocardia thouarsii was described by d’Orbigny in the “Voyage dans l’Amerique Méridionale…” page 580, and illustrated on plate 84. According to Sherborn & Griffin (1934) and Cowie & Thiengo (2003), that page was published in 1845; however, none of these papers refer to the date of publication of plate 84, and no other reference is available to date this plate. According to the ICZN (1999), Art. 21.7, the date of publication should be regarded as the “earliest date on which the work, or part of it, is demonstrated to be in existence as a published work”. Under this criterion, we consider that the date of publication for the species corresponds to the date of publication of the text.

Figures

FIGURE 3. Cyclocardia thouarsii. A, D: Holotype of Cardita thouarsii, Malvinas / Falkland Islands (NHMUK 1854.12.4.760); B, E, I: Cape Horn, 780 m (ZMB 103678); C, F, L: Cape Horn, 430 m (ZMB 103538); G, H: Syntype of Cardita congelascens, Burdwood Bank, 102 m (RSM 1921.143.711); J, K, M – O: Burdwood Bank, 286 m (MLP 13708). A – C, E – G: Outer view; A – C: Left valve; E – G: Right valve; D, H, J, K: Inner view; D, J: Left valve; H, K: Right valve; I: Dorsal view; L: Periostracal projections; M, N: Detail of hinge plate; M: Left valve; N: Right valve; O: Prodissoconch (right valve; arrow shows P- 1 / P- 2 boundary). Scale bars: A, C, D, F – H, J, K = 2 mm; B, E, I = 1 cm; L, O = 100 µm; M, N = 500 µm.