Tenuicorbula lyoni ( Pilsbry, 1897 )

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 42-43

Tenuicorbula lyoni ( Pilsbry, 1897)

Figure 13 .

Corbula lyoni Pilsbry, 1897: 294–295 ; pl.VII, fig. 21–23. Figueiras & Sicardi, 1970: 410–411; pl. V, fig. 77.

Corbula (Corbula) lyoni . Rios, 1985: 269; pl. 94, fig. 1328. Rios, 1994: 291; pl. 99, fig. 1421. Rios (2009): 584.

Aloidis lyoni . Carcelles, 1944: 291; pl. XIII, fig. 104. Castellanos, 1967: 269; pl. XXV, fig. 4–5.

Type material examined. Corbula lyoni Pilsbry, 1897 . ANSP 70538, lectotype designated herein, open pair, 12.7 mm length, 7.9 mm height; Maldonado Bay, Uruguay ( Fig. 13 A–D); paralectotype in the same lot, open pair, 8.6 mm length, 4.9 mm height. Pilsbry (1987) indicated the measurement of two specimens, one with 11.75 mm length and the other with 12.75 mm length. However, the syntype lot of the Academy of Natural Science of Philadelphia has two specimens of Corbula lyoni and a right valve of C. dietziana . The largest specimen of C. lyoni , chosen here as lectotype, is probably one of the specimens measured by Pilsbry and used for the drawing on plate VII, due to the similarity between the number and shape of the ribs represented in his design with the specimen. The other specimen of C. lyoni was not measured by Pilsbry, as it is 8.6 mm. The second specimen measured by Pilsbry may have been the right valve of C. dietziana .

Additional material. See Appendix. Corbula lyoni Pilsbry, 1897 . Brazil between Rio de Janeiro State and Chuí, Rio Grande do Sul State . Uruguay between Rocha Departament and Punta Ballena, Madonado. Argentina between Monte Hermoso to Puerto Madryn , Chubut.

Diagnosis: Shell small to moderate in size, thin to thick, subelliptical with posterior pronounced acute rostrum, aligned with the antero-posterior shell axis. Posterior slope broad and concave, set off by a high and sharp keel. Anterior dorsal margin short, convex, ventrally inclined, continuous with broadly convex anterior margin; posterior dorsal margin straight, ventrally inclined, longer than anterior dorsal; obliquely truncated posterior margin forming an acute angle in the confluence with the ventral margin; ventral margin straight or slightly convex with the posterior region straight and inclined dorsally. Sculpture similar in both valves, but different between the central and posterior slope; central area with wide commarginal ribs; posterior slope with narrower commarginal ribs.

Redescription. Shape. Shell small to moderate in size (length: 6.7– 17 mm; height: 4–8.2 mm), subelliptical, moderately heavy and inflated, subequilateral to inequilateral with posterior pronounced acute rostrum, aligned with the antero-posterior shell axis, right valve larger, higher and more inflated than left. Posterior slope broad and concave, set off by a high and sharp keel extending from the umbos to the posterior ventral margin; keel intersecting ventral margin; plane tangential to posterior slope forming a strong obtuse angle in relation to the plane tangential to central slope. Right valve surface, excluding the posterior slope, moderately and regularly convex; convex left valve surface with median inconspicuous radial sulcus. Umbos prosogyrous with beaks at about 31%–40% of shell length from anterior end. Lunular area depressed. Escutcheon elongate, narrow, slightly larger in the right valve, defined by a high and rounded elevation in right valve and by a slender radial rib in the left valve.

Anterior dorsal margin short, convex, ventrally inclined, continuous with broadly convex anterior margin; posterior dorsal margin straight, ventrally inclined, longer than anterior dorsal; obliquely truncated posterior margin forming an acute angle in the confluence with the ventral margin; ventral margin straight or slightly convex with the posterior region straight and inclined dorsally. Lateral siphonal plate rarely present; when present only in the left valve.

Ornamentation. External surface whitish with light brown periostracum preserved at the margins. Sculpture similar in both valves, but different between the central and posterior slope; sculpture on central slope constituted by irregular and high commarginal ribs whose top become wider near to posterior keel; sculpture on posterior slope constituted by high commarginal ribs, more regular and narrowest each other than in the central slope. Inner surface smooth, generally white with brown patches.

Hinge. Hinge axis parallel to antero-posterior shell axis on both valves. Right valve with cardinal tooth below the beak and a resilial socket sunken under umbo; right cardinal tooth pyramidal with dorsally curved apex, equilateral-triangle-shaped when viewed laterally. Left valve with a deep trigonal cardinal socket below the beaks, with lateral walls slightly wrapping around of opening, and a projected chondrophore perpendicular to the sagittal plane. Chondrophore broad, flattened and divided into anterior oval and posterior trigonal area by a radial ridge; posterior margin of chondrophore with low and moderately wider knob. Trough on right valve for reception of left valve extending around dorsal, posterior and posteroventral margins.

Muscle scars. Adductor muscle scars slightly well-impressed, in oblique angle in relation to anterior-posterior shell axis in both valves; anterior adductor scar ovate-elongate, posterior adductor scar rounded. Anterior pedal retractor muscle scar elongated; posterior retractor muscle scar rounded; anterior and posterior pedal retractor scars joining adductor scars. Pallial line far from valve margin, convex, nearly parallel to the ventral margin. Straight or slightly convex pallial sinus, parallel to the posterior margin.

Pre-accretion shell. Specimens in pre-accretion stage of T. lyoni do not differ in shape from the thickened shell (post-accretion stage). Thin hinge with poorly projected chondophore.

Distribution. Western Atlantic, from state of the Rio Janeiro, Brazil to Puerto Madryn, Chubut Province, Argentina, from depth of 14.8 m to 200 m. According Rios (2009) Tenuicorbula lyoni also occurs in state of Espírito Santo.

Remarks: Tenuicorbula lyoni is distinguished from Tenuicorbula tenuis (G. B. Sowerby I, 1833) from the Eastern Pacific by its sculpture, escutcheon and dorsal margin. Tenuicorbula lyoni has a wide commarginal ribs on the central slope and narrower commarginal ribs on the posterior slope, the escutcheon is narrow and the posterior dorsal margin is straight and ventrally inclined; T. tenuis have fine and dense commarginal ribs on the beaks, central and posterior slope ( Coan 2002), a comparatively wider escutcheon, principally on the left valve, and their posterior dorsal margin is concave.

Ihering (1907) included T. lyoni as a junior synonym of Corbula pulchella Philippi, 1893 and this decision has been followed by Huber (2010). After exhaustive searches of many museums, I was unable to find the holotype of C. pulchella to confirm the synonymy. Philippi’s description for C. pulchella is succinct and the figure is uninformative.

Figures

FIGURE 13. Tenuicorbula lyoni (Pilsbry, 1897). A–D. ANSP 70538, lectotype designated herein. A. external view of left valve. B. internal view of left valve. C. external view of right valve. D. internal view of right valve. E–J. CIB 1104. E. external view of left valve. F. internal view of left valve. G. external view of right valve. H. internal view of right valve. I. hinge of left valve. J. hinge of right valve. K. CIB 1102, dorsal view. Scale bars:A–H= 3mm; I–J= 1mm; K= 2mm. Legends: c= chondrophore; cs= cardinal socket; ct= cardinal tooth; k= knob; ke= keel; ps= pallial sinus; rs= resilial socket.