Caryocorbula luteola

Caryocorbula luteola (Carpenter, 1864)

Figure 7 A–F.

Corbula luteola Carpenter, 1864: 611, 637 (1872 reprint: 97, 123). Abbott, 1974: 539. Coan, 2002: 84–85; figs. 34–35. Corbula (Corbulomya) luteola. Lamy, 1941: 240–242.

Aloidis (Caryocorbula) luteola. Hertlein & Strong, 1950: 239.

Juliacorbula luteola. Coan et al., 2000: 479; pl. 102.

Caryocorbula luteola. Coan & Valentich-Scott, 2012: 870-872; pl. 272.

For other synonymies see Coan (2002 p. 84–85).

Type material examined. Corbula luteola Carpenter, 1864 — USNM 14897, lectotype designated by Coan (2002, p. 84–85, figs. 34–35) (validity of designation confirmed herein); one pair 10.2 mm length; 6.8 mm height; 4.5 mm width; United States, Los Angeles County, San Pedro (Fig. 7 A–B). USNM 73457, paralectotypes, one right and one left valves; right valve, 6.8 x 4.2 mm; left valve 5.7 x 3.9 mm; United States, Los Angeles County, San Diego. USNM 73457, paralectotypes, 3 right and 3 left valves; United States, Los Angeles County, São Pedro.

Additional material. See Appendix. Labeled as Corbula contracta Say, 1822. Brazil in Cabo Orange, Amapá.

Diagnosis: Shell small, thin to thick, subquadrate and more equivalve in specimens in pre-accretion stage, trigonal-ovate and subequivalve in thick one and with a slight posterior rostrum aligned with antero-posterior shell axis. Posterior slope flattened, set off by a low and rounded keel extending from the umbos to the posterior ventral margin; excluding posterior slope, valves surface regularly and weakly convex. Exterior surface white or light brown mottled with brown or purple near the umbo and ventral margin. Sculpture similar in both valves, comprising regularly spaced, low and rounded commarginal ribs, which become broader near the ventral margin. Left valve with conspicuous and well projected chondrophore.

Redescription. Shape. Shell small (length: 5–10 mm; height: 4–6.8 mm), trigonal-ovate to subovate, moderately heavy and inflate, subequivalve, subequilateral to equilateral, with a slight posterior rostrum aligned with antero-posterior shell axis. Posterior slope moderately broad, flattened, set off by a low and rounded keel extending from the umbos to the posterior ventral margin; posterior slope almost perpendicular to central slope. Excluding posterior slope, valves surface regularly and weakly convex. Umbos prosogyrous with beaks at about 47%–50% of shell length from anterior end. Lunular area shallowly concave. Escutcheon elongate, narrow, slightly larger in the right valve, defined by low ribs.

Anterior dorsal margin long, slightly convex and continuous with broad and convex anterior margin; anterior margin below median antero-posterior axis; posterior dorsal margin straight, as long as anterior dorsal margin; posterior margin truncated, almost aligned with the dorsal-ventral axis of the shell; ventral margin convex over anterior two thirds, straight and inclined dorsally near the posterior margin. Lateral siphonal plate not observed.

Ornamentation. Exterior surface white or light brown mottled with brown or purple near the umbo and ventral margin. Sculpture similar in both valves, comprising regularly spaced, low and rounded commarginal ribs, which become broader near the ventral margin; commarginal ribs with bases about two times broader than intercostal spaces near the umbo, and three times broader than intercostal spaces near the ventral margin. Inner surface smooth, white to yellowish.

Hinge. Hinge axis parallel to antero-posterior shell axis. Right valve with a cardinal tooth aligned with beak and a resilial socket sunken under umbo; cardinal tooth pyramidal, stout with dorsally curved apex, isosceles-triangleshaped when viewed laterally. Left valve with deep cardinal trigonal socket aligned with beak, and broad chondrophore projecting almost perpendicular to the sagittal plane; socket deeply retracted below umbo. Chondrophore shallowly excavated and divide into anterior and posterior area by a radially placed, shallow, narrow groove; anterior region of groove almost rectangular; low and inconspicuous posterior knob. Trough on right valve for reception of left valve continuous with hinge plate, extending around dorsal, posterior and postero-ventral margins.

Muscle scars.Adductor muscle scars strongly impressed in the valve; anterior adductor muscle scar pear-shaped; posterior adductor scar rounded; plane tangential to anterior adductor muscle scar almost parallel to antero-posterior shell axis; plane tangential to posterior adductor muscle scar perpendicular to antero-posterior shell axis. Anterior and posterior pedal retractor muscle scar trigonal-elongate joining adductor scars. Pallial line far from valve margin, oblique in relation to antero-posterior shell axis (higher anteriorly). Pallial sinus not invaginated forming a straight line.

Pre-accretion shell. Specimens in pre-accretion stage subquadrate, more equivalve with posterior margin longer than thick shell and parallel to dorso-ventral shell axis. Hinge plate thin with inconspicuous tooth-like knob.

Distribution: Eastern Pacific: Los Angeles County, California, south to Bahía Magdalena, Pacific coast of Baja California Sur, México (Coan & Valentich-Scott 2012). Western Atlantic: this is the first record of C. luteola in the Western Atlantic; Caryocorbula luteola was found in two lots identified as Corbula contracta Say, 1822, in the collection of the Prof. Eliézer Carvalho Rios Oceanographic Museum, collected in 1968 and 1970 at Cape Orange, located in the National Park Cape Orange, a Brazilian National Park (IUCN) in the northern region of the state of Amapá. The specimens were collected from depths of 80 and 85m.

Remarks. As with C. marmorata, further studies are needed to confirm that the C. luteola from Western Atlantic is the same as the Eastern Pacific species. The species most closely related to C. luteola are C. marmorata and C. contracta Say, 1822. Caryocorbula luteola is distinguished from C. marmorata by the rostrum aligned with the antero-posterior shell axis and the broad projected chondrophore, while in Caryocorbula marmorata the rostrum is ventrally curved and the chondrophore is short. Furthermore, C. marmorata has a shallow depression aligned with the umbo-ventral axis and another slight concavity just anterior to the keel. Caryocorbula luteola has neither the central depression nor the concavity anterior to the keel.

Compared to Caryocorbula luteola, C. contracta has an oval-subquadrate outline and a conspicuously inequivalve, inequilateral and inflated shell with a straight ventral margin almost parallel to the antero-posterior axis of the shell. The shell of C. luteola is trigonal-ovate and less inflated, and the ventral margin is convex, over 2/3 anterior and straight and dorsally inclined near the posterior margin. The species can also be differentiated by the form of the right cardinal tooth. In C. luteola the right cardinal tooth is isosceles-triangle-shaped, while in C. contracta the right cardinal tooth has a right-triangle shape. I have analyzed the type specimen of C. contracta and consider it to be a valid species. However, I did not find this species in Brazil.