Corbula patagonica d'Orbigny 1846

Corbula patagonica d’Orbigny, 1846 (in 1843–1847)

Figure 3

Corbula patagonica d’Orbigny, 1846 (in 1843–1847): 570; pl. LXXXII, fig. 18–20. Lamy, 1941: 217. Figueiras & Sicardi, 1970: 410; pl. V, fig. 76.

Corbula (Corbula) patagonica. Rios, 1985: 269; pl. 94, fig. 1329. Rios, 1994: 291; pl. 99, fig. 1422. Rios, 2009: 585. Aguirre, 1994: 366; pl. I, fig. 4. Aguirre & Whatley, 1995: 314–316; figs. BB–HH.

Aloidis patagonica. Carcelles, 1944: 291; pl. VIII, fig. 103.

Corbula uruguayensis Marshall, 1928: 5; pl. 4, fig. 7–9.

Type material examined. Corbula patagonica d’Orbigny, 1846 (in 1843–1847). BMNH 1854.12.4.753/1, lectotype designated by Aguirre (1994, p. 366, pl. I, fig. 4) (validity of designation confirmed herein), one pair, 10.5 mm length, 8.5 mm height, 6.5 mm width (Fig. 3 A–D); BMNH 1854.12.4.753/2–5, paralectotypes, two closed pair, one left valve and one right valve; San-Blas Bay, Patagonia, Argentina.

Corbula uruguayensis Marshall, 1928. USNM 368243, holotype (original designation), closed pair, 11 mm length, 7 mm height, 5 mm width (Fig. 3 E–F); Cape Santa Maria, Rocha, Uruguay.

Additional material. See Appendix. Usually labeled as Corbula sp. or Corbula patagonica. Brazil between Guarapari, Espírito Santo State and Chuí, Rio Grande State; Uruguay between off Palmares de la Coronilla, Rocha to Rio de la Plata; Argentina between Mar de Plata to Puerto Belgrano, Buenos Aires.

Diagnosis. Shell trigonal to trigonal-elongate, thin to very thick, inflated with well-produced posterior rostrum in thick shells. Posterior slope narrow set off by a high and rounded keel in right valve and a high and sharp keel in left valve. Anterior and posterior dorsal margins slightly convex, ventrally inclined; anterior end low with margin broadly convex; posterior margin short, obliquely truncated to slightly convex; ventral margin broadly convex, sometimes flattened posteriorly. Left valve with a deep trigonal cardinal socket and broad, flattened and well projected chondrophore.

Nepioconch (= pre-accretion shell) well distinguished with rounded, low, and regularly spaced commarginal ribs, with rib base broader than or equal to interspaces. Mesoconch sculpture different in both shell valves.

Redescription. Shape. Adult shell small to moderate in size (length: 6–17 mm; height 3.7– 12 mm), trigonal to trigonal-elongate, heavy, inflated, subequilateral and inequivalve with well-developed posterior rostrum, more pointed in left valve than right, aligned with anteroposterior shell axis; right valve larger, higher and more inflated than left. Umbos prosogyrous with beaks at about 36%–56% of shell length from anterior end. Escutcheon flattened, 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 and posterior dorsal margins straight to slightly convex, ventrally inclined; anterior end low with margin broadly convex; posterior margin short, obliquely truncated to slightly convex in the right valve, convex in the left; viewed from its inner surface, posterior end frequently extended by a lateral siphonal plate; ventral margin broadly convex, sometimes posteriorly flattened.

Posterior slope narrow set off by a high and rounded keel in right valve and a high and sharp keel in left valve; keel dies out before reaching the junction of posterior end of ventral margin; posterior slope forming a strong obtuse to almost straight angle with to the central slope.

Ornamentation. External shell surface white with red and deciduous periostracum covering mesoconch and forming overlapping foliations; color of periostracum lost in worn and dry shells. Mesoconch sculpture of right valve constituted by high, rounded, and commarginal ribs, regularly to irregularly spaced with bases larger than interspaces; secondary and slender commarginal ribs present near posterior-ventral margin. Mesoconch sculpture of left valve constituted by fine and irregularly spaced commarginal ribs, with bases narrower than interspaces; some large specimens with few and slender radial elevations on left valve. Inner surface smooth and white.

Hinge. Hinge axis parallel to antero-posterior shell axis on both valves. Right valve with cardinal tooth below the beak and a resilial socket moderately sunken under umbo; right tooth pyramidal, stout, with apex curved dorsally, equilateral-triangle-shaped when viewed laterally. Left valve with a deep, trigonal cardinal socket below beak, with lateral walls slightly wrapping around its opening and a chondrophore projecting perpendicular to the sagittal plane. Chondrophore broad, flattened and divided into anterior and posterior areas by a radial ridge; posterior margin of chondrophore with a stout, rounded, tooth-like knob. Lateral area of the hinge plate parallel to dorsal margins, thickened in large specimens, especially in right valve. Trough on right valve for reception of left valve continuous with hinge plate, well developed near internal dorsal margin of the shell, and extending around all the internal margins of the valve.

Muscle scars. Adductor muscle scars well impressed, nearly perpendicular to the anterior-posterior shell axis in the right valve and oblique in the left valve; pear shaped anterior adductor scar; rounded posterior adductor scar. Elongated anterior pedal retractor muscle scar and rounded posterior retractor muscle scar; pedal retractor scars joining adductor scars. Pallial sinus shallow or not invaginated forming an almost straight line parallel to dorsoventral axis, sometimes with a rounded scar at the confluence of the pallial line and intersecting the antero-ventral point of the posterior muscle scar.

Nepioconch and pre-accretion shell. Nepioconch well distinguished, separated from mesoconch by a growth step, conspicuous especially in left valve; sculpture of rounded, low, and regularly spaced commarginal ribs, with bases larger than or equal to interspaces (both valves). Specimens in pre-accretion stage comparatively more elongate, more equivalve and thinner than specimens in post-accretion stage, with a truncated rostrum and keel delimiting posterior slope, reaching the ventral margin; sculpture as in nepioconch with small radially arranged pustules, principally in very thin shells, apparently absent in the nepioconch of the thick-shelled specimens, probably eroded; rectangular chondrophore, less projecting compared to specimens in post-accretion stage; knob less conspicuous.

Distribution. Analyzed specimens are from the coast of the municipal region of Guarapari, in the state of Espírito Santo, Brazil to Monte Hermoso, Buenos Aires Province, Argentina, from depths of 3 to 150 m.

Corbula patagonica is extremely abundant along the coast of Uruguay, and specimens attached to each other by long byssus threads have been collected (F. Scarabino, personal communication). The southernmost record of this species is Golfo Nuevo, Chubut Province, Argentina (Figueiras & Sicardi 1970; Aguirre 1994; Rios 2009).

Remarks. The shell of C. patagonica changes from nearly equivalve to inequivalve during growth and becomes more trigonal. Specimens in pre-accretion stage can be confused with specimens in pre-accretion stage of Caryocorbula chittyana and post-accretion stage of Caryocorbula lavalleana. However, the pre-accretion shell of C. patagonica has a rectangular chondrophore with greater projection from the hinge (Fig. 3 M–N); in C. lavalleana and C. chittyana the chondrophore is trigonal and slightly projecting.

Aguirre (1994) and Aguirre & Whately (1995) consider Corbula sulcata and Corbula stolata (Iredale, 1930) from Australia as synonyms of C. patagonica. However, thick-shelled specimens of C. sulcata possess a posterior lateral tooth in the right valve and a chondrophore in the left valve with a resilifer-like pit below the hinge plate (see figures of Vokes 1945, plate 1 fig. 1–5).

Specimens in post-accretion stage of Corbula patagonica is similar to Corbula tunicata Hinds, 1843 (= N. vicaria Iredale, 1930), Corbula hydropica (Iredale, 1930) and Corbula stolata (Iredale, 1930), and a comparative study between these species is required. Corbula hydropica has a sculpture with radial lines (Iredale 1930); two specimens of C. patagonica analyzed in this study from Bacia de Campos (oil exploitation site), in the state of Rio de Janeiro (Brazil), also show this sculpture (a small number of radial, two or three lines on the left valve) (Fig. 3I).

Carcelles (1944) considered C. uruguayensis Marshall, 1928, to be a synonym of C. swiftiana C.B. Adams, 1852, and his opinion was accepted by Rios (1994; 2009) and Rosenberg (2009) but Huber (2010) considered this species as valid. After examining the holotype of C. uruguayensis I consider this species to be synonymous of C. patagonica. The holotype of C. uruguaensis is larger than the lectotype of C. patagonica, yet, it is a pre-accretion shell, while the smaller lectotype of C. patagonica is a specimen in post-accretion stage. It seems that there is variability in the shell size for the beginning of the accretion and thickening process for C. patagonica.