Juliacorbula bicarinata (G. B. Sowerby I, 1833)

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 39-41

Juliacorbula bicarinata (G. B. Sowerby I, 1833)

Figure 12

Corbula bicarinata G. B. Sowerby I, 1833: 35 ; Lamy, 1941: 128–129. Hertlein & Strong, 1950: 238.

Corbula (Caryocorbula) bicarinata . Keen, 1958: 208–209; fig. 523.

Corbula (Juliacorbula) bicarinata . Keen, 1971: 266–268; fig. 684. Coan, 2002: 64–66. Coan & Valentich-Scott, 2012: 1077; pl. 5.

For other synonymies see Coan (2002: 64).

Type material examined. Corbula bicarinata G. B. Sowerby I, 1833 . BMNH 1966567/1, lectotype designated by Coan (2002, p. 64–66) (validity of designation confirmed herein), closed pair, 10.6 mm length, 8.5 mm height, 6.6 mm width ( Fig. 12 A–C). BMNH 1966567/2–3, paralectotypes, 2 open pairs. Playa Kobbe, Panamá Province, Panamá. Type locality clarified by Coan (2002, p. 64).

Additional material. See Appendix. Usually labeled as Corbula caribaea or Corbula cubaniana . Mexico in Guaymas, Sonora. USA in Texas State. Brazil between Amapá State and Guarapari, Espírito Santo State.

Diagnosis. Shell small, subquadrate, quite equivalve with weak rostrum. Posterior slope with a slight radial groove and set off by a high and sharp keel that becomes somewhat rounded ventrally and dies out before reaching the junction of posterior and ventral margin. Valve surface irregularly convex with anterior slope convex and the central slope with a slight radial groove in median-ventral area. Sculpture similar in both valves comprising conspicuous regular and sharp commarginal ribs; base width of commarginal ribs larger than interspaces. Hinge axis oblique in relation to the antero-posterior shell axis in both valves. Right valve with a cardinal tooth pyramidal, stout with curved dorsally apex, isosceles-triangle-shaped when viewed laterally. Left valve with a short chondrophore projecting almost parallel to the posterior dorsal margin. Chondrophore shallowly excavated and divided into anterior and posterior trigonal area by radial ridge; posterior margin of chondrophore with an inconspicuous rounded tooth-like knob.

Redescription. Shape. Shell small (length: 4.5–10.6 mm; height: 3.4–8.5 mm), trigonal to subquadrate, heavy and inflated, quite equivalve, subequilateral to equilateral with weak rostrum. Posterior slope moderately broad, flat with a slight radial groove, set off by a high and sharp keel that becomes somewhat rounded ventrally and dies out before reaching the junction of posterior and ventral margin; posterior slope nearly perpendicular to central slope. Excluding posterior slope, valve surface irregularly convex with anterior slope convex and the central slope with a slight radial groove in median-ventral area; right valve slightly larger than left. Umbos prosogyrous with beaks at about 23%–50% of shell length from anterior end. Lunular area sunken. Escutcheon sublanceolate, larger in the right valve and delimited by a narrow and sharp ridge on both valves.

Anterior dorsal margin short straight to slightly concave, ventrally directed, continuous with a widely convex anterior margin, the latter situated below to the median longitudinal shell axis; posterior dorsal margin slightly convex to straight, slightly sloping ventrally; long and obliquely truncated posterior margin.

Ornamentation. Exterior surface withish with light brown periostracum. Sculpture similar in both valves with conspicuous regular and sharp commarginal ribs; base width of commarginal ribs larger than interspaces. Inner surface smooth, white or yellow.

Hinge. Hinge axis oblique in relation to the antero-posterior shell axis in both valves. Right valve with a cardinal tooth right below the beak and a resilial socket sunken below umbo; cardinal tooth pyramidal, stout with curved dorsally apex, isosceles-triangle-shaped when viewed laterally. Left valve with a deep, trigonal cardinal socket just posterior to beaks, and a short chondrophore projecting almost parallel to the posterior dorsal margin. Chondrophore shallowly excavated and divided into anterior and posterior trigonal area by radial ridge; posterior margin of chondrophore with an inconspicuous rounded knob. Trough on right valve for reception of left valve continuous with hinge plate, extending around internal anterior and ventral margins of the valve.

Muscle scars: Adductor muscle scars well-impressed; anterior adductor scar oval-elongate nearly perpendicular to the antero-posterior shell axis; posterior adductor muscle scar rounded, oblique to the antero-posterior shell axis and on top of well-elevated callosity. Anterior pedal retractor muscle scar elongated; posterior retractor muscle scar rounded; anterior and posterior pedal retractor scars joining adductor scars. Pallial line far from free border in both valves. Straight or rounded palial sinus, intersecting to antero-ventral point of the posterior muscle scar.

Pre-accretion shell: not observed.

Distribution. This is the first occurrence of this species in the Western Atlantic and Brazil. I found Juliacorbula bicarinata in lots collected from Texas (Gulf of Mexico, USA), from state of Amapá to the municipal region of Guarapari in the state of Espírito Santo ( Brazil) and from Abrolhos and Trindade Islands ( Brazil), from the intertidal zone to 70 m depth. The similar morphology occurs in the Eastern Pacific, from the Gulf of California, Sonora, México to Zorritos, Tumbes, Perú; Santa Maria Island, Galapagos Islands; intertidal zone to 110 m depth ( Coan & Valentich-Scott 2012).

Remarks. Like Caryocorbula marmorata and C. luteola , Juliacorbula bicarinata also occurs in the Eastern Pacific, and additional studies are needed to confirm if populations from Eastern Pacific and from Western Atlantic belong to the same species. Western Atlantic J. bicarinata show beaks located at 23%–50% from the anterior end whereas in Eastern Pacific specimens the beak is located between 38%–40% from the anterior end ( Coan 2002). Also, specimens from Brazil are rather variable in shape, from trigonal to square outlines. Thicker shells show more prominent commarginal ornamentation than thinner ones.

Juliacorbula bicarinata is similar to J. aequivalvis . The former is distinguished from the latter by its subquadrate outline and by a shell with a radial groove in the median-ventral area of the central slope. Juliacorbula aequivalvis has a trigonal to subtrapezoidal outline and a regularly convex shell surface, without a radial groove in the central slope. Comparing specimens of the same size, J. bicarinata has broader and higher commarginal ribs. These species can also be differentiated by the position of the hinge plate and the shape of the chondrophore. In J. bicarinata the hinge plate is oblique in relation to the antero-posterior shell axis, the chondrophore has an inconspicuous to small knob and is short and shallowly excavated, with the dorsal face divided into two trigonal areas by a low, slender ridge; in J. aequivalvis the hinge plate is almost parallel to the antero-posterior shell axis and the chondrophore is more projected from the free border of the hinge plate with the dorsal face divided into two areas by a high ridge, and the anterior area deeply excavated. Viewed laterally, the right cardinal tooth is right-triangle shaped in J. aequivalvis and isosceles-triangle shaped in J. bicarinata .

Figures

FIGURE 12. Juliacorbula bicarinata (G. B. Sowerby I, 1833). A–C. lectotype BMNH 1966567/1 (Coan 2002). A. external view of left valve. B. internal view of left valve. C. external view of right valve. D–G. José Carlos Tarasconi Private Collection, Guarapari–SC. D. external view of left valve. E. internal view of left valve. F. external view of right valve. G. internal view of right valve. H–N Adolph Birman Private Collection, Nova Almeida–ES. H. external view of left valve. I. internal view of left valve. J. external view of right valve. K. internal view of right valve. L. dorsal view. M. hinge of left valve. N. hinge of right valve. Scale bars:A–C, H–K = 3mm; D–G, L = 2mm; M–N= 1mm. Legends: c= chondrophore; cs= cardinal socket; ct= cardinal tooth; k= knob; ke= keel; ps= pallial sinus; rs= resilial socket.