Corbula tryoni Smith, 1880

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 19-21

Corbula tryoni Smith, 1880

Figure 4

Corbula tryoni Smith, 1880: 321 . Figueiras, 1973: 299–302.

Corbula (Corbula) tryoni . Rios, 1975: 251, pl. 80; fig. 1203. Rios, 1985: 269; pl. 94, fig. 1330. Rios, 1994: 291; pl. 99, fig. 1423. Rios, 2009: 585.

Type material examined. Corbula tryoni Smith, 1880 . BMNH 1879.10.15.165-170/1, lectotype designated herein, probably the specimen measured by Smith (1880), one pair, 6.2 mm length, 5.2 mm height, 4.0 mm width ( Fig. 4 A–D); BMNH 1879.10.15.165-170/2-6, 5 closed pair; East of Uruguay, 32 o 45’S 50 o 39’W, 87.78 m depth.

Note that the syntype lot has six specimens, but Smith (1880) did not figure any of them, he did not mention specimen numbers and only provided the measurement of one specimen .

Additional material. See Appendix. Usually labeled as Corbula sp. or Corbula tryoni Smith, 1880 . Brazil between São Francisco de Itabapoana, Rio de Janeiro State and Chuí, Rio Grande do Sul State; Uruguay between Punta del Diablo, Rocha Departament and Maldonado; Argentina in Puerto Madry.

Diagnosis. Shell small, thin to very thick, oval to subquadrate in specimens in pre-accretion stage, oval to subtrigonal in thick one, inflate, equilateral, with moderately developed truncated posterior rostrum. Posterior slope narrow set off by a sharp keel in the left valve and rounded in the right that dies out before reaching the ventral margin. Anterior and posterior dorsal margins slightly convex and ventrally inclined with same length; anterior end low with margin broadly convex; posterior margin long, truncated, parallel to the dorsal-ventral axis; sometimes posterior margin slightly convex in the right valve; ventral margin broadly convex in the right valve and weakly convex in the left.

Nepioconch well distinguished with regular, flattened, and low commarginal ribs crossed by radial striae. Mesoconch sculpture different in both shell valves. Left valve with narrow chondrophore projecting almost perpendicular to the sagittal plane; resilium-like pit generally enlarged and attached to lower surface of chondrophore in very tick shells.

Redescription. Shape. Shell small (length: 3.8–8.8 mm; height: 2.7–6.5 mm) subtrigonal to oval, heavy, inflated, equilateral, inequivalve, with moderately developed truncated posterior rostrum, aligned with the anteroposterior shell axis. Posterior slope narrow, set off by a sharp keel in the left valve and rounded in the right that dies out before reaching the ventral margin; plane tangential to posterior slope in obtuse angle to plane tangential to central slope. Umbos prosogyrous with beaks at about 33%–48% of shell length from anterior end. Lunule area depressed. 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 slightly convex and ventrally inclined with same length; anterior end low with margin broadly convex; posterior margin long, truncated, parallel to the dorsal-ventral axis; sometimes posterior margin slightly convex in the right valve; posterior end frequently extended by the lateral siphonal plate in the left valves; ventral margin broadly convex in the right valve and weakly convex in the left.

Ornamentation. External surface white with deciduous brown periostracum covering the mesoconch and forming overlapping foliations, mainly on the left valve. Mesoconch sculpture of right valve constituted by high and rounded commarginal ribs, regularly spaced with bases larger than interspaces. Mesoconch sculpture of left valve constituted by fine and irregular commarginal ribs, narrower than interspaces. Internal surface smooth and white.

Hinge. Hinge axis parallel to antero-posterior shell axis in both valves. Right valve with cardinal tooth below beaks, and a resilial socket sunken under umbo; right cardinal tooth pyramidal, stout, with apex curved dorsally, equilateral-triangle-shaped when viewed laterally. Left valve with a deep trigonal socket with lateral walls wrapping around its opening, and a narrow chondrophore projecting almost perpendicular to the sagittal plane. Dorsal face of chondrophore rectangular, flattened with free ventral margin straight when viewed dorsally and with a posterior margin with a stout, rounded tooth-like knob; resilium-like pit generally enlarged and attached to lower surface of chondrophore. Trough on right valve for reception of left valve continuous with the hinge plate, extending around all 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; anterior adductor scar ovate-elongate; posterior adductor rounded. Anterior pedal retractor muscle scar elongate; posterior retractor muscle scar rounded; anterior and posterior pedal retractor scars joining adductor scars. Pallial line far from valve margin, convex, parallel to ventral margin. Pallial sinus shallow, weakly concave in the right valve and strongly concave in the left one, extending beyond anterior edge of posterior adductor muscle scar.

Nepioconch and pre-accretion shell. Nepioconch sculpture constituted by regular, flattened and low commarginal ribs crossed by radial striae; base of commarginal ribs narrower than intercostals spaces. Specimens in pre-accretion stage oval to subquadrate, more equivalve than thick shell and with more conspicuous keel; plane tangential to posterior slope perpendicular to plane tangential to central slope; sculpture as in nepioconch, sometimes with most conspicuous radial striation; hinge plate thin with less conspicuous tooth-like knob; resilium-like pit absent.

Distribution. The specimens of C. tryoni analyzed here were collected from the state of Rio de Janeiro, Brazil to Puerto Madryn, Chubut Province, Argentina, from depths of 8 to 160 m. The specimens have been abundantly collected in the states of Rio de Janeiro and São Paulo. However, Rios (1975; 1985; 1994; 2009) registered C. tryoni from the northeast of Brazil to Uruguay, and the same author (1975) stated that the species was recorded off the states of Pernambuco and Sergipe (northeast of Brazil). I have analyzed lots with eroded shells collected in 1968 and 1970 from the states of Pará and Amapá (North of Brazil) (MORG 14835, MORG 15774, MORG 50795) that may be C. tryoni , but because of the poor preservation of the shells, it is not possible to be certain. Other than these records, I did not find any other lots of C. tryoni from the north and northeast of Brazil.

Remarks. Figueiras & Sicardi (1970) considered C. tryoni to be a synonym of C. patagonica . Comparing specimens of similar size, C. tryoni is distinguished from C. patagonica by the truncated and longer posterior margin parallel to the dorsal-ventral axis, and by the narrower and less projecting chondrophore. The posterior margin of C. patagonica is oblique and short in relation to shell height; its chondrophore is broad and projects significantly into the shell interior. Corbula tryoni can be also differentiated by the sculpture of the nepioconch which is constituted by regular, flattened, and low commarginal ribs crossed by radial striae. In C. patagonica , the nepioconch has rounded, low and regularly spaced ribs, with larger bases than interspaces and without radial striae.

Figures

FIGURE 4. Corbula tryoni Smith, 1880. A–D. lectotype designated herein BMNH 1879.10.15.165-170/1. 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–K. CIB 1129. 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. external view of left valve, closed shell. J–K. left valve hinge of shells with different thickening; note the resilium-like pit attached to lower surface of chondrophore in 2K. L–M. CIB 1127 specimen in pre-accretion stage. L. external view of left valve. M. external view of right valve. N. CIB 1129, dorsal view. Scale bars: A–I, N= 2 mm; J–M = 1mm. Legend: c= chondrophore. cs= cardinal socket. ct= cardinal tooth; k= knob; ke= keel; lsp= lateral siphonal plate; n= nepioconch; p= resilial-like pit; pl= pallial line; ps= pallial sinus; rs= resilial socket.