Varicorbula philippii (E. A. Smith, 1885 )

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 45-47

Varicorbula philippii (E. A. Smith, 1885)

Figure 14

Corbula philippii E. A. Smith, 1885: 33 –34 ; Pl. VII, fig. 4–4b. [in text erronneously as Pl. VIII]

Varicorbula philippii . Mikkelsen & Bieler, 2001: 288; fig. 27. Mikkelsen & Bieler, 2007: 384. [See Mikkelsen & Bieler (2001, p. 288) for a more complete synonymy list].

Corbula (Varicorbula) operculata . Rios, 1975: 251; pl. 80 fig. 1205. Rios, 1985: 270; pl. 94, fig. 1334. Rios, 1994: 292; pl. 99, fig. 1427. Rios, 2009: 586.

Type material examined. Corbula philippii E. A. Smith, 1885 . BMNH 1967966/1, lectotype designated herein, closed pair, 6 mm length, 5.5 mm height, 3.8 mm width ( Fig. 14A ). BMNH 1967966/2, paralectotype, open pair, 5.8 mm length, 5.3 mm height, 3.8 mm width, Haiti ( Fig. 14 B–D). Smith (1885) did not say how many specimens were used to describe C. philippii , but the original information lists two localities: off Bermuda, collected during the Challenger Expedition and Haiti, the material of which comes from the Cuming Collection. The BMNH 1967966 lot (now NHM London), registered as the syntype lot, has two specimens from Haiti of Cuming Collection, one closed and one open specimen. The closed specimen has the measurements closest to those presented by Smith (1885). The specimens of lot 1887.2.9.2390 from the NHM London were collected off Bermuda are also syntypes of C. philippii (Natural History Museum Data Portal 2020), but this lot can not be analyzed.

Additional material. See Appendix. Labeled as Corbula operculata Philippi, 1848 , or Corbula sp. USA between Dustin Florida and Florida Keys. Brazil, between Amapá State and Ilha Rasa, Rio de Janeiro.

Diagnosis. Shell small, thin to thick, strongly inequivalve. Right valve larger, higher and more inflated, trigonal, strongly convex, with truncated posterior rostrum and posterior slope set off by a rounded to sharp keel. Left valve slightly convex, non rostrate and trigonally suboval with posterior slope flattened, set off by a rounded keel. Sculpture discrepant in the right and left valves; right valve with high and rounded commarginal ribs, wider than interspaces; left valve with fine commarginal lines crossed by narrow and widely spaced radial ribs that extend from umbonal region to ventral margin; radial ribs absent on left posterior slope.

Redescription. Shape. Adult shells small (length: 2–10 mm; height: 2.34– 10 mm), heavy, strongly inequivalves. Right valve larger, higher and more inflated, trigonal, strongly convex, with truncated posterior rostrum aligned with anteriorposterior shell axis; posterior slope set off by a rounded to sharp keel that dies out before reaching the junction of posterior and ventral margins; posterior slope forming an obtuse angle to the central slope; posterior slope with radial sulcus. Left valve slightly convex, non rostrate and trigonally suboval, with posterior slope flattened, set off by a rounded keel that dies out before reaching the junction of posterior and ventral margins; posterior slope forming an obtuse angle to the central slope. Umbos high, prosogyrous, with beaks 30%–47.5% of shell length from anterior end. Lunule area concave. Right escutcheon depressed, defined by a high and rounded ridge, larger than left one and sometimes with light red to brown patches; left escutcheon flattened, defined by a narrow and faint ridge.

Anterior and posterior dorsal margins straight and ventrally inclined; posterior dorsal margin longer than anterior; anterior margin low and broadly convex; posterior margin truncated, parallel to the dorsal-ventral shell axis; ventral margin broadly and regularly convex. Lateral siphonal plate rarely present; when present, only in the left valve.

Ornamentation. Exterior surface white with deciduous, light brown periostracum that forms overlapping foliations and overhanging shell margin in the left valve. Sculpture discrepant in the right and left valves; right valve with high and rounded commarginal ribs, wider than interspaces; left valve with fine commarginal lines crossed by narrow and widely spaced radial ribs that extend from umbonal region to ventral margin; radial ribs absent on left posterior slope. Inner surface smooth, white with brown patches.

Hinge. Hinge axis oblique in relation to the anterior-posterior shell axis in both valves (posteriorly elevated). Right valve with a cardinal tooth below the beak and a resilial socket sunken under umbo; cardinal tooth pyramidal with its apex not curved dorsally, isosceles-triangle-shaped when viewed laterally. Left valve with a deep, trigonal cardinal socket just posterior to beak with lateral walls wrapping around its opening, and a broad chondrophore projecting perpendicular to sagittal plane; dorsal face of chondrophore divided into anterior and posterior region by a conspicuous ridge, being the anterior region rectangular, excavated, and the posterior region with a stout, rounded tooth-like knob. Trough on right valve for reception of left valve extending around all margins of the valve.

Muscle scars. Adductor muscle scars slightly impressed, nearly perpendicular in relation to the anterior-posterior shell axis in the right valve, and in oblique angle in the left valve. Anterior adductor muscle scar oval-elongate; posterior adductor muscle scar rounded. Anterior and posterior pedal muscle scar fused dorsally with adductor muscle scars. Pallial line far from valve margin, convex, parallel to the ventral margin. Pallial sinus shallow, slightly convex.

Pre-accretion shell. Specimens in pre-accretion stage less inequivalve than post-accretion shell; right valve less inflated; left valve more flattened. Right hinge with a cardinal tooth without resilial socket in very thin shells that become thicker as grow older. Left valve with chondrophore and knob little thick that becomes conspicuous in thicker specimens.

Distribution. According Mikkelsen & Bieler (2007), V. philippii occurs from North Carolina to Florida, Bermuda, Caribbean Islands, Gulf of Mexico, Central America and South America. I have analyzed lots with specimens collected from Florida ( USA), Amapa State ( Brazil) to Ilha Rasa, Rio de Janeiro State, Brazil, from a depth of 20 m to 105 m.

Remarks. Mikkelsen & Bieler (2001) revised the Varicorbula species from the Western Atlantic and recognized three morphospecies based on conchological characters: Varicorbula limatula ( Conrad, 1846) (= Corbula disparilis d’Orbigny, 1853 ), Varicorbula krebsiana (C. B. Adams, 1852) and Varicorbula philippii (E. A. Smith, 1885) , and relegated the most common species name, Varicorbula operculata ( Philippi, 1848) as a nomen dubium. Rios (1975; 1985; 1994; 2009) considered the species V. operculata for the Brazilian littoral. Based on the analyses I have performed for various lots from different regions of the Brazilian littoral, including lots of Varicorbula deposited in the Eliézer C. Rios Oceanographic Museum institution where Professor Rios worked, I consider that V. philippii occurs in Brazil, as described by Mikkelsen & Bieler (2001). According to these authors, V. philippii is distinguished from V. limatula and V. krebsiana due to being more truncated, based on the ratio between posterior margin and height (height ratio), having stronger radial ribs on the left valve that are distributed from the umbonal region to the ventral margin, and by the presence of a thick periostracum. The presence of a high, rounded ridge delimiting the escutcheon on the right valve in V. phillippi is another difference that distinguishes this species from V. limatula . Varicorbula philippii and V. krebsiana are also differentiated by the commarginal ribs on the right valve, which are higher and more conspicuous on the valves of V. philippii .

Figures

FIGURE 14. Varicorbula philippii (E. A. Smith, 1885). A. BMNH 1967966/1, lectotype designated herein, external view of left valve from closed shell. B–D. paralectotype BMNH 1967966/2. B. internal view of left valve. C. external view of right valve. D. internal view of right valve. E–H. CMPHRM 2396. 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–L. MZSP 22851, specimen in pre-accretion stage. I. external view of left valve. J. internal view of left valve. K. external view of right valve. L. internal view of right valve. M–N. left valve hinge of shells with different thickening. M. MZSP 22851. N. CMPHRM 2396. O. Adolph Birman Collection, dorsal view. Scale bars: A–H, O= 2mm. I–N = 1mm. Legend: c= chondrophore. cs= cardinal socket. ct= cardinal tooth; k= knob; ke= keel; ps= pallial sinus; rs= resilial socket.