Varicorbula, Grant & Gales, 1931

Arruda, Eliane P., 2020, Taxonomic revision of the recent marine Corbulidae (Mollusca, Bivalvia) from Brazil, Zootaxa 4851 (1), pp. 1-59 : 44-45

Genus VARICORBULA Grant & Gales, 1931

Varicorbula Grant & Gales, 1931 : type species Corbula gibba ( Olivi, 1792) by original designation; Recent, Western Europe and Mediterranean Sea. For synonymies see Mikkelsen & Bieler (2001) and Vokes (1945).

Diagnosis. Shell oval to trigonal, thin to thick, inequivalve with right valve larger, higher and more inflated, rostrate posteriorly, with internal margins grooved (like a trough) for the reception of the left valve; left valve smaller, flatter and less rostrate than right valve. Umbo prosogyrous, higher and more inflated in right valve. Nepioconch not distinguished. Sculpture of right valve with pronounced commarginal ribs; left valve with fine commarginal striae or ribs, often crossed by faint, narrow and widely spaced radial ribs. Umbonal region with less pronounced commarginal ribs.

Remarks: Varicorbula species are more inequivalve than Corbula species, less rostrate and without a distinguished nepioconch. In general, the left valve of the Varicorbula species is less rostrate than in the other Corbulidae species. Stenzel et al. (1957), Weisbord (1964), Jung (1969), Warmke & Abbott (1961), Rios (1975; 1985; 1994) and Morris (1973) consider Varicorbula and Notocorbula to be synonyms based on the presence of a “subdued” nepioconch in the type-species of Varicorbula , V. gibba . According to Mikkelsen & Bieler (2001), V. gibba occasionally has irregular growth stoppages that could be interpreted as nepioconch, but their presence is sometimes only evident on the larger right valve and the sculpture is the same on the remainder of the valve.

According to Anderson & Roopnarine (2003), Varicorbula is divided into two sister lineages, the Old-World lineage, European species represented in the study by V. gibba , and the New World lineage, American species represented by V. limatula ( Conrad, 1846) . Based on this, Anderson et al. (2010) and Hallan et al. (2013) considered the New World species of Varicorbula to belong to Vokesula Stenzel & Twining in Stenzel et al. (1957) , whose type species is Vokesula smithivillensis Harris, 1895 , from the Eocene to Oligocene of North America. Nevertheless, the characteristics described by Stenzel & Twining for Vokesula do not differ from those of the genus Varicorbula , except for the left valve nearly smooth with growth wrinkles near to the ventral margin and with some fine concentric ribs on the umbonal region ( Stenzel et al. 1957). However, the figures of Vokesula smithvillensis presented by Stenzel & Twining in Stenzel et al. (1957, pl. 21) demonstrate that this species has a left valve with well-defined commarginal ribs crossed by radial striae in the umbonal region. Anderson et al. (2010) and Hallan et al. (2013), meanwhile, do not mention other characteristics that could help distinguishing these two genera. Mikkelsen & Bieler (2001) also recognize that the two genera are similar and that further studies are needed to clarify their relative status. Presently, I consider the following species to be part of the Varicorbula genus.