Rheodreissena guru n. sp. from the Orinoco River (Bivalvia: Myida: Dreissenidae)

Abstract

The study of recently collected Dreissenidae specimens from the rocky substrate of the main channel of the Orinoco River, on the Colombia–Venezuela border, has allowed for the description of Rheodreissena guru n. sp. This species has biphasic brooding (ctenidial and pallial), with larvae passing through prodissoconch 1 and 2 stages to juvenile dissoconchs inside the pallial cavity, before being released into the environment. This reproductive strategy is characteristic of Rheodreissena Geda et al., 2018, confirming that the new species belongs to this genus. The recently described Rheodreissena is endemic to inland, clear, fast-flowing, freshwater river systems of northern South America. Rheodreissena guru has a triangular shell, with a dorsal wing; about two-thirds of the posterior ventral region is convex, while roughly one-third of the anterior margin is concave near a deep byssal notch. The pseudolunular fields are relatively long, obliquely forward-sloping, and with the byssus slightly downward directed. This morphology allows the species to position itself obliquely in relation to the substrate, adhering only the anterior-ventral third of the shell. Shell mesosculpture is absent on the wing, but when present, it forms granular spots below the wing or radial and zigzag bars on the carina. Anatomical features of the new species are similar to other Rheodreissena species, except for the more elongate stomach and sorting areas SA3 and SOA, which contain more folds. Differences among Rheodreissena species are presented in a table, including shell and branchi morphology, as well as reproductive and environmental characteristics.