Morphological assessment of Rheodreissena (Bivalvia: Veneroida: Dreissenidae) with an updated diagnosis of the genus, descriptions of two new species, redescription of R. lopesi, and the first account of larval brooding in New World dreissenids

ABSTRACT. Rheodreissena Geda et al. 2018, a recently described genus of Dreissenidae, is represented by living species that inhabit inland freshwaters of Brazil and Venezuela. The type species of Rheodreissena is R. hoeblichi (Schütt 1991a) described from the lower Orinoco basin, Venezuela. Three species are added in this study: R. lopesi (Alvarenga & Ricci, 1989) from the Tocantins and Xingu basins; R. cordilineata n. sp. from the Madeira, Trombetas and Xingu basins, and R. xinguana n. sp. from the Tapajós and Xingu basins. Those three species are recorded from Volta Grande, the stretch of the Xingu River impacted by the Belo Monte dam complex. All Rheodreissena typically attach in shallow clusters to clean, rocky substrates associated with rapids in the main channels of large clear water rivers. Species of Rheodreissena are distinguished by aspects of shell morphology and soft anatomy that presumably reflect adaptations to a rheophilic lifestyle. In Xingu and Tocantins populations of Rheodreissena, larval broods were observed in the ctenidial (R. lopesi and R. xinguana) and pallial cavities of females (all three species). Examples of pallial broods included a few prodissoconch-1 (D-shaped) larvae in R. xinguana n. sp., 24 larvae/juveniles representing three different stages (prodissoconch 1 and 2, dissoconch) in R. cordilineata and up to 65 larvae/juveniles representing the same three stages in R. lopesi. In R. cordilineata and R. lopesi, the prodissoconch is attached to the parental mantle via a modified velum (adhesivelum), and dissoconchs attain lengths of 800 µm and 987 µm, respectively. For larvae brooded in the pallial cavity, a biphasic growth pattern was demonstrated statistically in R. lopesi and determined by observation in R. cordilineata. Juveniles are presumably released gradually into the environment at the crawl-away dissoconch stage. Biphasic brooding (ctenidial and pallial) was previously described for the European cave-dwelling dreissenid species Congeria kusceri; differences between brooding in Congeria and Rheodreissena are noted. Various morphological and lifehistory traits of species of Rheodreissena are summarized and compared to the other living species of Dreissenidae: Mytilopsis leucophaeata, M. sallei, Dreissena polymorpha, D. rostriformis (ex. bugensis), and the three nominal species of Congeria.