Integrative taxonomy and biogeographic affinities of the first freshwater sponge and mollusc association discovered in tropical Asia

Abstract

Diverse associations of freshwater sponges with molluscs were recently described from the Xingu River, Amazon Basin. However, such associations in other parts of the world are almost unknown. Here, we report on the discovery of epifaunal associations of a freshwater sponge (Corvospongilla, Spongillidae) with a byssus-attaching clam (Limnoperna, Mytilidae) and a freshwater mussel (Hyriopsis, Unionidae) in tropical Asia (Mun River, Thailand). We used this association as a model system to show how the application of an integrative taxonomic approach can change modern views on the taxonomic status and biogeographic affinities of tropical invertebrates in a rather small sample. The freshwater sponge from the Mun River morphologically corresponds to the nominal taxon Corvospongilla siamensis but it is identical to Corvospongilla ultima from the Kaladan River, western Myanmar, based on the sequences of five DNA markers and the PTP species delimitation modeling. Hence, the first species is considered here to be a junior synonym of C. ultima. Conversely, the Limnoperna clam from the Mun River is found to be distant phylogenetically from the invasive golden mussel Limnoperna fortunei based on the sequences of three DNA markers. We therefore restore Limnoperna siamensis stat. rev. as a valid species. Our time-calibrated multi-locus phylogeny reveals that the split between L. siamensis and L. fortunei most likely occurred at the Miocene—Pliocene boundary. Finally, the freshwater mussel Hyriopsis khoratensis is an abundant species endemic to the Mun River. Our novel data highlight that associations of freshwater sponges with bivalves are present in Asia but were historically overlooked there and that the Mekong's sponge-mollusc association can be considered analogous to those discovered in South America.