Associations of mayfly larvae with Corbicula clams

Currently, the parasitic and endosymbiontic fauna of Corbicula clams remain poorly studied throughout their range. Here, using samples from the Mekong basin, we describe two Symbiocloeon species new to science: Symbiocloeon corbiculinus sp. nov. and Symbiocloeon laoensis sp. nov. Our results and a review of available published data indicate that freshwater bivalve- associated mayflies are narrow host specialists, being associated with one or a few closely related species of freshwater bivalves. The bivalve-associated mayfly larvae have several specific morphological traits compared with free-living species, which include a reduction of integument chitinization and a significant reduction of the surface structures on segments. An increase in area of the respiratory surface of larval tergalia was also recorded. The possible positive and negative effects of mayflies on the clam hosts are discussed. This study is an example of the many possible hidden associations between aquatic species that remain to be described.