Occurrence of chironomid larvae (Paratanytarsus sp.) as commensals of dreissenid mussels (Dreissena polymorpha and D. bugensis)

Up to 38% of zebra mussels (Dreissena polymorpha) and 10% of quagga mussels (Dreissena bugensis) collected from the upper St. Lawrence River in July 1993 were invaded by larvae of the tanytarsine chironomid Paratanytarsus sp. Third- and fourth-instar larvae were found living in the mantle cavity around the gills, gonads, and siphonal tissues. The larvae were never observed feeding on these tissues, and no tissue damage was detected. Most frequently, a single Paratanytarsus sp. larva occurred in a mussel; otherwise, two to six larvae were found. Invaded mussels were significantly larger than co-occurring non-invaded mussels. No chironomid larvae were found in young-of-the-year dreissenids. This is the first case of a large-scale endosymbiotic association, apparently a form of inquiline commensalism, between chironomid larvae and dreissenid mussels. Paratanytarsus sp. larvae also occurred in unionid bivalves (Elliptio complanata, Lampsilis radiata, Anodonta cataracta), but at relatively lower frequencies.