DIMOCK, RV, Jr*; ALDRIDGE, DC; Wake Forest University, Winston-Salem, NC; University of Cambridge, UK: Role reversal in symbiosis: functional morphology of larval fish in a mussel’s gills

The life cycle of most freshwater mussels (Bivalvia: Unionidae) includes a larva, the glochidium, which must become parasitic on a fish host to complete its development. In a reversal of this conventional vertebrate host-invertebrate symbiont association, the European bitterling Rhodeus sericeus (Cyprinidae) undergoes fertilization and early development within the water tubes of the gills of several species of mussels. Female bitterling use an elongated ovipositor to place ova in the host mussel’s gills. Males subsequently release sperm into the inhalant siphon of the host, following which fertilization occurs in situ. We have employed video endoscopy to image the distribution and functional morphology of larval bitterling in the European mussel, Unio pictorum. Early embryos are located deep within the water tubes. The polarity of development results in larval fish being oriented head downward. Prominent lateral projections of the embryonic yolk sac become wedged into the tissue of the water tubes, facilitating retention of developing larvae. Mechanical stimulation can induce rapid flexion of fish tails projecting into the mussel’s suprabranchial cavity. Larval fish can occasionally be induced to leave the gills upon exposure to hypoxia. A cost of harboring fish to host mussels in respect to compromised irrigation of the gills and perhaps suspension feeding, although as yet undetermined, seems highly likely. Video imaging will accompany this oral presentation.