Fritts, MW, AK Fritts, SA Carleton, and R Bringolf. 2012. Shifts in stable isotope signatures of freshwater mussel glochidia during attachment to host fish. In Review Journal of Moluscan Studies.
Freshwater mussels (Bivalvia: Unionacea) are characterized by complex lifecycles composed of multiple, distinct life-stages, including a larval stage referred to as a glochidium. The parasitic nature of larval freshwater mussels (i.e., the role that the fish hosts play in providing nutritional resources to the developing glochidia) is still uncertain. While previous work provided unique descriptions of developing glochidia while they were transforming on fish hosts, earlier studies have not explicitly documented the flow of nutrition to glochidia as they develop to the juvenile stage. Therefore, our objective was to evaluate the feasibility of using stable isotope analysis to quantitatively document the flow of nutrients between fish hosts and mussel glochidia. Glochidia were collected from nine adult plain pocketbook mussels (Lampsilis cardium) and used to inoculate largemouth bass (Micropterus salmoides) (n=27) that produced juvenile mussels for the trial. Adult mussel tissue samples, glochidia, transformed juvenile mussels, and fish gill tissues were analyzed for δ 15N and δ13C ratios. We used a linear mixing model to estimate the fraction of juvenile mussel tissue derived from the host fish’s tissue during attachment. Our analyses indicate a distinct shift in both C and N isotopic ratios from the glochidial stage to the juvenile stage during mussel development. Linear mixing model analysis indicated that 90.0% of the δ13C and 57.4% of the δ 15N in juvenile tissues was obtained from the host fish. This work provides new evidence that larval unionids are true parasites that derive nutrition from a host fish during their metamorphosis into the juvenile stage; however, future studies should investigate the relative nutritional contribution of host fishes to mussels that exhibit different glochidial morphologies and brooding periodicities. Additionally, the effects of diet and food quality on fractionation patterns in individual tissues in mussels and the relative contribution of assimilated DIC on the δC13 values of molluscan tissues should be considered.