Evidence of phenotypic plasticity in the response of unionid mussels to turbidity

Abstract

Abstract Increases in total suspended solids (TSS) reduce feeding and reproductive success of unionid mussels, but mussels in turbid rivers are less affected than those in clear rivers, probably due to differences in gill and palp morphology. This study was designed to determine whether the differences observed between adult mussels in populations from turbid versus clear rivers are due to phenotypic plasticity. Parasitic larvae (glochidia) of Lampsilis siliquoidea (Fatmucket) obtained from a low turbidity river (TSS < 5 mg/L) were transformed on Ambloplites rupestris (Rock Bass) in the laboratory to obtain juvenile mussels for rearing under clear and turbid conditions in the laboratory (i.e. nominally 0 versus 50 mg/L suspended river sediment). Juveniles obtained were reared under these contrasting conditions until age 3–4 weeks, when they were examined in a feeding experiment under a range of TSS concentrations (0, 5, 10, 15, 20, 25 mg/L). The clearance rate (volume cleared of particles per unit time) of algae by juveniles from both rearing conditions was similar for the no‐TSS control and declined with increased TSS. The rate of decline was, however, lower in the group reared under turbid conditions, which is consistent with reports for adult mussels. Our results indicate that differences in the clearance rate response to high TSS observed in mussels in clear and turbid rivers are probably driven in part by phenotypic plasticity. This finding provides a mechanism to explain how freshwater mussels thrive under turbid conditions as well as informs conservation efforts involving reintroduction of mussels in this highly imperilled taxon.