Stress-induced variation in host susceptibility to parasitic freshwater mussel larvae

An increasing number of studies demonstrate the critical role of the host-parasite relationship for the persistence and distribution of freshwater mussels. Laboratory experiments are a powerful tool for quantifying the physiological compatibility between parasitic mussel larvae and fish hosts and are clearly applicable to species conservation. Recent findings, however, indicate potential need to control for biases caused by infection intensity and host stress responses. We tested glochidia metamorphosis success and host plasma cortisol response using Lampsilis siliquoidea glochidia on Lepomis macrochirus. The main aims were to (1) compare metamorphosis success in response to infection intensity (number of attached glochidia per fish) and (2) compare metamorphosis success between typical and reduced-stress approaches to the handling and infection of host fish. We found no effect of the glochidia bath density used to infect the fish (1000-4000-8000 glochidia l(-1)) or the resulting infection intensity on metamorphosis success, although host plasma cortisol was correlated with infection intensity at 24 h post infection. Small but statistically significant differences in metamorphosis success were observed between the typical and reduced-stress approaches. Overall, typical host compatibility testing methods appear to be robust to these variables, but more emphasis on standardizing laboratory protocols may provide more repeatable data.