The role of substratum stability in determining zebra mussel load on unionids

Data reported herein do not support the existence of preference for or attraction to unionids by settling or migrating zebra mussels compared to alternative hard substrata. Despite claims and inferences in the Dreissena literature suggesting that unionids are preferred substrata, higher Dreissena loads on unionids compared to alternative hard substrata can be explained by mechanisms other than preference. Our data indicate that substratum conditions are often critical in determining the relative zebra mussel loads that accrue on unionids. On stable and relatively hard lake/river bottoms, zebra mussel loads on unionids tend to be similar to those on other hard substrata. However, on bottoms mostly composed of very soft or unstable substrata, discreet hard objects become silted-over and/or buried, hence sub-optimal for zebra mussels. Under such conditions, unionids develop a higher load of zebra mussels due to their ability to maintain position in relation to the sediment/water interface. We conclude that high Dreissena loads on unionids relative to other substrata are not a matter of preference for or attraction to the unionids, but are the outcome of differential survival/emigration of the Dreissena due to unstable or changing bottom conditions.