Epizoic invertebrate communities on upper Mississippi River unionid bivalves

Based on the Great Lakes experience with Dreissena polymorpha, it is clear that the native freshwater mussel (family Unionidae) fauna will be one of the riverine communities most severely impacted if high zebra mussel densities occur. Negative impacts on unionids will also affect the epizoic invertebrate communities that live on the shells of the unionids. In 1991 and 1992 (prezebra mussel infestation) we determined the composition and density of epizoic invertebrates on individual unionids from the upper Mississippi River. The 1991 samples were from a location with a strong current; the numerically dominant epizoic invertebrates at this location were three species of hydropsychid caddisflies and the chironomid larvae: Polypedilum convictum, P. scalaenum group, Rheotanytarsus sp., Microtendipes pedellus group and Thienemannimyia group. Strong correlations existed between effective surface area (ESA) (amount of shell surface area above the sediment-water interface) of the individual unionids and: (1) number of epizoic chironomid larvae (r = 0.81); (2) number of hydropsychid caddisflies (r = 0.73), and (3) total number of epizoic invertebrates (r = 0.78). Sampling in a slower current area in 1992 revealed an epizoic composition dissimilar from that observed in 1991. Glyptotendipes nr. lobiferus was the most abundant epizoic larval chironomid, and, although caddisfly larvae were common, they mostly belonged to families other than the Hydropsychidae. Again, strong correlations existed between ESA of the unionids and: (1) total number of epizoic invertebrates (r = 0.64) and (2) number of epizoic chironomid larvae (r = 0.57). A strong correlation also existed between the size of the unionids and the number of epizoic taxa present (1991: r = 0.78; 1992: r = 0.77). Fifty-three invertebrate taxa were collected from the unionids; mean epizoic densities were ca. 9600 and 6400 invertebrates/m(2) of ESA in 1991 and 1992, respectively. These results indicate that unionids are important substrates for epibenthic invertebrates, especially in rivers in which other large clean substrates are in short supply.