Impact of the zebra mussel, Dreissena polymorpha, on populations of Unionidae (Bivalvia) in Lake St. Clair

Abstract

Both density (number/m2) and species diversity of unionid populations in the southwestern region of Lake St. Clair have declined since the introduction of the epizoic zebra mussel (Dreissena polymorpha). The unionid population in the offshore waters of Puce, Ontario, was surveyed from 1990 to 1992. The density of living unionids decreased significantly [Formula: see text] from 1.75/m2in 1990 to 0.06 in 1991. The number of living species declined from 11 in 1990 to 4 in 1991. No living unionids were found at the Puce site in 1992. The decline in the unionid population corresponded to a significant increase in the number of attached zebra mussels. The average number of zebra mussels per unionid increased from 143 in 1989 to 642 in 1990. The unionid population in the offshore waters of Grosse Pointe Farms, Michigan, was surveyed in 1991 and 1992. The population declined from 1.98 unionids/m2 in 1991 to 1.36 in 1992, although this was only significant at p = 0.059. The average number of zebra mussels per unionid significantly increased from 9 in 1991 to 155 in 1992. Although thin, alate species of unionids appeared to decline sooner than species with more inflated and rounded shells, all species were affected, with an average of 64% of all dead unionid shells collected containing either living zebra mussels or their byssal threads.