Impact of Zebra Mussels (Dreissena polymorpha) on Fingernail Clams (Sphaeriidae) in Extreme Southern Lake Michigan

The demography of the zebra mussel (Dreissena polymorpha) and its impacts on native fingernail clams (Sphaeriidae) in Lake Michigan near Michigan City, Indiana were studied from 1992 to 1997 at 5, 10, and 15 m depths. Zebra mussel densities ranged from 0 to 6,209/m², and were greatest at deeper stations. Size-frequency distributions suggest that initial colonization of the Michigan City area occurred in 1991, with adults typically living for 2 to 3 years. Abundance of adult populations may be limited by the sandy habitat typical of the areas studied and the occasional high-energy wave action in the shallow (5 m) near-shore zone of this area. Densities of fingernail clam ranged from 0 to 1,312/m² and were also greatest at deeper stations. Approximately half of the fingernail clams shells between 1 and 2 mm in size were used as substrate for attachment by juvenile and adult zebra mussels, while over 90% of the clams > 2 mm showed attachment by zebra mussels. Overall median densities of Sphaeriidae decreased from 832/m² to 13/m² at the 15 m depth and from 234/m² to 0/m² at the 10 m depth during the study. It appears that zebra mussel colonization caused a dramatic reduction of Sphaeriidae density by 1997 that may eventually result in their loss from the area.