Feeding interactions between native freshwater mussels (Bivalvia : Unionidae) and zebra mussels (Dreissena polymorpha) in the Ohio River

The effects of zebra mussel infestation on the feeding of native unionids i n the Ohio River were evaluated through gut contents and available food in the water column. In 1996, heavily infested Amblema plicata (Say, 1817) and Quadrula pustulosa (I. Lea, 1831) had significantly less (p < 0.01) organi c matter in their guts (1.4 and 0.6 mg ash-free dry weight [AFDW], respecti vely) than lightly infested specimens (4.6 and 1.8 mg AFDW, respective ly), and heavily infested Q. pustulosa had a significantly lower (p < 0.05) mea n algal cell number(1.8 x 10(4)) in the guts than lightly infested specimen s (3.9 x 10(5)). However, mean algal cell numbers in the guts of heavily in fested and lightly infested A. plicata (5.7 x 10(5) versus 9.1 x 10(5), res pectively) were not significantly different (p = 0.17). In 1997, significan t reductions (p < 0.05) in total algal cells and organic matter in gut samp les again occurred for heavily versus lightly infested individuals of both species. in addition, gut contents of individual A. plicata from one of two sites contained significantly less (p < 0.05) organic matter (0.92 versus 4.55 mg AFDW) and fewer algal cells (9.4 x 10(4) versus 2.3 x 10(5)) than t he combined gut contents of all zebra mussels (18-33 mm in length) attached to their shells. Gut analyses also revealed significant diet overlap betwe en native unionids and infesting zebra mussels. Water samples collected fro m just above the mussel beds in 1997 showed that algal densities and total suspended solids at the heavily infested site (> 360 zebra mussels/m(2)) we re reduced by more than 50%, when compared to samples collected from the su rface. Thus, competitive interactions or interference by zebra mussels like ly reduced the availability of algal and detrital food resources for consum ption by unionids.