SIZE-SELECTIVE PREDATION ON ZEBRA MUSSELS (DREISSENA-POLYMORPHA) BY CRAYFISH (ORCONECTES-PROPINQUUS)

Zebra mussels (Dreissena polymorpha) are now a common component of benthic communities in the lower Great Lakes, but information regarding North American predators remains scant. I measured predation by crayfish (Orconectes propinquus) on different size classes of zebra mussels in aquarium experiments. Crayfish showed a strong preference for small (3-5 mm) mussels, although larger individuals (8-10 mm, 12-14 mm) were also eaten when presented simultaneously. Feeding rates on different mussel size classes were affected by crayfish size and sex, although results varied between experiments. Survival of zebra mussels was enhanced between 3 and 30% when crayfish were offered alternative macrophyte foods (Potamogeton, Vallisneria); however, size selective predation patterns on mussels remained virtually unchanged in the presence of alternative foods. Time-lapse videographic studies revealed that both rejection rate and handling time increased with mussel size. Despite high predation rates on 'small' mussels, crayfish attacked medium and large mussels more often than small individuals. Results of this study indicate that predation by O. propinquus on D. polymorpha will be limited primarily to small- and medium-sized mussels.