Lethal and sublethal effects of sponge overgrowth on introduced dreissenid mussels in the Great Lakes St Lawrence River system

Freshwater sponges in the Great Lakes - St. Lawrence River system overgrow and kill introduced zebra (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) on solid substrates. Sponges overgrow and smother mussel siphons, thereby interfering with normal feeding and respiration. We tested the significance of sponge-enhanced mussel mortality by repeated sampling at several sites where both organisms were abundant in the upper St. Lawrence River and on an artificial reef in central Lake Erie. A small proportion (<10%) of the dreissenid population at each site was overgrown by sponge. Mussel colonies that were completely overgrown for 1 or more months invariably contained a significantly greater proportion of dead mussels than local uncovered populations. Mussels that survived prolonged periods (4-6 months) of overgrowth suffered significant tissue weight losses. Laboratory experiments and field observations suggest that dreissenids are not able to colonize sponges; therefore, sponges should always dominate competitive overgrowth situations. The overall impact of sponges on dreissenid populations in the Great Lakes - St. Lawrence River system will probably be negligible because of the high rate of mussel recruitment and the environmental constraints on sponge growth; however, our results suggest that sponges may control mussel abundance locally.