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 over grow and kill introduced zebra (Dreissena polymorpha) and quagga musse ls (Dreissena bugensis) on solid substrates. Sponges overgrow and smot her mussel siphons, thereby interfering with normal feeding and respir ation. We tested the significance of sponge-enhanced mussel mortality by repeated sampling at several sites where both organisms were abunda nt in the upper St. Lawrence River and on an artificial reef in centra l Lake Erie. A small proportion (<10%) of the dreissenid population at each site was overgrown by sponge. Mussel colonies that were complete ly 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 suf fered significant tissue weight losses. Laboratory experiments and fie ld observations suggest that dreissenids are not able to colonize spon ges; 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 negligib le because of the high rate of mussel recruitment and the environmenta l constraints on sponge growth; however, our results suggest that spon ges may control mussel abundance locally.