SHUKLA, V.*; KING, K.N.; RAM, J.L.; Wayne State Univ., Detroit, MI: Zebra mussels at the freshwater/sea interface: Ionic and osmotic challenges to oocyte structural integrity

The zebra mussel (ZM), Dreissena polymorpha, is a highly prolific biofouling freshwater mollusk accidentally introduced into North America about 15 years ago. Understanding its reproductive mechanisms may give insight into adaptive mechanisms and possible control methods. Exposure of ZM oocytes to low concentrations (5% to 10% of full strength) of seawater (SW), as might be encountered in marine estuaries, causes oocyte fragmentation within 3 hr (Fong et al., 1995). Structural changes may resemble apoptotic responses in aging oocytes and other cell types. To study causal agents (which ions? osmotic or other effectors?) and mechanisms (apoptosis? necrosis?), the effect of various solutions on oocyte integrity were tested. ZM were induced to spawn with serotonin (Ram et al., 1993). The percent of oocytes fragmenting after 3 hr exposure to low SW concentration were: control, 11�3%; 2% SW, 18�2%; 5% SW, 47�4%; and 10% SW, 53�4% (ANOVA, p<0.001). NaCl or CaCl₂ concentrations comparable to amounts present in 5% and 10% SW also induce oocyte breakdown (e.g., control, 12�1%, 50 mM NaCl, 49�1%; 1 mM CaCl₂, 52�1%, p<0.001). However, NaCl and Ca mechanisms may differ because: a) fragmentation morphology differs (NaCl resembles SW; Ca does not), b) osmotically equivalent NaCl, Na-phosphate, and glucose yield similar results, and c) NaCl osmotically equivalent to CaCl₂ causes significantly less fragmentation. Apoptosis inducers (emetine, anisomycin) are also toxic to ZM oocytes. NaCl-mediated responses are probably osmotically driven; Ca responses may depend upon specific Ca-activated mechanisms; and both may be involved in the response to SW. Supported by NIH #5T34-GM08030-19 and the Detroit Public Schools.