Zebra mussels at the freshwater/sea interface: Ionic and osmotic challenges to oocyte integrity

Summary

The zebra mussel, Dreissena polymorpha, is a highly prolific biofouling freshwater mollusk accidentally introduced into North America in the 1980s. Understanding its reproductive mechanisms may give insight into its dispersion, adaptive mechanisms, and possible control methods. The spread of zebra mussels into tidal regions and estuaries could be affected by the responses of zebra mussel gametes to seawater. This study investigated the effect of dilute seawater (Instant Ocean, IO) and several of its components on zebra mussel oocyte viability. Zebra mussels were induced to spawn with serotonin. Oocytes exposed to various concentrations of IO or its component ions began to break down within 3 h, usually after a latency of 1–2 h. The percentages of oocytes fragmenting after 3-h exposure to low IO concentration were: control, 11±3%; 2% IO, 18±2%; 5% IO, 47±4%; and 10% IO, 53±4% (ANOVA, p<0.00l). NaCl or CaCl₂ concentrations comparable to amounts present in 5% and 10% IO also induced oocyte breakdown (e.g., after 3 h, control, 12±1%; 50 mM NaCl, 49±1%; 1 mM CaCl₂, 52±1%; ANOVA, p<0.001). However, NaCl and calcium-mediated mechanisms may differ because: (a) fragmentation morphology differs; (b) osmotically equivalent NaCl, Na-phosphate, and glucose yield similar results, suggesting that these solutions all work primarily by an osmotic effect; and (c) CaCl₂ caused twice the oocyte fragmentation of osmotically equivalent NaCl, suggesting that oocyte breakdown may also depend upon specific Ca-activated mechanisms. Both osmotic and calcium-mediated mechanisms may be involved in the breakdown of oocytes in response to seawater. Structural changes of zebra mussel oocyte fragmentation may resemble breakdown of oocytes described in other organisms in response to aging and oxidative stress that have been suggested to be mediated by apoptosis.

Key words:

• Oocytes
• osmotic effects
• salinity
• zebra mussels