Spawning in the zebra mussel (Dreissena polymorpha): activation by internal or external application of serotonin

The zebra mussel, Dreissena polymorpha, was recently introduced accidentally into the Great Lakes and, due in part to its prodigious reproductive capacity, is spreading rapidly in temperate fresh waters of North America. The present studies examine some of the mechanisms that regulate spawning in this animal. In August and September 1990 and in May 1991 injection of serotonin (5-hydroxytryptamine, 5-HT) induced ripe male, but not female, zebra mussels to spawn. During mid-summer 1991, 5-HT induced spawning in both males and females, and 5-HT could produce spawning responses by either injection or external application. External pH over a broad range (6.0 to 9.1) had no effect on spawning, neither inhibiting induction of spawning by 5-HT nor significantly eliciting spawning itself. With external application, 10(-3) M and 10(-4) M 5-HT caused spawning, but 10(-5) M and 10(-6) M did not. Cyproheptadine, a 5-HT receptor antagonist, reduced the response of both males and females by more than half. Spawning in response to 5-HT was blocked at 4 degrees C, but not at 12 degrees C, 20 degrees C, or 27 degrees C. For male zebra mussels morphological criteria for judging gonadal maturity were well-correlated with probability of spawning in response to 5-HT. For females, the likelihood of spawning in response to 5-HT was not tightly coupled to morphological maturity of the gonad, with many morphologically ripe females failing to spawn and some apparently immature animals releasing oocytes. Prior spawning reduced subsequent responsiveness and intensity of spawning of animals to 5-HT. These experiments support a role for 5-HT in regulating reproduction in zebra mussels and help define conditions by which zebra mussel spawning may be stimulated or inhibited.