Species-specific sperm attraction in the zebra mussel, Dreissena polymorpha, and the quagga mussel, Dreissena bugensis

Abstract

The occurrence of species-specific agents that attract sperm to spawned oocytes of zebra and quagga mussels might explain both the high fecundity of these species and their apparent inability to hybridize in nature. Ethanol extracts of freshly shed oocytes were prepared, dried, and redissolved in aquarium water and tested for sperm-attracting activity by micrometer-controlled pipetting into highly diluted suspensions of recently released sperm. The time course of the sperm responses was quantified by stop-frame video morphometric analysis of sperm density at various distances from the pipette tip. Controls included aquarium water, 10−3 M serotonin solutions used to induce release of sperm and oocytes, and ethanol extracts of aquarium water. Injection of oocyte extracts from both zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels elicited a significant increase in the density of actively motile conspecific sperm, usually within 10 s of extract release. No significant increase in sperm density was observed in response to any of the control solutions, nor was there any significant change in sperm kinematic parameters. Positive sperm responses were also obtained to whole-gonad acid extracts stored for 1 year or boiled. Although full-strength extracts attracted sperm of both species, serial half-dilution of the two extracts showed that a 100-fold higher concentration of attractant was required to attract sperm of the heterotypic compared with the homotypic species. Serotonin had no effect on the kinematics of either species' sperm. This is the first demonstration of sperm attraction to egg extracts in a bivalve.