Physiological aspects of zebra mussel reproduction: maturation, spawning, and fertilization1

Abstract

SYNOPSIS. The prolific reproductive capabilities of the zebra mussel, Dreissena polymorpha, have facilitated the rapid spread and high densities of this biofouling organism since its accidental introduction into North America less than 10 years ago. Research on its reproductive mechanisms and capabilities may be valuable not only in predicting its further spread, but also in investigating basic mechanisms of reproduction and development and in developing new strategies to mitigate its impact. Since zebra mussels are dioecious and fertilization occurs externally, coordinated maturation, spawning, and other mechanisms have evolved to increase the probability of successful fertilization. The zebra mussel undergoes an annual cycle of gonadal growth and gamete maturation, culminating in one or more spawning events in late spring or early summer. Temperature, rates of temperature change, food availability, and effects of neighboring mussels seem to be critical variables that determine reproductive responses. Serotonin is a biogenic amine which is implicated in spawning behavior and can reliably trigger spawning. Serotonin is present in the gonad in neural varicosities that encircle groups of gametes, and specific serotonergic ligands can mimic or block spawning caused by serotonin. In females, serotonin reinitiates meiosis causing maturation from prophase I to metaphase I prior to spawning. Spawned oocytes contain substances that are species specific sperm chemoattractants. The sequence of binding, entry, and subsequent nuclear movements have been observed with fluorescence and scanning microscopy. Despite their negative ecological and economic impacts, zebra mussels have also provided a new and easily obtainable resource for studies of reproductive mechanisms.