PHYSIOLOGICAL-ASPECTS OF ZEBRA MUSSEL REPRODUCTION - MATURATION, SPAWNING AND FERTILIZATION

The prolific reproductive capabilities of the zebra mussel, Dreissena polymorpha, have facilitated the rapid spread and high densities of th is biofouling organism since its accidental introduction into North Am erica less than 10 years ago. Research on its reproductive mechanisms and capabilities may be valuable not only in predicting its further sp read, bur also in investigating basic mechanisms of reproduction and d evelopment and in developing new strategies to mitigate its impact. Si nce zebra mussels are dioecious and fertilization occurs externally, c oordinated maturations spawning, and other mechanisms have evolved to increase the probability of successful fertilization. The zebra mussel undergoes an annual cycle or gonadal growth and gamete maturation, cu lminating in one or more spawning events in late spring or early summe r. Temperature, rates of temperature change, food availability, and ef fects of neighboring mussels seem to be critical variables that determ ine reproductive responses. Serotonin is a biogenic amine which is imp licated in spawning behavior and call reliably trigger spawning. Serot onin is present in the gonad in neural varicosities that encircle grou ps of gametes, and specific serotonergic ligands can mimic or block sp awning 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 che moattractants. 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 mussel s have also provided a new and easily obtainable resource for studies of reproductive mechanisms.