ANNUAL REPRODUCTIVE-CYCLE OF THE ZEBRA MUSSEL IN THE OHIO RIVER - A COMPARISON WITH LAKE ERIE

In order to evaluate the effect of temperature and trophy on zebra mus sel reproduction, the annual reproductive cycle of Dreissena polymorph a was determined by histological examination of the gonads. Population s from Lake Erie, its tributaries, and the Ohio River were compared si nce they varied systematically in these parameters. The timing of gona dal changes in Lake Erie mussels and that of its eutrophic tributaries were identical. Gonadal development commenced overwinter. By August, spawning had commenced and ovarian tubules contained oocytes of varyin g stages and sizes, males possessed fully developed testes, and mature sperm were evident. Mussels from Lake Erie spawned only once during a season but approximately 30 % of mussels from Lake Erie tributaries ( Sandusky Bay, Huron River, Old Woman Creek) had an additional spawn. B y October, the gonads of all mussels were regressed and empty, and it was difficult to determine sex histologically. In contrast, complete g onadal regression was not seen in the Ohio River population. Some fema les contained yolked oocytes throughout the year, and some males posse ssed spermatids during the winter months. Females were generally gravi d from May through September although there was considerable variation among individuals at any point in the year. Spawning in the river pop ulations was estimated to occur from March through September. Evidence of two sets per year was observed in the middle Ohio River mussels. T emperature appeared to regulate the onset of major reproductive events , but food must have played a role in determining the frequency of rep roduction.