On the Biology of Sexual Reproduction of Hydromedusae, Genus Phialidium Leuckhart

Abstract

The genus Phialidium is nearly ubiquitous in the coastal waters of the temperate zone. The medusae occur at Friday Harbor certainly from April through September and disappear rather suddenly in October, under circumstances which need investigation. Drifting with tides and currents, they occur in swarms which greatly vary in density. During the present investigation no swarms were seen in which individuals, on the average, were closer to each other than a few inches; usually they were many inches to several feet apart. Under these circumstances it would seem that fertilization becomes somewhat of a problem. In a population which, as a whole, is moving passively, and in which individuals appear to have no affinity to each other, ovulation and spermiation must be closely and appropriately timed and the properties of eggs and sperms evolved to insure a high rate of fertility, the results of which are seen in the wide distribution and in the tremendous numbers of colonies of the sessile stages of the species, the hydroid polyps. The present paper attempts to clarify some of the factors which insure the high reproductive capacity of hydromedusae under what, on first sight, appear to be difficult circumstances.