PRIMARY SEXUAL PHASES IN THE OVIPAROUS OYSTER (OSTREA VIRGINICA)

Abstract

Examination of more than 6000 young of this al-ternosexual species in their 1st spawning period, obtained from various localities from Xew England to the Gulf of Mexico, has shown a high variability in the ratio of the 2 sexual phases in different situations and in different yrs. Under conditions unfavorable for rapid growth at northern localities the proportion of individuals functioning as [female][female] at their 1st spawning season was less than 8 for each 100 [male][male]; at the same localities under more favorable nutritive conditions the [female] ratio exceeded 20. Farther south the [female] ratio averaged nearly 40. The ratio may change somewhat during the 1st spawning season, since in the primary sexual phase [female] development may be either direct or indirect. In the 2d and later spawning periods, the proportion of [female][female] increases with age until there are more [female][female] than [male][male], since more of the [male][male] change to the [female] phase than the number of individuals which experience a sex change in the opposite direction. Change of sexual phase follows the cytolysis of most of the sexually differentiated reproductive cells after spawning, leaving mainly undifferentiated gonia as residual cells, but in many individuals cells of one or both sexual types remain. Local races with differing genetic factors as well as environmental conditions may account for the diverse sexual ratios. Two generations in 12 months may occur in southern localities.