Reproduction in Australian Pearl Oysters (Lamellibranchia). IV. Pinctada margaritifera (Linnaeus)

Abstract

The gonad of Pinctada margaritifera (L.) is similar to that of P. albina in appearance, development, and histological structure; minor differences are described and figured. Although the primary gonad may begin to develop within 12 months, complete and universal gonad maturity does not occur until the second year; compared with P. albina, then, the species is late-maturing.

At first maturity, less than 30 per cent, are females; in later seasons this shows a gradual rise to 50 per cent. Both protandric and protogynic sex-changes are occasionally seen in gonad sections and the evidence suggests that the process is a rapid one. It is therefore probable that sex-changes are common, and that the early increase in the percentage frequency of females is due to the more frequent occurrence of protandric change.

During 1956-57, major spawnings occurred in early January and late July, and lesser spawnings in other months. There were two distinct cycles: March to August, and September to February. It is considered, therefore, that breeding takes place predominantly in summer and winter, and may continue, at a reduced intensity, over the intermediate periods.

It is shown that the ripe oocyte becomes activated immediately prior to spawning. The germinal vesicle breaks down and meiosis proceeds to the metaphase of the first maturation division, at which stage there is a further arrestment. At fertilization, a second activation occurs and meiosis continues to completion. The maturation divisions and cleavage stages are described and figured.