Reproduction in Australian Pearl Oysters (Lamellibranchia). V. Pinctada fucata (Gould)

Abstract

>Pinctacla fucata, in common with P. albina and P. margaritifera, is hermaphrodite. The gonad develops in a series of phases, in which germ cells are formed anew from indifferent "stem cells" which remain after spawning. Each phase is unisexual, but sex may change from phase to phase. There is a tendency towards protandry, but this is less marked than in P. albina. The percentage of individuals in female phase increases until middle life, by which time the sexes are equally common. Both protandric and protogynic sex-changes have been recorded in the field and, on one occasioii, a bisexual, transitional condition was observed in gonad sections. The evidence shows that sex change is not restricted to the wintering period.

Primary gonad differentiation begins 2-3 months after spat settlement, and within 6 months sexual maturity is reached. Frequently, there is a second phase of gonad development towards the end of the first year.

The typical adult breeding cycle consists of: a period of gonad differentiation from September to November; a spawning period from December to May; and a resting period from June to August. During the spawning season, there are usually two periods of maximum intensity: January-February ("summer spawning") and April-May ("autumn spawning"), the latter being the more consistent of the two. However, lesser spawnings occur outside the major spawning season, and spatfalls have been recorded throughout the year.

The macroscopic and histological structure of the gonad is similar to that of P. albina and P. margaritifera. The following points are noted: gonad colour is an unreliable indication of sex; the white mantle layer covering the gonad is predominantly mucoprotein, and is probably concerned with shell secretion ; there are no yolk nuclei visible in the cytoplasm of the oocyte, as in P. margaritifera; activation of the ripe oocyte takes place within the gonad follicles immediately prior to spawning; as in P. albina spawning is frequently incomplete, and phagocytosis plays a major role in the resorption of unspent gonad products; the haploid chromosome number is six.