Sexual Morphology and Physiology

Pinctada margaritifera are protrandrous hermaphrodites with the ratio of males to females tending to 1:1 with increasing age.  Both male to female and female to male sex changes can occasionally be seen in gonad sections where hermaphroditic phases are transitional and not functional. Change in sex can occur in all members of the genus after male maturity has been reached within the second year. No differences in shell morphology are associated with a change in sex. Gonad coloration distinguishes sex with ovaries being pinkish, creamy or yellow and granular (Tranter 1958). Testes are white and smooth (Reed 1966). The gonad is not a discrete organ but is found between the connective tissue at the base of the foot and the intestinal loop. As ripening gonads increase in size, follicles and germ cells extend through the connective tissue, filling the cavity between the foot and the byssal gland, around the retractor muscle and digestive tract. A ripe individual is identifiable superficially by the size of the gonad and microscopically by the abundant gametes and fewer germ cells in the follicles.

Spawning results from muscular contractions and oocytes are activated immediately prior to spawning in the follicle (Tranter 1958). Sperm and eggs are released into the water where fertilization takes place.

The overall pattern of reproduction in pearl oyster populations is synchronous, with male and females undergoing sequential processes that lead to a simultaneous breeding period. This is a cyclical process with three general phases:

1. Vegetative phase, during which the oysters display no evidence of sexual activity.

2. Cellular differentiation phase, during which the proliferation and differentiation of stem cells lead to mature gametes and spawning.

3. Recovery phase, during which there is degeneration of the gonad after spawning and recovery of nutrients from unspawned gametes for recycling.