On the nature of bisexuality

Crassostrea have no less an interesting sex life, albeit it is slightly different. They are intersexual. They can also change sex at will. They will spawn either as a male or produce eggs as a female and the seed and eggs are ejaculated to fertilise in the water. The scientific word is protandric; the male organs develop first and are then inhibited to allow the development of the female.. They begin life as male, and then change to females the following season. Then they seem to prefer to stay female most of the time but will revert from time to time like unreformed alcoholics. The older the oyster the more likely it will be female. The ratios on a bed at any given time can vary wildly. One study showed 100 females to 73 males, but in another account it was 133 young males for the same level of females.

Not incubating the larvae means as much as 80 per cent of the crassostrea’s bodyweight may be turned to sperm or eggs.

A male ejaculation in the colony seems to trigger other males to follow suit and brings a response in the females who sense what is happening and immediately start to release their eggs too. Soon the whole reef can be covered in white clouds of floating eggs and sperm.

In the context of a single oyster, fertilisation seems almost impossible. How can a sperm find an egg when they are let loose in the wilds of thetide? The answer is oysters congregate naturally. They spawn across each other. They send out hundreds of thousands of sperm and eggs at the same time, not unlike pollen. The shells of the parents become the breeding grounds for the next generations. In many osyterages, the shells are deliberately recycled back to the waters to create the cultch for the next generation and attract the larvae.