Saturday, 10 May 2008

Oysters spawn in the spring, when the waters start to warm. The young larvae propel themselves along using cilia or hairs. When it gets older and stationary it will use these same cilia to sort its food. The spat can travel quite long distances, perhaps two miles up or out of the estuary. After a week they start to move towards the ocean floor as if some genetic homing device is attracting them downwards. By then it will have started to grow its shell and under a microscope can be seen to have taken an oyster shape. After 14 to18 days it will clasp on to a hard object where it will settle forever, unless man transplants it to different waters to fatten off. Some will take two years to reach marketable size, others five or six years. Left to themselves, an oyster can live to be 50 or more. Oysters found in Lake Bras, Canada, have been 100 years old.

Ostrea are bisexual meaning they can be whatever sex they feel like. Hermaphroditic suggests sterility, which oysters certainly are not. They change sex seemingly as they like, even during a single breeding season. The female lays her eggs in the shell. The male ejaculates and the sperm is carried to her on a wave of plankton. Quite what makes an oyster change sex is a mystery, although as in most important events in an oyster’s life, a change in water temperature often triggers a dramatic event.

Amazingly the scientific literature says it was not until 1937, that J.H.Orton in the UK and R Sparek in Denmark realised that oysters were changing sex at all…although certainly there are other references to the oyster as a hermaphrodite before that in literature and less learned journals, so maybe the science was just playing catch up.

In Orton’s laboratory, a batch marked females started producing sperm. Amazed, Orton drilled holes in the shell. He watched through a microscope as the females changed into males over a few days. It seemed that as soon as she had discharged her eggs, so she transformed back into a male quite quickly.

The change from male to female took much longer, weeks, even months irrespective of the conditions. Technically, this is perhaps unsurprising. Producing sperm is relatively easy and makes fewer demands on the body, while producing eggs and generating enough yolk style substance to nurture the young is considerably more onerous.

Being bisexual makes for easy reproduction. Biologically the oyster’s anatomy is almost abstract. The female has no glands for making albumen or need for a womb to protect the eggs because she has her shell; nor does the male need a penis or anywhere safe to stash his sperm. Nor is there any paraphernalia needed for courtship, which simplifies things even further. There is just less to get in the way of a sex change, than in other creatures. The follicles produce eggs or sperm or vice versa, a minor issue for a bivalve.

It seems usually the young ostrea will become sexually mature as a male, then slowly change to become female. She will guard the larvae in her shell for 12 days and then release them into the estuary to swim off in search of a safe place to settle.

No comments: