Monday, 26 May 2008


Oysters are sensitive. In case of danger, it closes its shell. It reacts to light, to levels of salt in the water, to temperature, to shadow and sound. They have much more complex responses to changes in the environment.

Walk along the beach at low tide near an oyster cove and you can hear a series of short sharp spitting sounds as the oysters react and close their shells, even from quite long distances away.

They are also amazingly powerful. Trials have shown that it would take a pull of more than 9kg to open up a 10 cm mature oyster that has clamped its shell shut.

Its anatomy is encase in the mantle which has three folds or ridges, each of which has a particular function.

The outer ridge secretes the calcareous material to build and repair the shell. In the centre ridge are two rows of fine, dark tentacles or feelers which are the sensors. Any disturbing change in the water or even just a passing shadow stimulates the nerves of the mantle and causes the adductor muscle to close the shell. The inner ridge, the largest of the three, is muscular and mobile. It pumps water in and out of the shell, washing the body and keeping it constantly bathed. So long as the shell stays shut and retains this liquid, it can ignore most predators and survive in polluted waters or exposure at low tide and being transported.

The mouth is by the hinge, the lips surrounded by gills, sometimes called the beard. These are used to breathe and to collect and sort food. They are covered with more tiny, lashing hairs which create an incoming current of water from which food particles are filtered and oxygen absorbed.

They breathe like fish, using both the gills and the mantle. A small, three-chambered heart, lies under the adductor muscle, and pumps colourless blood, with its supply of oxygen, through the body. Also it has two kidneys to purify the blood.

The food is sorted by the lips and passed into its digestive system and then through a coiled intestine to be expelled through the rectal chamber.

They continue to grow through their life, although more slowly as they get older. Some, as can easily be seen in early still lifes can grow to nearly 40cms long and weigh nearly one and half kilos.