Contact Person: Prof. Dr. Bela H. Buck
The Biology of Oysters (Crassostrea gigas, Ostrea edulis)
Oysters are two shelled molluscs of the family Ostreidae, including the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola or Saccostrea. Examples are the Edible Oyster, Ostrea edulis, Eastern Oyster Crassostrea virginica, Olympia Oyster Ostreola conchaphila, or the Pacific Oyster Crassostrea gigas (see Fig. 1).
Adult oysters are sessile benthic animals, living on the seafloor. The larvae are planktonic. They are common in shallow water and intertidal environments, but some species also inhabit deeper waters. As a keystone species, oysters provide habitat for an extensive array of marine life. The native oyster usually inhabits water depths of between 2 and 8m. The hard surfaces of oyster shells and the nooks between the shells provide places where a host of small animals can live. Hundreds of animals such as anemones, barnacles, and hooked mussels use oyster reefs as habitat. An oyster reef, with its many convolutions, can encompass 50 times the surface area of an equally extensive flat bottom. An oyster's mature shape often depends on the type of bottom to which it originally attached. It orients itself with its outer, flared shell tilted upward. One valve is cupped and the other is flat. The submerged shell opens periodically to permit the oyster to feed.
Oysters feed by filtering seawater over their gills. The gills are the largest organ in the oyster's body and consist of four folds of tissue. Along with the mantle, it is the chief organ of respiration. Oysters are able to create water currents, collect food particles and move food particles down to the labial pulps for sorting. Filtered phytoplankton and other particulates are entrapped and bound in mucus. Strings of mucus carry the particulate matter to the labial palps, where they are passed into the mouth, or rejected as pseudofeces (waste from the oyster, which has not passed the gut). Oysters are important to marine ecosystems because of their ability to filter up to 200 Litres per day, removing phytoplankton and other particulates from the seawater.
Oysters breathe, using both gills and mantle. The mantle is lined with many small, thin-walled blood vessels which extract oxygen from the water and expel carbon dioxide. A small, three-chambered heart, lying under the abductor muscle, pumps colorless blood, with its supply of oxygen, to all parts of the body. At the same time a pair of kidneys located on the underside of the muscle purifies the blood of any waste products it has collected.
Reproduction of Oysters
There is no way of determining male oysters from females by examining their shells. While oysters have separate sexes, they may change sex one or more times during their life span. The gonads, organs responsible for producing both eggs and sperm, surround the digestive organs and are made up of sex cells, branching tubules and connective tissue. Oysters usually mature by one year of age. They are protandric, which means that during their first year they spawn as males (releasing sperm into the water). As they grow larger over the next two or three years and develop greater energy reserves, they release eggs, as females. Bay oysters are usually prepared to spawn in early summer. An increase in water temperature prompts a few initial oysters to spawn. This triggers a spawning 'chain reaction', which clouds the water with millions of eggs and sperm. A single female oyster can produce up to 100 million eggs annually. The eggs become fertilized in the water and develop into larvae, which eventually find suitable sites to settle. Attached oyster larvae are called 'spat'.
Oysters are a very popular delicacy in many countries. Due to the high demand, the practice of oyster farming has become a large industry all over the world. The Pacific Oyster is the most heavily farmed species of oyster in the world. That is why a lot of research is focused on its biology, ecology and genetics.
Europe has a long history in oyster cultivation. Seaside resort like Cancale in France or Kentish Flats in the United Kingdom are noted for its oysters which date from Roman times. In the early nineteenth century, oysters were cheap and were mainly eaten by the working classes. However, increasing demands from the rapidly-growing cities led to many of the beds running short. To increase production, foreign varieties were introduced and this soon brought disease which, combined with pollution, resulted in oysters becoming rare. This scarcity increased prices leading to their current status as a delicacy. Due to this process the native European oyster (Ostrea edulis) is nearly extinct.