General information
regarding oysters and oyster cultivation
The oyster is a mollusc (shellfish) and is of the class Bivalvia (with two shells or valves).
The female oyster is able to produce more than 150 million eggs which, once fertilised, begin swimming within 24 hours and remain free-swimming for approximately 21 days, depending on temperature etc.
An oyster is in the most advantageous position of being able to change its sex – the older oysters become the greater the percentage of females.
The most important oysters on the South African coast are:
- Soccostrea cuccullata, which is found from the Transkei northwards.
- Crassostrea margaritacea, which is found from the Transkei southwards to False Bay.
- Ostrea atherstoni (the red oyster), which is found from Algoa Bay to False Bay and is also found on the west coast.
- Ostrea algoensis (the weed oyster), which is found from Algoa Bay to False Bay. This oyster is not good eating and is too small to be of commercial value.
- Pinctada capensis (the pearl oyster) ,which forms the true pearl. Pearls are formed by other oysters but are not uniform in shape and lack the lustre of a commercial pearl.
The three most important edible oysters on the Southern African Coast are:
- Crassostrea margaritacea, which is the common rock oyster on the Cape coast.
- Saccostrea cuccullata, which is similar in shape and taste to the imported Crassostrea gigas.
- Ostrea atherstoni has an extremely strong taste when eaten raw, but is most palatable when cooked.
The main difference between Ostrea and Crassostrea oysters is in their breeding habits. The genus Ostrea fertilise their eggs internally, incubate their lavae and release them at a size of approximately 160 micron. The genus Crassostrea release their eggs and sperm – fertilisation therefore takes place externally.
24 hours after fertilisation the larvae are able to swim by using an organ know as a velum, which is covered with cilia (fine hairs) that propel the organism.
After 48 hours the larva will begin feeding on plankton (minute organisms) and this will be its diet together with bacteria and detritus (decaying organic matter) for the rest of its life. The size of the larvae at this stage is approximately 45 micron.
During the advanced stages of larval development an “eyespot” is formed, which is sensitive to light, specific gravity etc. and is used to select a suitable place to settle.
A “foot” is developed (similar to that of a snail). This foot is used to crawl with and to excrete the glue required to attach it to the surface chosen to settle on. The foot is also used to control its vertical movement whilst swimming – when retracted the larva will drop.
All of the above organs are absorbed by the oyster during metamorphosis and they become redundant after settling.
In a laboratory various culch (materials used for the larvae to settle on) are used – the most successful being thin PVC sheets or crushed oyster shell with a particle size of approximately 400 micron. The larvae settle at a size of approximately 320 micron.
Owing to an overcrowding problem, the spat (post larval or juvenile oysters) are scraped off the sheets every 24 hours and are then cultivated loose to facilitate grading and density control. Spat settled on crushed shell are automatically loose as the particle size is equal to that of the larvae.
Cultivated oysters tend to produce a far superior (cupped) and uniform shape as an attached oyster will, as it grows, adopt the shape of the surface that it is adhered to, creating varying shapes and sizes.
