SHELLFISH 95 



A North European oyster acts alternately as female 

 and male. It produces eggs as many as a million in 

 a season and a fortnight after the eggs have been shed, 

 the same oyster produces millions of spermatazoa, which 

 form a cloud of fine dust in the water. These sper- 

 matazoa rapidly scatter in all directions, and, entering 

 the tubular reproductive sacs of oysters that are 

 producing eggs, fertilize them. 



American and Portuguese oysters are definitely male 

 and female, the eggs being discharged by the female and 

 fertilized subsequently in the sea by the male. 



The eggs remain attached to the parent's gill plates, 

 and in a day or so develop into minute, shell-less oysters. 

 The parent oyster is then said to be " white-sick." 

 About two days later the young oysters have become 

 dark-coloured and are found to have formed minute 

 convex shells, rather like those of a cockle. The parent 

 is then " black-sick." A week later the young oysters 

 escape and rise in thousands to the surface water, 

 swimming by means of fine hairs or cilia that are attached 

 to the upper edge of the shells. They are carried far 

 and wide by tides and surface currents. Many are 

 eaten by young fish and shrimps. As they grow the 

 shells become heavier, and after a time they sink to the 

 sea bottom. This is known as the " fall of spat. " If 

 they fall on stony ground, where they will be well 

 irrigated and nourished through the movement of the 

 water, they will thrive. Many, however, fall on soft, 

 unsuitable ground and perish. 



The European oysters spawn in the summer (from 

 May to September) . They become mature in three years, 

 are at their prime in from five to seven years, and 

 rarely live longer than ten years. 



Oysters are gathered from natural beds or from 

 artificial grounds The oyster breeders place movable 



