THE OYSTER INDUSTRY OF THE WORLD 555 



conditions show that in clean, unpolluted water adult oysters keep their shells 

 open and with brief interruptions pump water for 20 to 22 hours a day. The rate 

 of pumping varies from a few quarts to a maximum of about 30 quarts an hour, 

 with the average for the summer temperatures of about 14 to 16 quarts per hour. 

 On that basis a single adult oyster would require about 80 gallons of sea water 

 daily for feeding and respiration. These observations show that overcrowding of 

 oysters should be avoided and that the grounds selected for planting of oysters 

 should have a good exchange of water. 



Digestive gland. The large, dark-colored mass of tissue easily seen if the oyster 

 is cut across by a knife, is the digestive gland, or so-called liver. Its color is usu- 

 ally obscured by the mantle and by gonads, which in ripe oysters form a thick 

 white layer of sex cells. In thin, watery oysters the dark color of the liver shows 

 clearly through the surrounding tissues and frequently causes the purchaser not 

 familiar with oyster anatomy some concern. 



Sex and Propagation. There are two different methods of propagation. In some 

 species, as for instance in the European flat oyster (O. edulis), or native Pacific 

 Coast oyster (O. lurida), the eggs are fertilized inside the oyster and the ensuing 

 larvae are retained for about 10 days. At this age they leave the parents to swim 

 freely in the water and after a week or more settle and attach themselves to a 

 hard surface. The other method is well illustrated by the American oyster (O. vir- 

 ginica) and the Japanese oyster (O. gigas). By vigorous snapping of the shell the 

 spawning female discharges its eggs into the water where they are fertilized by 

 sperm. Simultaneous shedding of eggs and sperm is essential for the success of 

 fertilization, which is accomplished by mutual stimulation of the two sexes. Sperm, 

 present in water, induces the spawning of the ripe female, while eggs shed by the 

 female oyster secrete a substance which provokes spawning in the male (GaltsoflE, 

 1938, 1940). Fertilization takes place before the eggs settle to the bottom, and 

 free-swimming larvae develop within a few hours. 



The sex of an oyster is unstable. Although at one instant the majority of the 

 individuals are either male or female, changes frequently occur, especially in the 

 oysters of the first group in which hermaphrodites are often found. In the second 

 group the change of sex takes place in only a few individuals after the completion 

 of the spawning and before the new gonad is developed. Young O. virginica, less 

 than 1-year old, are predominantly male; but by the end of the first year the two 

 sexes have established a 50:50 ratio. 



Oysters are very prolific. Female O. virginica and O. gigas may discharge up to 

 100 million eggs at one shedding and may spawn several times during one sea- 

 son. Adverse conditions, such as freshets, abnormal temperature, and pollution 

 of water may inhibit or completely prevent the development of gonad. Degree 

 of gonad development and ripeness of the oyster can be easily observed by mak- 

 ing a cut across the body of the oyster and noticing the thickness and consistency 

 of the creamy gonad layer. 



Fatness. The quality of oyster meat is primarily dependent on the percentage 

 of solids and the content of glycogen, a carbohydrate stored as reserve food in the 

 mantle and other organs. 



Good oysters, with a high yield of meat per bushel, usually contain around 20 

 per cent solids, while in poor, watery ones the solids drop to less than 10 per 

 cent. High solids content is usually accompanied by high glycogen content. Such 



