The length of the spawning period of oysters 

 depends upon the climate. In New England 

 waters it extends on the average from 2-1/2 

 to 3 months, while in Florida waters oysters 

 with ripe eggs or spermatozoa can be found 

 most of the year. Therefore, until recently, 

 in New England waters and similar areas 

 experiments on larvae of oysters and most 

 other bivalves were confined exclusively to the 

 short period of natural propagation. However, 

 biologists of the Fish and Wildlife Service, 

 Bureau of Connmercial Fisheries, recently 

 found that, by using proper methods, normal 

 development of gonads and spawning can be 

 induced during late fall, winter, and spring 

 (fig. 5). 



Conditioning oysters to develop mature spawn 

 during winter is relatively simple. Place the 

 moUusks in warm water and then gradually 

 increase the temperature to the desired level. 

 Toward spring, instead of slowly conditioning 

 the nnollusks by gradually increasing the 

 v/ater temperature, the oysters can be placed 

 directly in water of about 70° F. and kept 

 there until they become ripe, which normally 



takes from 3 to 5 weeks. At higher tempera- 

 tures they will ripen more rapidly. Upon 

 ripening, the oysters can be induced to spawn 

 in the laboratory by quickly raising the tem- 

 perature to about 770 F. and adding a suspen- 

 sion of eggs or spermatozoa to the water 

 (fig. 6). The spawn obtained in this way is no 

 less viable than that released by oysters in 

 the usual manner during sunnmer. 



Fertilizable eggs can be obtained by strip- 

 ping ripe females, but since oysters spawn so 

 readily in response to chemical and thermal 

 stinnulation, this method is seldom needed. 

 Moreover, among the eggs obtained by induced 

 spawning there are always considerably fewer 

 abnormal ones than among eggs obtained by 

 stripping. 



In early fall, when Long Island Sound oysters 

 cannot be conditioned because they have not 

 recovered from summer spawning, ripe spawn 

 can be obtained from individuals placed, in 

 spring, in water sufficiently warm to allow 

 the eggs to develop to maturity but not high 

 enough to permit spawning. This has been 

 done successfully with Long Island Sound 



Figure 5. — Milford Biological Laboratory, one of the centers where extensive research on oysters and other commercial 



moUusks is conducted by the Bureau of Commercial Fisheries. 



11 



