6 U. S. BUREAU OF FISHERIES. 



The quantity of spawn found in the oysters is determined largely 

 by the time of spawning and the previous weather conditions. Oys- 

 ters one year old are capable of producing mature spawn, though 

 for artificial propagation oysters three to five years old are con- 

 sidered the best. In sorting over the brood stock to be placed on 

 the trays in the spawning tank only those oysters are selected that 

 are free from the boring sponge, drill spawn, mussels, barnacles, 

 hydroids, and tube-building worms, and which, from a noticeable 

 amount of new growth a,round the edges of the shell, appear to be 

 healthy. If necessary they can be scrubbed with a stiif brush to 

 remove any excessive growths that may cover them. 



SPAWNING. 



The act of spawning consists in the discharge of the ripe re- 

 productive elements into the surromiding water by both sexes of the 

 oyster. The actively moving spermatazoa swarm about and fertilize 

 the eggs, a. single spermata^oon penetrating each egg membrane, the 

 head of the spermatazoon passing on in after the tail has dropped 

 off. The material of the head unites with that of the egg and causes 

 the development of the egg cell into an embryo oyster. 



The general method, since Brooks's initial experiment, for obtain- 

 ing fertilized eggs has been to artificially mix the spermatazoa and 

 eggs after stripping them from the ripened oysters, which have 

 been opened. Tliis method is unreliable, crude, wasteful, unnatural, 

 and in most cases unnecessary, and undoubtedly accounts for many 

 of the failures in the various attempts tO' artificially propagate 

 oysters. It is impossible to determine how nearly ripe the spawn 

 may be in any group of oystere without opening them, and conse- 

 quently many are wasted before specimei^s apparently suitable for 

 stripping are found. The experiments conducted at Milford have 

 proved without a doubt that an enormous number of fertilized eggs 

 can be produced by a natural process and the adults saved for future 

 use. This process consists in placing the oysters in tanks of slowly 

 running water, in the sunlight, where spawning is induced by the 

 rising temperature. In every attempt made from July 10 to Sep- 

 tember 1 the oysters spawned readily and produced a greater amount 

 of spawn than could be taken care of with the small-scale equipment. 

 The spawning occurred at temperatures ranging from 68 to 75° F., 

 and lasted over intervals of 15 to 30 minutes (see fig. 3). At higher 

 temperatures than 75° the spawn was released slowly and immediately 

 settled to the bottom of the tank, whereas normally it would be for- 

 cibly ejected into the water and would float about for some time. It 

 was observed that the female oyster discharged the eggs in puff's at in- 

 tervals of about 30 to 50 seconds, while the products from the male 

 were emitted continuously in a slender, white, thread-like stream. In 

 this way, when the trays of oysters were placed near the surface 

 of the water, it was possible to proportion the numbers of each sex 

 when spawning began, and one male was found to produce sufficient 

 spermatazoa to fertilize the eggs from four females. The genital 

 products can also be distinguished with the naked eye as the eggs 

 appear as clouds of tiny white specks when discharged into the 

 water, while the spermatazoa appear tO' fade away almost immedi- 

 ately after leaving the oyster. From these experiments it is evident 

 that if oysters have reached sexual maturity, it is far better to have 



