;:::m^^ 



Figure 290. — Spawn of C. gigas carried down by the tidal river near Vancouver, British Columbia. Courtesy of D. 



Quayle. Black and white enlargement of Kodachrome slide. 



kept on floats and among specimens living under 

 natural conditions on oysters beds near low tide 

 level. The spawning of an entii'e oyster popula- 

 tion can be artificially initiated by mincing tlie 

 meats of several sexually mature oysters and 

 spreading them into the waters of the oyster bed. 

 This method, based on my laboratory experiments, 

 has been applied on a commercial scale by oyster 

 growers in British Columbia (Elsey, 19.36). 



Simultaneous spawning of oyster populations is 

 essential for the production of a large brood of 

 oyster larvae and for obtaining setting of com- 

 mercial value. In the estuaries where the ma- 

 jority of oyster beds are located, the tides carry 

 the released spawn for some distance before the 

 eggs sink to the bottom. A transport of oyster 

 spawn by a strong ciurent can be seen in the 

 photogi'aph (fig. 290) taken by Quayle near Van- 

 couver, British Columbia, and kindly given to me 

 for reproduction. The white streak in the fore- 

 ground of the clear river water was formed bj- 

 billions of eggs and sperm discharged by a popula- 

 tion of C. gigas several miles up river. 



The method of discharging sperm and eggs is 

 also of considerable significance. vSpermatozoa 

 carried away by the respiratory cmTent remain in 

 suspension for several hours. When eggs are dis- 

 charged in the same manner by some sex-reversed 

 oysters, they rapidly sink to the bottom only a 

 few inches away from the fenude. Lalioratory 

 observations indicate that under such conditions 



only a very small percent of them are fertilized 

 or have even a slim chance of developing. On the 

 other hand, eggs discharged in the usual manner 

 through the gills and forcibly ejected from the 

 mantle cavity, have a much better chance to be 

 fertilized and survive. Furthermore, because of 

 the specificity of female response to sperm, eggs 

 are ejected only when the water contains free 

 spermatozoa of the same species. The female 

 spawning reaction is an adaptation of an oviparous 

 organism to the conditions of its existence and 

 assures the survival of the species. 



BIBLIOGRAPHY 



Amemiya, Ikusaku. 



1929. On the sex-change of the Japanese common 

 oyster, Ostrea gigas Thunberg. Proceedings of the 

 Imperial Academy (of Japan), vol. 5, No. 7, pp. 

 284-2S6. 



1936. Effect of gill excision upon the sexual differ- 

 entiation of the oyster, Ostrea gigas Thunberg. 

 [Abstract No. 643.] Japanese .Journal of Zoology, 

 Transactions and Abstracts, vol. 6, No. 4, pp. 84-85. 

 Antheunisse, L. J. 



1963. Neurosecretory phenomena in the zebra 

 mussel Dreissena polymorpha Pallas. Archives 

 Neerlandaises de Zoologie, tome 15, fascicule 3, 

 pp. 237-314. 

 AwATi, P. R., and H. S. Rai. 



1931. Ostrea cucuUata (the Bombay oyster). The 

 Indian Zoological Memoirs on Indian Animal 

 Types, III. Methodist Publishing House, Luck- 

 now, India, 107 pp. 



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