Prionodesmacea 537 



Class Pelecypoda, Order prionodesmacea 



SPAWNING AND FERTILIZATION OF THE OYSTER, 



OSTREA VIRGINICA 



Paul S. Galtsoff, V. S. Bureau of Fisheries 



THE spawning season of the oyster in the North Atlantic States 

 begins during the second half of June and ends by the 15th of 

 August. Delayed spawning of the oysters in deep water (30-40 feet) 

 sometimes occurs in Long Island Sound late in September. In the 

 Middle Atlantic and South Atlantic States, and in the Gulf of Mexico 

 the season begins earlier and extends over a longer period of time. In 

 Texas and Louisiana, for instance, ripe oysters may be found from 

 March until October (Hopkins, 1931). 



The sexes of the oyster are separate but secondary sexual characters 

 are wanting. The identification of sexes may easily be made by drill- 

 ing* a small hole in the anterior half of the shell or by cutting a shell 

 with a hacksaw, pinching off a small piece of gonad and examining the 

 tissue under the microscope. This operation seems to have no ill effect 

 on the organism. 



In both males and females the well developed gonads completely 

 embrace the visceral mass, surrounding it with a thick creamy layer. 

 A ripe adult female may produce several hundred million eggs all of 

 which are discharged during one spawning season (Galtsoff, 1930a). 

 The branching genital ducts, well developed in ripe specimens, are easily 

 noticeable on the surface of the gonad. The openings of the gonad, one 

 on each side, may be seen by making an incision through the wall of the 

 cloaca and exposing the posterior end of the gonad. By exerting slight 

 pressure a small amount of sperm or eggs may be forced through it. 



Spawning never takes place at temperatures below 20.5 C. and is 

 controlled by a number of factors among which the mutual stimulation 

 of sexes by eggs or sperm plays an important role (Galtsoff, 1930). 

 Males are more responsive than females to stimulation by rise in tem- 

 perature and almost always initiate spawning. Sperm discharged into 

 water causes ovulation in the females and the presence of eggs in turn 

 stimulates other males. The process once started spreads over the 

 entire oyster bed. In order to prolong the use of the material for ex- 

 perimental purposes in the laboratory, the sexes should be separated 

 early in the summer and males and females kept apart in different tanks. 



The spawning reaction of the females is specific in the sense that it 

 may be provoked only by the sperm of the genus Ostrea. The males 

 are, however, entirely non-specific. Shedding of sperm may be stim- 



* For drilling the shell the use of an electric "Handee Grinder" (Chicago Wheel & 

 Manufacturing Company) is recommended. 



