Figure 278. — Spawning of large femalo C. virginica, photographed in the laboratory. 



by sperm drawn in from the outside with the 

 respiratory current and are extruded as well-de- 

 veloped larvae. The process is called "swarming" 

 (Korringa, 1941). Careful studies of shell move- 

 ments of 0. luri'la or 0. edulis during the repro- 

 ductive period nuiy uncover some peculiarities of 

 the behavior of their adductor muscles associated 

 with swarming. 



If the shell movements of a spawning female are 

 prevented by cutting off a piece of valve between 

 the adductor muscle and the hinge, the eggs can- 

 not pass through the gills and are discharged 

 through the cloaca. This has been demonstrated 

 in the experiments illustrated in fig. 280, A and B. 

 In both cases fully mature Cape Cod oysters 

 were placed in finger bowls under a low-power 

 binocular microscope. In oyster A the gills were 

 exposed by cutting off a piece of the right valve 

 without injuring the adductor nniscle. Its shell 

 movement remained normal. In oyster B the en- 

 tire dorsal half of the right valve above the muscle 

 attachment was removed, and in this way shell 

 movements were prevented. During the spawning 

 of oyster A the released eggs (e) passed through 



the gills, while in oyster B they were discharged 

 through the cloaca. 



Female spawning of C. gigas and 0. cucullaia 

 follows the same pattern as the American oyster 

 (Galtsoff, 1932). It is apparent that the mantle, 

 gills, and adductor muscle of Crassostrea species 

 temporarily assume the role of accessory sex organs 

 and through coordination and adjustment of their 

 activities perform a specific role which is distinct 

 from their primary functions. 



The release of sex cells from sexually mature 

 oysters often requires a stimulus which causes a 

 triggerlike effect and initiates spawning. Very 

 often this effect is associated with a sudden rise 

 in the temperature of the water. Numerous 

 ecological observations show that under natural 

 conditions oysters spawn at rising temperature. 

 This led to the concept of "critical temperature," 

 and for many years the temperature of 20° C. was 

 considered the lowest at which spawning takes 

 place. It was postulated that "once this critical 

 temperature' of 20° C. is reached a trigger mecha- 

 nism is released which requires some hours for its 

 consummation" (Nelson, 192Sa). Further obser- 



306 



FISH AND WILDLIFE SERVICE 



