Centimeters 



Figure 280. — Experiment showing the role of shell movements in the discharge of spawned eggs of C. virgivica through 

 the gills. A. — Portion of the right valve was removed to expose the gills; the adductor muscle was not injured and 

 shell movements during spawning were normal. Eggs (e) pass through the gills. B. — Portion of the right valve 

 between the adductor muscle and the hinge was cut off to prevent shell movements. Eggs (e) pass through the 

 cloaca. Drawn from life. 



spawning oyster are recorded on a kymograph, 

 and the presence of discharged eggs or sperm in 

 the water is ascertained by microscopic examina- 

 tion of samples taken at frequent intervals. In 

 the case of heavy spaw^ning so many se.x cells may 

 be shed that the water becomes milky and opaque; 

 when there is light spawning the presence of eggs 

 should be checked by collecting material which 

 settles on the bottom of the tanks. 



Spaw'ning of sexually ripe females of C. mrgrnica 

 may be induced by warming the water from 18° 

 to 20° C. to 22° to 23° C. and maintaining this 

 temperature for several hours. Relatively few 

 oysters respond to this mild stimulation. A more 

 effective method, which in my experience gave 

 positive results in about 40 percent of the oysters 

 tested, consisted in rapidly raising the temperature 

 of the water from about 20° C. to 33° to 34° ('. 

 The remaining 60 percent of the oysters which 

 did not respond to thermic stimulus required 



additional stimulation by live sperm. Using this 

 technique I found that the population of oysters 

 from a single small bed in Onset, tested within a 

 few days, consisted of individuals which greatly 

 varied in the degree of their response to spawning 

 stimuli. The tests were made at intervals of 

 2° C. The females that failed to spawn at 22° to 

 23° C. spawmed at this temperature when sperm 

 was added to the water. Some of the oysters 

 spawned at 2.5° to 27° C, but still others required 

 the addition of sperm to induce ovulation at this 

 temperature level. Similar results were obtained 

 at 29° to 31° C. and 32° to 33° C. In each of the 

 groups tested there were specimens which did not 

 respond to the rise of temperature and required 

 additional stimulation by live sperm. All the 

 oysters used in these experiments were mature; 

 they had fully developed gonads, the eggs were 

 lertili/.able, and spawning, when induced, was 

 Copious. 



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FISH AND WILDLIFE SERVICE 



