The threshold temperature of spawning is not 

 a "critical" temperature in the sense that it 

 automatically induces the discharge of eggs in all 

 physiologically ripe oj-sters. The success or failure 

 of thermic stimulation depends on the responsive- 

 ness of the organism. It would be more appro- 

 priate to speak of the "critical condition" of the 

 organism which makes it responsive to stimulation 

 rather than of critical temperature of spawning. 

 Within broad limits between 15° and 32° to 

 34° C, spawning of C. lirginica may occur at any 

 temperature; mass spawning of an oyster popu- 

 lation is more likely to take place in warm water 

 above the 22° to 23° C. level. 



Stimulation by live sperm is of great importance 

 in the reproduction of Crassostrea oysters. In 

 the Woods Hole experiments the time elapsed 

 between the addition of sperm suspension and the 

 beginning of shedding of eggs varied between 6 

 and 38 minutes. At about 20° C. the sperm added 

 to the pallial cavity passed through the giUs and 

 was expelled from the cloaca within 7 to 8 seconds. 

 The latent period of spawning reaction lasts 

 several minutes. This suggests that possibly the 

 sperm acts upon the female organism after it has 

 been absorbed by the cells of the water transport 

 system or by the digestive tract. Direct evidence, 

 however, is absem since attempts to prevent the 

 penetration of sperm into the digestive tract by 

 plugging the mouth and esophagus were not 

 successful. 



Rhythmic contractions of the adductor muscle 

 are associated with the release of eggs from the 

 ovary and are not directly stimulated by tempera- 



ture or by any known chemical agent. This 

 becomes clear from the observations which show 

 that spawning contractions proceed in the same 

 manner whether the spawning was induced by 

 temperature or by sperm. Two kymograph 

 tracings of shell movements of the two females 

 shown in fig. 281 are similar in spite of the fact that 

 in one of them (upper line) spawning was induced 

 by the addition of sperm, while in the other by 

 rapidly warming the water from 21.6° to 30.2° C. 



Experiments were made to determine whether 

 some substances causing the contraction of the 

 adductor muscle are released into the blood stream 

 during spawning. A female was induced to 

 spawn by thermic stimulation, and a sample of its 

 blood withdrawn from the pericardium was 

 immediately injected into the visceral mass and 

 into the circulatory system of a sexually mature 

 but nonspawning female. Six e.xperiments of this 

 type were made with negative results. 



Shell movements during female spawning are so 

 typical that they cannot be nristiken from any other 

 type of muscular activity. Spawning reaction is 

 recognized by the duration of the latent period of 

 not less than several minutes; the uniformity of 

 the tonus level at the points of relaxation; regular 

 rhythm of the contractions, particularly at the 

 beginning of the reaction; and the presence of a 

 small "plateau" about half-way on the relaxation 

 curve (see fig. 281). The plateau is indicative of 

 the slowing down of the relaxation phase; its 

 significance is due to the fact that it coincides 

 with the oozing out of eggs through the ostia of 

 gill filaments (Galtsoff, 1938b). This type of 



22.1 



Figure 281. — Kymograph records of shell movements during spawning of two female C. virginica. Upper line — spawning 

 induced by the addition of sperm at 22.1° C; lower line — spawning induced bv rapid rise of water temperature from 

 26.1° to 30.2° C. Time interval, 1 minute. 



ORGANS OF REPRODUCTION 



309 



