hour, and those in which the mean oxygen con- 

 sumption was higher varying from 4.0 to 5.8 mg. 

 of oxygen per hour per oyster. These differences 

 could not be associated with sex or sexual maturity. 

 The three females of the group (Nos. 4, 8, and 11) 

 had a high metabolic rate, but an even higher 

 value was recorded for one of the males (No. 7). 

 Subsequent tests showed that all 11 oysters were 

 sexually mature and upon stimulation spawiied 

 copiously. 



For comparison the metabolic rates of six 

 oysters from Onset, Mass., were determined. 

 The same technique was used but the duration 

 of each observation was extended to 7.5 to 8.5 

 hours (table 24). The tests were completed 

 during the last week of July and the first week 

 of August. The mean oxygen uptake in these 

 oysters varied from 2.99 to 4.24 mg. per hour per 

 oyster, and the mean value for the entire group 

 was 3.47. After the test was finished the oysters 

 were examined and found to be partially spawned, 



Table 24. — Oxygen uptake in mg. of oxygen per hour per 

 oyster of six adult oysters from Onset, Mass. 



[Tests made between July 12 and .\ugust 7 after the completion of spawning. 

 Temperature 25.1° C] 



whicli suggested that their lower metabolic rate 

 possibly was associated with the loss of sex cells. 



To clarify this point three tests were made with 

 11 adult New England oysters of approximately 

 the same sizes as those of the Onset animals. 

 After the fu-st test, made early in July, the oysters 

 were induced to spawn several times in the labora- 

 tory and were tested again 2 weeks and then 1 

 month after spawning. The data shown in table 

 25 are the mean values of six consecutive readings. 



A significant decrease in the uptake of oxygen 1 

 montli after spawning was found in all these 

 oysters. The difference was less pronounced 2 

 weeks after spa\vning, possibly because of the 

 incomplete discharge of sex cells. The rapid 

 decrease in the metabolic rate is shown by plotting 

 the rates of oxygen consumption as percentages 

 of the initial rate observed before spawning (fig. 

 187). This inference that basal metabolic rate 

 decreases after spawning is substantiated by data 

 on seasonal changes in the composition of oyster 

 meat (ch. XVII) which shows that the lowest 



WEEKS AFTER SPAWNING 



NOTE: Mean of all observations 3.47±0.48 mg. oxygen per oyster per hour . 



' Partially closed from 1.5 to 2.0 hours; fully open at 2.6 hours. 

 ' Oyster partially closed from 6.0 to 6.6 hours. 



Figure 187. — Decrease in the rates of oxygen con.sumption 

 in 11 adult C. virginica from Long Island Sound after 

 spawning. The changes are plotted as percentages of 

 the initial oxygen uptake before spawning. 



T.A.BLE 25. — Mean oxygen uptake, in mg. per hour, per oyster, determined early in July before spawning, at 2 weeks and then 



1 month after spawning 

 The figures are the mean values of each test and were computed from six consecutive readings made at half hour intervals at temperature 24.0 to 25.0° C . 



TRANSPORT OF WATER BY THE GILLS AND RESPIRATION 



207 



