solid content of oysters occurs shortly after the 

 spawning; season. It will be shown later (ch. 

 XIV) that the gonads of sexually mature oysters 

 may constitute as much as 40 percent of the body 

 weight and volume, exclusive of the shell. The 

 loss of a considerable portion of gonad tissues may 

 acct)unt for the lower oxygen uptake. 



The amount of oxygen consumed by an orga- 

 nism during a unit of time depends on its weight. 

 In the oyster this relationship is obscured by 

 wide fluctuations in the proportion of solids to 

 water. In a series of tests made during the 

 second half of August at Woods Hole oysters of 

 different weights were selected from an oyster 

 bottom at Onset at the head of Buzzards Bay, 

 Mass. The total weight of individual oysters 

 varied from 80 to 203 g. and the wet weight of 

 their tissues ranged from 11.35 to 23.25 g. The 

 oysters had already spawned but still retained a 

 substantial amount of sex cells, with the exception 

 of one oyster in which the gonad was empty and 

 its sex was not recognizable. The data given in 

 table 26 are the mean values of oxygen uptake 

 computed for each oyster from 10 consecutive 

 readings made at half-hour intervals. The rate 

 of oxygen consumption per oyster per liour 

 varied from 3.97 to 7.29 mg. of oxygen. The 

 o.xygen consumption of the three heaviest oysters 

 (Nos. 1, 2, and 3) was higher than for the others, 

 but there are no significant dift'erences in oxygen 

 uptake per unit of dry weight according to oyster 

 size (fig. 188). The oxygen demand expressed in 

 this way varied between 2 and 3 mg. per hour. 



In tlie majority of tests the initial oxygen 

 consumption measured within tiie first hour after 

 placing the animal in the respiration chamber 

 was noticeably higher than in the successive 

 samples. This phenomenon recorded for five 



T.\BLE 26. — Oxygen uptake per hour of adult C. virginica 

 from Onset, Mass. 



3 7 



a 6 



,0, UPTAKE PER OYSTER 



Oj UPTAKE PER GRAM OF DRY WEIGHT 





13 



15 17 19 21 



WET WEIGHT IN GRAMS 



Figure 188. — Mean uptake of oxygen expressed in mg. 

 per oyster (circles) and mg. per 1 g. of dry weight per 

 hour (triangles) in relation to the wet weight of tissues. 

 Each item is a mean value of 6 to 10 consecutive deter- 

 minations of oxygen consumption of a single oyster. 

 Temperature 24.5° C. 



out of the total of six oysters (table 24), represents 

 the effect of an oxygen debt incurred during the 

 time the oysters were closed while being prepared 

 for the test. The rate of oxygen uptake usually 

 reaches a more or less stable level after a variable 

 period of adjustment to the new situation. In 

 figure 189 the rate of metabolism recorded con- 

 tinuously for 8.5 hours is plotted against time. 

 Partial closure of the valves was accompanied 

 by a decrease in oxygen consumption (fine B). 

 Both oysters A and B reached a steady level of 

 oxygen uptake after the initial periods of adjust- 

 ment, which in the case of oyster A required 

 4 hours. 



The rate of oxygen uptake decreases with a 

 partial closing of the valves, presumably because 

 of the decrease in the rate of water transport. 

 In the test summarized in figure 190 the two 

 oysters A and B remained in a steady state for 

 nearly 4^2 hours. One of them (oyster A, solid 

 line) then began to reduce the opening between 

 the valves and completely closed them at l)i 

 hours. The decline in oxygen consumption cor- 

 responded to the shell movement and registered 

 zero at the moment the shell was closed. No 

 measurable changes in o.xygen content in the 

 water were noted after the valves remained 

 tishtlv closed for some time. 



208 



FISH AND WILDLIFE SERVICE 



