measurable values were recorded by Gaarder and 

 Eliassen even at temperatures approaching 0° C. 

 It may be assumed that under normal conditions 

 the valves would be closed at this low temperature 

 and ventilation of the gill stopped. 



Another experiment by Gaarder dealt with the 

 effect of oxygen tension on oxygen consumption. 

 The "critical oxygen tension" at which a decrease 

 in the oyster oxygen consumption becomes 

 apparent was found to be 4 ml. of oxygen per 1. 

 (at 22° C). If the figure is correct, it would 

 indicate that 0. edulis has a higher "critical 

 point" than the one reported for C. virginica in 

 which the rate of oxygen consumption begins to 

 diminish when the oxygen tension is reduced to 

 2.5 cm.^ per 1. or lower (Galtsoff and Whipple, 

 1931). Gaarder and Ehassen disagree with Peder- 

 sen's (1947) conclusion that 0. edulis can hve for 

 quite a while in water poor in oxygen. They 

 think this species shows a rather high "critical 

 point" of oxygen tension. In both 0. edulis and 

 C. virginica the uptake of oxygen is independent of 

 oxygen tension above the respective critical points. 



0. circumpicta, observed in a closed chamber 

 system of about 8-1. capacity containing a thick 

 layer of liquid paraffin, was found by Nozawa 

 (1929) to consume oxygen at the rate of about 3.2 

 ml. of oxygen per hour per 10 g. of wet tissues. 

 This rate is computed from Nozawa's published 

 data with an assumption that his figures of oxygen 

 uptake represent the cm.' of oxygen. The R.Q. 

 values of this species gradually increased during 

 the 22 hours of observations from 0.85 to 2.8. 

 The vahdity of the latter figure is questionable 

 and is probably due to accumulation of metab- 

 olites. Nozawa claims that oxygen consump- 

 tion of 0. circumpicta is independent of oxygen 

 tension until the latter is reduced to 0.1 percent 

 of its normal content in water. The figure 

 appears to be too low to be accepted without 

 further verification. 



UTILIZATION OF OXYGEN 



Bivalves use only a small portion of the oxygen 

 dissolved in the water which they transport 

 through the gills. The percentage of oxygen 

 consumed is the measure of the intensity of 

 utilization of oxygen. In most cases less than 

 10 percent of the oxygen available is removed 

 from the water (Hazelhoff, 1938). In comparison 

 to gastropods and cephalopods, which utilize up 

 to 80 percent of the available oxygen, the oxygen 



demand by bivalves is very low. The actual  

 figures of utilizaton vary depending on the condi- \ 

 tions of the moUusks. In Mya arenaria and in i 

 fresh-water Anodonta the normal utilization ranges 

 from 2 to 10 percent (Van Dam, 1938). The 

 low rate of utilization is due to the rapid transport 

 of water which both mollusks have to maintain 

 in order to obtain a sufficient supply of food. 

 Van Dam reports (1937, 1938) that in many 

 cases when the respiratory current was slowed 

 down or when before making the test the mollusks 

 were left in the air for 20 hours, as much as 97 

 percent of the oxygen was utilized. 



The rate of oxygen consumption is usually 

 higher after a period of interruption of respiratory 

 current or after exposure of mollusks to air. This 

 compensation by oysters for an oxygen debt has 

 also been observed in Mya arenaria, and in 

 Anodonta cygnea (Koch and Hers, 1943). The 

 authors maintain that the rate of ventilation of 

 the gills of Anodonta is regulated both by the need 

 of the animal for oxygen and by the availability 

 of oxygen in water. Oxygen determination in 

 their experiments was made by means of a polaro- 

 graph. By this method is was possible to record 

 photographically the continuous changes in the 

 oxygen content of water of the exhalant current. 

 The inference the authors draw from their observa- 

 tions is that the regulation of the branchial current 

 in Anodonta by contraction of the exhalant siphon 

 has relation to the intensity of metabolic processes. 

 They found that the periods of closure of siphons 

 are longer in water rich in oxygen and become 

 shorter when the oxj'gen content is low. The 

 technique of dropping mercury electrodes 

 (Petering and Daniels, 1938; F0yn, 1955; Brezina 

 and Zuman, 1958) appears to be promising and 

 should be applied in further study of respiration 

 in mollusks. 



The coefficient of oxj-gen utilization in the 

 oyster (percent of oxj'gen removed from the water 

 as it passes through the gills) has not been deter- 

 mined. The data of the metabolism tests cannot 

 be used for this purpose because the actual rate of 

 water transport cannot be measured in an oyster 

 kept in the respiratory chamber. The flow of 

 water through the chamber was maintained at a 

 rate lower than the expected rate of water transport 

 through the gills and, consequently, it is reasonable 

 to expect that the water in the chamber passed 

 through the gills several times before it reached 

 the outlet. 



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FISH AND WILDLIFE SERVICE 



