Figure 186. — Van Dam's method of sampling sea water 

 from the exhalant current of sea scallop. Samples are 

 taken by syringe pipette P (shown in part, left side) 

 from the end of siphon with the lower part introduced 

 into the cloaca, CI. The horizontal arm of the siphon 

 is about 5 mm. inside the cloaca. S — scallop. From 

 Van Dam, 1954. 



study of respiration of the scallop. The presence 

 of the tip of the siphon in the cloaca does not in- 

 terfere with the normal propulsion of water, pro- 

 vided the needle does not touch the scallop. Be- 

 fore the first samples were taken Van Dam allowed 

 the water from the cloaca to pass through the 

 siphon for about 1 hour at the rate of 1.5 ml. per 

 minute. Such a slow rate of collecting was con- 

 sidered a guarantee that the sample was not con- 

 taminated with outside water. The difference 

 between tne oxygen content of the water that 

 entered the gill and of the water of the exhalant 

 current showed the percentage of utilization of 

 oxygen. 



In bivalves with long and narrow siphons, as 

 for instance Alya or Mercenaria, water leaving the 

 exhalant aperture probably has a uniform content 

 of oxygen. In species lacking siphons the cloaca 

 opens as a wide cone-shaped slot and the stream 

 of water escaping from the cloaca contains con- 

 siderable and variable amounts of outside water 

 depending on the distance from the epibranchial 

 chamber. This introduces uncertainty in inter- 

 preting the results of the test. Van Dam (1954) 

 found that in bay scallops the oxygen content of 

 the samples taken at different positions within the 

 cloacal current varied from 28.5 to 66.6 percent 

 of the oxygen content of the inhaled water. Be- 

 cause of this uncertainty the method does not 

 appear to be suitable for measuring tne true oxygen 



utilization of bivalves unless the tip of the col- 

 lecting syi'inge is introduced deep inside the 

 epibranchial chamber of the gills. Since the rate 

 of propulsion of water and the volume of water 

 transported are not known, the total quantity of 

 oxygen used cannot be determined. This greatly 

 limits the usefulness of the method. The main 

 advantage of microdetermination methods is that 

 the mollusks may be kept in running sea water 

 under conditions which closely approach their 

 normal environment. 



The selection of a method for respiratory studies 

 must be governed by the purpose and conditions 

 of the experiments. All closed system methods 

 are suitable for tests that should be completed 

 before the depletion of oxygen begins to affect the 

 respu-atory rate. The microanalytical methods 

 are suitable for determining the utilization of 

 oxygen by bivalves (i.e., the percent of oxygen 

 removed by tissues during the transport of water) 

 provided the samples of the exhalant current are 

 not contaminated with outside water. This is 

 nearly impossible to avoid in species like scallops 

 and oysters, which have no siphons. The use of 

 the respiratory chamber with a constant rate of 

 flow of water seems to be the most satisfactory 

 technique for long-term observations on oysters. 

 The method gives reproducible values of the 

 oxygen uptake over a period of many hours. The 

 following discussion of the respiration of the 

 oyster is based primarily on results obtained in 

 the Bureau's shellfish laboratory with this method. 



OXYGEN UPTAKE 



The rate of oxygen uptake is influenced by 

 several extrinsic and intrinsic factors. The first 

 group includes seasonal and diurnal changes in 

 the temperature and salinity of water, and the 

 occasional presence of contaminants or other 

 envu'onmental changes, such as an abundance of 

 unicellular algae which may depress the rate of 

 respiration. The existence of the intrinsic factors 

 becomes apparent in observations of differing 

 metabolic rates in oysters of known origin and 

 uniform size and age kept under constant condi- 

 tions (see: p. 207). Some of the intrinsic factors 

 are associated with differences in the contents 

 of water and glycogen in the tissues; with the 

 loss of solids due to the discharge of sex cells 

 during spawning; and with generally poor condi- 

 tion of the oysters. In a comparative study of 

 respiratory rates several precautions are necessary 

 to minimize the extent of individual variations. 



TRANSPORT OF WATER BY THE GILLS AND RESPIRATION 

 733-851 O— 64 14 



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