Baldwin (1935) suggested that the enzyme 

 arginase may be involved in the elaboration of 

 both urea and uric acid in gastropods. He found 

 no evidence of its presence in Mytilus edulis or 

 Pecten (rpercularis although the enzyme was found 

 in fresh-water Anodonfa. Brunei (193S) points 

 out that many investigations have shown that 

 uric acid and allantoine (the product of oxidation 

 of uric acid) are not found in bivalves, and 

 Needham (1935) considers that its absence in 

 that class and its presence in land snails are 

 adaptations to terrestrial life by uricotelic orga- 

 nisms, which often can not find sufficient water for 

 their needs and avoid toxemia by converting 

 poisonous ammonia into the insoluble and rela- 

 tively innocuous uric acid. The body of a marine 

 bivalve is, as a rule, permeable to water and to 

 small molecules. Consequently, the ammonia 

 formed during catabolism easily escapes by 

 diffusion into the external environment. 



Table 32. — Excretion of nitrogen in three species of bivalves 

 in grams of nitrogen per total nitrogen excreted 



[From Handbook of biological data, 1956, Spector, ed., p. 43] 



OSMOREGULATION 



It is generally considered that marine bivalves 

 have little power of osmoregulation when placed 

 in diluted sea water (Morton, 1958), and can pre- 

 vent loss of salts only by closing their valves. 

 There is, however, an indication tiiat the excretory 

 system may serve as an osmoregulating organ. 

 Kumano (1929) reported almost complete agree- 

 ment between the ionic concentrations of the 

 blood and pericardial fluid of O. circumpicta and 

 the sea water in which the oyster was kept. The 

 exception was reported only for magnesium and 

 sulfate ions, which were higher in the blood liv 1 1 

 and 7 percent respectively than in the sea water. 

 The corresponding values for the pericardial fluid 

 were 5 percent higher for magnesium and 2 

 percent lower for sulfate. 



Robertson (1949, 1953) found that marine bi- 

 valves and gastropods {Pecten, Mya, Ensis, Pleu- 

 rohranchus, Neptiinca) accumulate potassium and 

 calcium and eliminate sulphate to a small degree. 



The range of values expressed in percentage of 

 ionic concentration in the surrounding sea water 

 was as follows: sodium, 97 to 101; calcium, 103 to 

 112; magnesium, 97 to 103; chlorine, 99 to 101; 

 sulphate, 87 to 102. 



Robertson defined ionic regulation as the main- 

 tenance of ionic concentrations in the body fluid 

 differing from those found in a passive equilibrium 

 witii the external medium. Accordingly, he made 

 an analysis of a body fluid obtained from a mol- 

 lusk and determined its ionic concentration. A 

 second determination was made using the same 

 fluid after it had been dialyzed in a collodion sac 

 against the original sea water. The results repro- 

 duced in table 33 show that Mytilus and Ostrea 

 exert little ionic regulation in their coelomic fluid 

 or plasma, apart from the accumulation of potas- 

 sium. Magnesium remains within 3 to 4 percent 

 of the equilibrium while calcium sometimes ex- 

 ceeds this by a few percent. The accumulation of 

 sulphates by Mytilus galloprodmialis was re- 

 garded by Robertson as a rare feature in marine 

 invertebrates. The protein content of blood 

 plasma was found to be low (0.2 to 0.3 percent) in 

 0. edulis and significantly higher in M. gallopro- 

 vincialis. 



Table 33. — Ionic concentrations in plasma as percentage 

 of concentrations in dialyzed plasma {according to 

 Robertson, 1953) 



[Data based on pooled samples] 



The suggestion that some sort of osmoregu- 

 lation is present in C. virginica was made by 

 Fingerman and Fairbanks (1957, 1958), who 

 placed Louisiana oysters in sea water of three 

 different salinities. After periods up to 14 days, 

 samples were taken from the ventricles, the peri- 

 cardial cavity, the reservoir of the excretory 

 system, the secretory portion of the nephridia, 

 and the mantle. The autliors claim that at the 

 highest concentration of 515.6 milliequivalents of 

 chloride per 1. all fluids were hypotonic and at low 

 salinities (161.7 milliequivalents of chloride per 1.) 

 all body fluids were hypertonic to the environ- 

 ment. The original data are not given, and the 

 results are presented as average differences. The 

 "t" test of sia;nificance ransrins; in the values of P 



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FISH AND WILDLIFE SERVICE 



