THE REGULATION OF WATER AND SALT BY THE FIDDLER CRABS, UCA PUGNAX AND UCA PUGILATOR

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Abstract
Analyses were made of the serum, urine, gill and stomach fluids for total osmotic concentration and the electrolytes Na, Mg, K, Ca, NH4, Cl and SO4 in U. pugnax and U. pugilator when these species were kept in 100% and 175% sea water (S.W.). The sera of crabs from both media were hypotonic to their saline environments and differed in different ways in their ionic com-position from these environments. Urine osmotic and electrolyte con-centrations were significantly higher than the corresponding serum concentrations for crabs in both media. All electrolytes were regulated by the antennary gland by crabs in 175% S. W. and all except Ca in normal S. W. In the high salinity medium the antennary gland excretion of Na was significantly lower and Mg significantly higher than in normal S. W. In both groups of crabs the stomach fluids-were more concentrated than the corresponding medium. While Na24 was taken up more rapidly by crabs in the concentrated medium, urine specific activities were the same for both groups; Na24 specific activity for the gill fluid was lower for crabs in 100% S.W. and Na24 counts from the gill tissues of these crabs was higher than for those in 175% S.W. These findings indicate Na excretion by the gills.

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