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

Abstract

1. 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 Uca pugnax and U. pugilator when these two species were kept in 100% and 175% sea water.2. For crabs in 100% sea water the serum electrolyte values for Na, Mg and Cl are lower and those for K, Ca, NH4 and SO4 higher than in the medium; for crabs in 175% sea water the serum electrolyte values of Na, Mg, K, Ca, NH4 and Cl are lower and only SO4 higher than the values in the medium. The sera of crabs from both media are hypotonic to their saline environment.3. The electrolyte values of sera from crabs in normal sea water differ significantly from the gill fluid electrolytes for Mg, Ca and Cl only; while similar sera values from crabs in concentrated sea water differ significantly for Na, Mg, Ca, Cl and SO4. In all cases except for Ca from crabs in normal sea water the significant gill fluid electrolyte concentrations are greater than the corresponding sera values.4. Crabs in normal and concentrated sea water maintain their stomach fluids more concentrated than the external medium. Sera electrolyte concentrations from crabs in 100% sea water are significantly lower than stomach fluid concentration for Mg, K, Ca, NH4 and SO4. In crabs from 175% sea water corresponding serum electrolyte significance is found for Mg, Ca, NH4, Cl and SO4.5. All electrolytes are regulated by the antennary gland by crabs in the high salinity medium and all except Ca in the normal sea water; Mg and NH4 are especially controlled by the antennary gland. In concentrated media the antennary gland excretion of Na is significantly lower than in normal sea water while the Mg excretion is markedly elevated.6. Ammonia appears to be secreted by both the antennary gland and the stomach but its appearance in the gill fluid is attributed to diffusion.7. Urine osmotic and electrolyte concentrations are significantly higher than the corresponding serum concentrations for animals in both media.8. For crabs in 100% sea water the average fluid osmotic concentrations are equivalent to the following moles of NaCl: serum, 0.497; urine, 0.583; gill fluid, 0.506 and stomach fluid, 0.758; for crabs in 175% sea water the corresponding values are: serum, 0.587; urine, 0.683; gill fluid, 0.860 and for stomach fluid, 0.828.9. By the use of Na24, the relative specific activities of serum and gill fluid from crabs in 175% sea water are shown to be significantly higher than the corresponding serum and gill fluid values from crabs in 100% sea water while the RSA values of the urines are not significantly different. Na24 counts in gill tissue from the 175% sea water crabs are significantly lower than in the 100% sea water crabs. Active excretion of Na by the gills is indicated.10. The low isotopic concentration of the mid-gut gland from crabs in concentrated sea water, comparable to that of gill tissue, suggests a Na secretory mechanism for this organ. The high isotopic Na concentrations found in muscle and stomach tissues of crabs in 175% sea water indicate that these tissues may be serving as storage depots during periods of serum Na stress.11. The data show that the chief sites of entrance of water and electrolytes into these fiddler crabs are the stomach and the gills. They show that the chief sites of regulation are the antennary glands and the gills with some regulation by the stomach and possibly the mid-gut gland.