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Ion regulation in two species of inter-tidal crabs. Carefoot, Thomas

Abstract

Estuarine animals are faced with environmental conditions which favour dilution of the body fluids by water uptake and ion loss. The mechanisms employed by two species of estuarine crabs, Hemigrapsus nudus and Hemigrapsus oregonensis, to maintain ionic stability in sea water of varying salinity were investigated. The concentrations of sodium, potassium, calcium and magnesium ions in the blood and urine of these species were measured at eight salinities (6-175% sea water), three temperatures (5°, 15° and 25°C.) and in summer and winter. In dilute salinities (6-75% sea water), concentrations of sodium, potassium and calcium ions in the blood of both species were considerably hypertonic to the media concentrations. In high salinities (100-175% sea water), the concentrations of these ions approached isotonicity with the media. Hypertonic ion regulation appears to be primarily effected by extra-renal mechanisms since the urine-blood ratios (U/B) approximate unity. It is suggested that the principal site of ion uptake in dilute sea water may be the lamellar cells in the gill epithelium. Magnesium ion concentrations in the blood were maintained in both species at constant, hypotonic levels in all salinities above 12% sea water. This appeared to result solely from kidney activity since the U/B magnesium ratios were markedly greater than unity over most of the salinity range. It is thought that low blood magnesium levels are necessary to facilitate neuromuscular impulse transmission. Changes in blood and urine concentrations of sodium, potassium and calcium ions in 25, 75 and 125% sea water required at least 24 hours to be completed, whereas the response of blood concentrations of magnesium ion was comparatively rapid, the major changes occurring after 12 hours. Although seasonal differences in the ability of Hemigrapsus to regulate sodium and potassium were not consistent, in dilute salinities calcium was found to be more effectively regulated by winter animals than by summer ones. Magnesium levels in the blood of both species were equivalent in the two seasons. No effect of temperature on the body fluid concentrations of sodium, potassium, or calcium was demonstrated at either season in Hemigrapsus. The magnesium regulatory mechanism in both species, however, appeared to be gradually impaired as the temperature increased from 5° to 25°C. This may have resulted from a decrease in available energy, since all other metabolic processes were accelerated. Hemigrapsus oregonensis was more effective in regulating winter blood concentrations of sodium in high salinities than H. nudus. The ability to regulate blood magnesium levels was almost identical in the two species. Higher urine magnesium concentrations in H. oregonensis, as compared to H. nudus, were related to the more permeable exoskeleton in the former species.

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