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A study of the osmoregulatory role of the antennary glands in two species of intertidal crabs Stone, Dmitry David

Abstract

Two species of intertidal crabs, Hemigrapsus oregonensis and H. nudus ocour in large numbers at Spanish Bank, Vancouver, British Columbia. The area is characterized by sea water of high temperature and low salinity in summer and low temperature and high salinity in winter. The crabs osmoregulate strongly in low salinities, keeping blood considerably hypertonic to the external medium. They do not regulate strongly in salinities higher than those normally found in the field in winter (70-80% sea water). This study attempts to establish the role of the excretory organs (antennary glands) in osmoregulation. Effects on their function of seasonal adaptation, temperature, osmotic stress and body size were also investigated. Experimental temperatures of 5°, 15° and 25° C and media of 6%, 12%, 25%, 75%, 100%, 125%, 150% and 175% sea water were used (100% sea water: 31.88% salinity). Animals were brought into the laboratory and equilibrated in 75% sea water for 36-48 hours at the experiments temperature. After equilibration, groups of 10-15 animals were transferred to each of the experimental salinities. After 3, 24 and 48 hours, 10 urine samples were drawn from each group, sealed in separate capillary tubes and quick-frozen. Osmotic concentration was measured by the method of melting point determination. Identical series of experiments were carried out, summer and winter. Procedures differed only in that summer animals were damp-dried and weighed before sampling. For each species and experimental temperature, a series of urine osmotic response curves was drawn. Data for summer-adapted animals at 15° C and winter-adapted animals at 5° C were used for most comparisons. These approximated seasonal mean field temperatures. Osmotic gradients between urine and media at 4-8 hours formed the basis for comparison of seasonally-adapted responses. Data were analysed for salinity and temperature effects and seasonal differences by means of Student's “t" test, which was used also to evaluate differences between urine and blood concentrations. Differences attributable to weight, and interspecific differences in U/B ratios were analysed by means of Wilcoxon's Matched-Pairs Signed-Hanks test. Concentration of urine was found to fall in dilute, and rise in concentrated media at rates, directly related to osmotic stress, which declined with time and were influenced by the seasonal adaptation of the animals and the experimental temperature. New equilibria were generally established by 48 hours, at levels, particularly in concentrated media (above 100% sea water), which were considerably higher in summer- than in winter-adapted animals. Hyper-osmotic regulation was achieved in summer-adapted animals with the production of blood-isotonic urine, implicating extra-renal mechanisms. In winter-adapted animals, hyper-osmotic regulation was enhanced by production of blood-hypotonic urine. Summer-adapted animals appeared to resist blood change in 100-150% sea water by producing blood-hypertonic urine, and although this resistance was maintained longest in 100% and 125% sea water, blood soon became hypertonic. In general, cooling retarded, and warming stimulated salt absorption and regulation. Winter-adapted animals in high salinities did not effectively resist blood change, and both urine and blood quickly became hypertonic. Effects on urine concentration of cooling or warming summer-adapted animals and warming winter-adapted animals were significant only in low and intermediate salinities. Body size had, in some cases, significant effects on urine concentration. Small H. nudus. taken from summer field conditions, had urine significantly hypertonic to that of large animals. This was also true of H. oregonensis at 15° in concentrated media. In winter-adapted animals, H. oregonensis had total osmotic U/B ratios significantly higher (nearer unity) than H. nudus for the whole range of experimental salinities at 50 C. In summer-adapted animals at 15° C, U/B ratios approached unity in both species. Seasonal adaptation of osmoregulatory mechanisms in both species altered the balance of active processes so that urine was lower, both in absolute concentration and relative to blood, in winter than in summer.

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