Body fluid volume regulation in elasmobranch fish.

This review addresses an often overlooked aspect of elasmobranch osmoregulation, i.e., control of body fluid volume. More specifically the review addresses the impact of changes in blood volume in elasmobranchs exposed to different environmental salinities. Measurement of blood volume in the European lesser-spotted dogfish, Scyliorhinus canicula, following acute and chronic exposure to 80% and 120% seawater (SW) is reported. In 80%, 100% and 120% SW-adapted S. canicula, blood volume was 6.3+/-0.2, 5.6+/-0.2 and 4.6+/-0.2 mL 100 g(-1) body mass, respectively. Blood volume was significantly higher and lower in 80% and 120% SW-acclimated animals compared to 100% SW controls. Comparisons are made between these results and previously published data. The role of drinking and volume regulation in elasmobranchs is discussed. For the first time measured water reabsorption rates and solute flux rates across the elasmobranch intestinal epithelia are presented. Water reabsorption rates did not differ between 100% SW-adapted bamboo shark, Chiloscyllium plagiosum, and fish acutely transferred to 140% SW. For the most part net solute flux rates and direction for both the 100% and 140% SW groups were the same with the exception of a net efflux of chloride and potassium in the 140% group and influx of these ions in the 100% adapted group. The significance of the intestine as part of the overall elasmobranch osmoregulatory strategy is discussed as is the role of the kidneys, rectal gland and gills in the regulation of body fluid volume in this class of vertebrates.