Physiological studies on supercooled killifish (<i>Fundulus heteroclitus</i>) II. Serum organic constituents and the problem of supercooling

Abstract

Abstract To gain insight into the mechanism permitting Fundulus heteroclitus to survive subzero temperatures in a supercooled state, previously published data on inorganic components of the serum were complemented by studies on serum organic constituents in parallel groups of adult male killifish acclimated to various temperatures (30°C, 20°C, 10°C, 4°C, 2°C, −1°C and −1.5°C) in salt water under otherwise constant laboratory conditions. When the acclimation temperature was lowered from 20°C to −1.5°C, there was little or no change in levels of serum total non‐protein nitrogen, total amino acids, urea and non‐glucose free carbohydrates. Total serum protein levels were usually unaffected by the cold, but occasionally showed a decrease. Serum total cholesterol levels increased by 69% in the subzero cold, presumably due to an increased lipid metabolism. Whereas most serum constituents (organic and inorganic) showed only minor changes in supercooled F. heteroclitus , serum glucose levels increased from three‐ to sixfold. Serum glucose levels of 68 mg% in fish at 20°C increased to 460 mg% in sexually regressed fish at −1.5°C but to only 280 mg% in supercooled mature fish. It was suggested that the elevation of serum glucose in the subzero cold permitted survival in a supercooled state, presumably by retarding ice crystal growth on any ice nuclei that might form spontaneously in the blood.