HEAT CONSERVATION IN TUNA FISH MUSCLE

Abstract

In 1835, Davy,' noting the mammalian-like red meat and copious blood of the bonito (Thynnus pelamys Cav.), measured its temperature and found it to be 100C higher than the water. From conversations with intelligent fishermen he learned that the bluefin tuna also has a high body temperature. There have been numerous other measurements showing that for various species and water temperatures the tuna may be from 0 to 140 higher than the water.'-5 Marlin are also said to have a temperature elevated by several degrees,4' 6 but other fish examined were at most a fraction of a degree above ambient temperature. The benefits which a fish might obtain by being at a temperature above that of its environment include increased rates of processes such as digestion, conduction of nerve impulses, and speed of muscle contraction. Despite the possible advantages, most fish cannot be appreciably warmer than the water because all blood passes through the gills before reaching the tissues. If equilibration of gases occurs between the blood in the gills and the surrounding water, then temperature equilibration will also occur, for thermal diffusion is more rapid than molecular diffusion. On leaving the gills, the blood is thus at the same temperature as the water. Metabolic heat will warm the blood when it reaches the tissues, but metabolism is limited by the amount of oxygen carried by the blood. One milliliter of fish blood with a high oxygen-carrying capacity can bind about 0.2 ml of oxygen. The caloric yield from respiring this amount of oxygen is about one calorie, the amount of heat necessary to warm 1 ml of blood by 10C. Since the cooling and oxygen uptake are linked by the physical process of the passage of blood through the gills, it would seem impossible for a fish to maintain its body temperature more than one degree above that of its environment by metabolic processes alone.