Meeting Abstract
In Pacific bluefin tuna, vascular counter-current heat exchangers composed of numerous parallel arterioles and venules conserve metabolic heat, allowing temperatures of the slow-oxidative (red) locomotor muscle and visceral organs to be elevated above water temperature (= regional endothermy). We used a thermocouple thermometer to measure maximal tissue temperatures immediately after capture in juvenile Pacific bluefin tuna specimens ranging in size from 18.4 to 62.5 cm fork length (FL), encompassing the size range over which the capacity for regional endothermy develops. We quantified the amount and distribution of the red muscle in frozen, sectioned, whole individuals, and examined the counter-current heat exchangers using standard paraffin histology. The temperature of the red muscle and the viscera were elevated significantly above ambient water temperature in individuals > 33.3 cm and 59.7 cm FL, respectively. The amount of red muscle increased isometrically with fish mass. Red muscle was contiguous with the skin in the smallest individuals, but in larger individuals white muscle fibers were positioned between the skin and the red muscle. The number of blood vessels within the heat exchangers increased with fish size. Increases in both the mass of tissues that produce heat and the size of the counter-current heat exchangers that retain heat contribute to the transition to regional endothermy in the Pacific bluefin tuna.
