(full paper is archived in the Miller Library)
Title: Morphological and biochemical evidence for countercurrent heat exchange in the tuna caecum
Student Author(s): Jaron, Tamara
Faculty Advisor(s): Block, Barbara
Location: Final Papers Biology 175H
Date: June 1998
Abstract: The viscera of bluefin tuna, Thunnus thynnus, are warm, and internal temperatures as high as 25 degrees C have been recorded in 5 degrees C water (Block, pers. comm.). The source of this heat and the visceral site of vascular heat conservation are not clear. Measurements taken after feeding of bluefin implicate the caecum as the warmest visceral organ, however, little is known about the general anatomy of the tuna caecum or its method of heat production. Specimens of yellowfin, Thunnus albacares, and blackfin, Thunnus atlanticus, were used to study the anatomy, histology, and aerobic capacity of the tuna caecum. Anatomical comparisons were also made to the bonito, Sarda chiliensis, an ectothermic relative of the tuna. Injections of silicon into the caecum vasculature detected a parallel array of arteries and veins throughout the organ. Measurements of citrate synthase activity, an enzymatic indicator of tissue metabolic capacity, show a high degree of caecum metabolic activity, around 20 units/g tissue at 25 degrees C. This value is 4-5 fold greater than tuna white anaerobic muscle, but not as high as the aerobic red muscle. The high heat contribution from aerobic activity, along with that contributed by digestive processes within the caecum and stomach, may be able to account for much of the heat production within the viscera. The endothermic nature of the organ and the parallel architecture of the vasculature suggest that countercurrent heat exchange can occur. An increased visceral temp would lead to faster digestion and provide a continuous source of fuel for the tuna's high metabolic needs.