TUNAS AS SMALL AS 207MM FORK LENGTH CAN ELEVATE MUSCLE TEMPERATURES SIGNIFICANTLY ABOVE AMBIENT WATER TEMPERATURE

Tunas (family Scombridae) maintain muscle temperatures (T-m) elevated significantly above ambient water temperature (T-a) by using vascular counter-current heat exchangers (retia mirabilia) to conserve metaboli c heat generated by continuous contraction of red muscle fibers. Previ ous work has documented endothermy and both behavioral and physiologic al thermoregulation in tunas, but only individuals greater than approx imately 1 kg body mass had been studied. The purpose of this study was to measure T-m and to examine heat exchanger morphology in smaller fi sh in order to determine the minimum size at which tunas can elevate T -m significantly above T-a. Measurements of T-m made immediately after capture in field-caught and laboratory-raised tunas (Euthynnus lineat us, Auxis thazard, A. rochei and Thunnus albacares) spanning a size ra nge of 15.5-667 mm fork length (FL) show that tunas 207 mm FL (162.3 g ) and larger can elevate red muscle temperature by at least 3.0 degree s C above T-a E. lineatus, A. thazard and A. rochei of this size posse ss blood vessels serving the red muscle that are arranged as both late ral and central heat exchangers; the smallest tuna with evidence of a rete was a 108.5 mm FL E. lineatus. In the ectothermic scombrid Scombe romorus sierra (108.7-132.0 mm FL), T-m was elevated 0.2-0.5 degrees C above T-a, and the greatest T-m elevation reported in any ectothermic species is only 2.7 degrees C. Taken together, these data provide evi dence that tunas greater than 207 mm FL are endothermic.