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

Tunas (family Scombridae) maintain muscle temperatures (Tm) elevated significantly above ambient water temperature (Ta) by using vascular counter-current heat exchangers (retia mirabilia) to conserve metabolic heat generated by continuous contraction of red muscle fibers. Previous work has documented endothermy and both behavioral and physiological thermoregulation in tunas, but only individuals greater than approximately 1 kg body mass had been studied. The purpose of this study was to measure Tm and to examine heat exchanger morphology in smaller fish in order to determine the minimum size at which tunas can elevate Tm significantly above Ta. Measurements of Tm made immediately after capture in field-caught and laboratory-raised tunas (Euthynnus lineatus, Auxis thazard, A. rochei and Thunnus albacares) spanning a size range of 15.5­667 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°C above Ta. E. lineatus, A. thazard and A. rochei of this size possess blood vessels serving the red muscle that are arranged as both lateral and central heat exchangers; the smallest tuna with evidence of a rete was a 108.5 mm FL E. lineatus. In the ectothermic scombrid Scomberomorus sierra (108.7­132.0 mm FL), Tm was elevated 0.2­0.5°C above Ta, and the greatest Tm elevation reported in any ectothermic species is only 2.7°C. Taken together, these data provide evidence that tunas greater than 207 mm FL are endothermic.