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 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 T_m and to examine heat exchanger morphology in smaller fish 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 lineatus, Auxis thazard, A. rochei and Thunnus albacares) spanning a size range of 15.5–667mm fork length (FL) show that tunas 207mm FL (162.3g) and larger can elevate red muscle temperature by at least 3.0°C above T_a. 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 108.5mm FL E. lineatus. In the ectothermic scombrid Scomberomorus sierra (108.7–132.0mm FL), T_m was elevated 0.2–0.5°C above T_a, and the greatest T_m elevation reported in any ectothermic species is only 2.7°C. Taken together, these data provide evidence that tunas greater than207mm FL are endothermic.