Extraocular muscles as a potential heat source for cranial endothermy in tunas.

摘要

In endothermic tunas (family Scombridae), cranial counter-current heat exchangers, retia mirabilia, conserve metabolic heat, allowing cranial temperatures to be elevated above ambient water temperature (cranial endothermy). Although the retia have been described, little is known about the source of metabolic heat used in cranial endothermy in tunas other than Allothunnus fallai. It was hypothesized that one or more of the six extraocular muscles (EOMs) serve as the source of heat for cranial endothermy in those tunas. The specific activity of the enzyme citrate synthase (CS units g-1 of muscle) and muscle mass were measured as indices of heat production potential in all six EOMs of five scombrid species: three endothermic tunas -- Pacific bluefin (Thunnus orientalis), yellowfin (T. albacares), and skipjack (Katsuwonus pelamis) -- and two ectotherms -- eastern Pacific bonito (Sarda chiliensis) and Pacific chub mackerel (Scomber japonicus). The EOM with the greatest CS specific activity and relative mass within each species varied interspecifically. The medial rectus muscle in T. orientalis was the only EOM to have both a greater CS activity and mass relative to the other EOMs. In the majority of interspecific comparisons, the EOMs of the ectothermic S. chiliensis had a CS activity and relative mass greater than or equal to that of endothermic scombrids. Overall, this study did not provide evidence that the tunas studied have evolved an elevated heat production capacity in their EOMs for cranial endothermy. Furthermore, no histological evidence of modification for heat production was observed. Therefore, the presence of retia mirabilia alone appears to be sufficient for elevating cranial temperatures above ambient water temperatures in these tuna species.