Meeting Abstract
P2.176 Saturday, Jan. 5 Extraocular muscles as a potential heat source for cranial endothermy in tunas HOLTZ, S B*; DICKSON, K A; California State University Fullerton; California State University Fullerton sholtz@fullerton.edu
In endothermic tunas (family Scombridae) counter-current heat exchangers, retia mirabilia, associated with the prootic region of the skull conserve metabolic heat, allowing cranial temperatures to be elevated above ambient water temperature (cranial endothermy). In tunas, although the retia have been described, little is known about the source of metabolic heat used in cranial endothermy. We hypothesized that one or more of the six extraocular muscles serve as the source of heat for cranial endothermy in 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 extraocular muscles of five scombrid species: three endotherms– 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 extraocular muscle with the greatest CS specific activity and relative mass within each species varied interspecifically. The medial rectus muscle in T. orientalis was the only extraocular muscle to have both a greater CS activity and mass relative to the other extraocular muscles. In the majority of comparisons, the extraocular muscles 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 tunas have evolved an elevated heat production capacity in their extraocular muscles for cranial endothermy. Furthermore, no histological evidence of modification for heat production was observed. Therefore, the presence of retia mirabilia alone may be sufficient to elevate cranial temperatures above ambient water temperatures.
