Meeting Abstract
P3.140 Thursday, Jan. 6 Extra-ocular muscles as a possible source of heat for cranial endothermy in lamnid sharks KEHRIER, C.L.*; DICKSON, K.A.; California State University Fullerton; California State University Fullerton c_kehrier@msn.com
In the lamnid sharks orbital, retia mirabilia conserve metabolic heat, allowing eye and brain temperatures to be elevated above ambient water temperature. Warm blood transported from the locomotor red muscle to the orbital retia contributes some heat, but an additional source of heat is needed to maintain cranial temperatures at the levels previously reported in these sharks. We hypothesized that one or more of the extra-ocular muscles would serve as a heat source for cranial endothermy in the shortfin mako shark (Isurus oxyrinchus). To test this hypothesis, the total activity of the enzyme citrate synthase (CS activity g-1 of muscle * mass of each muscle) was measured for each of the six extra-ocular muscles as an index of heat production capacity. The metabolic heat production potential of each individual extra-ocular muscle in the shortfin mako shark was compared to that of the ectothermic blue shark (Prionace glauca). In the shortfin mako, the medial rectus muscle has a significantly greater CS specific activity than the inferior oblique and superior rectus. As a percentage of total eye mass, all six extra-ocular muscles in the shortfin mako were greater than those of the blue shark. The CS specific activities of the medial rectus and inferior rectus muscles were significantly greater in the shortfin mako than in the blue shark. These results suggest that contraction of the medial rectus muscle is a secondary heat source for cranial endothermy in the mako shark. Interspecific comparisons suggest that muscle mass contributes more than CS specific activity to differences in heat production capacity between the shortfin mako and the blue shark.
