Aspects of the Evolution and Adaptive Significance of Regional Endothermy in Fishes
This dissertation addressed questions related to the selective advantages of regional endothermy and its independent evolution among several fish groups. The chapters integrate laboratory and field physiological measurements to provide a better understanding ofthe evolution of regional endothermy and its importance as an adaptation to the pelagic environment. Studies on the swimming energetics of the eastern Pacific bonito (Sarda chiliensis) revealed that the bonito had a similar cost of transport to that of similar sized yellowfin tuna (Thunnus albacares) and that the bonito had a significantly lower standard metabolic rate than tuna. Thermal studies on live slender tuna, Allothunnus fallai, were the first to document both red muscle (RM) and eye and brain endothermy for this species and establish regional endothermy as a synapomorphy of the Tribe Thunnini. Additional findings for A.fallai revealed this species to possess a brain heater, a thermogenic organ composed of portions of all four of the rectus muscles. These findings demonstrate that the use of a heater tissue to elevate cranial temperatures has evolved independently in at least three fish groups.
Movement studies on the mako shark (Isurus oxyrinchus) used acoustic telemetry to record the in-vivo stomach temperatures and depth distribution off the coast of Southern California. This work showed that the tracked makos remained in the upper 12 m of the water column during up to 80% of the track period and that larger makos dove to greater depths. Stomach temperature data were recorded for all tracked makos (including young of the year), and these data were used to document feeding events in five of the seven tracking sessions.
A study on the muscle morphology of three thresher shark species (common thresher, Alopias vulpinus, pelagic thresher, A. pelagicus, and bigeye thresher, A. superciliosus) revealed interspecific differences in the RM transverse position and distribution along the body. The common thresher is the only alopiid to possess a RM arrangement similar to that of the endothermic lamnid sharks, suggesting that this species is likely the only thresher to be capable of RM endothermy.