Meeting Abstract
Despite disparate phylogenetic histories, swordfish (Xiphias gladius) and bigeye thresher sharks (Alopias supercilious) share a unique, pelagic ecology characterized by routine, long duration dives from surface waters (18-20°C) to well beneath the thermocline (5-10°C). Interestingly, swordfish red muscle morphology is comparable, albeit simplistic, to that of regionally endothermic fishes, suggesting red muscle operating temperatures are somewhat elevated above the environment, whilst that of bigeye thresher sharks experience dive-driven fluctuations. The thermal effects on contractile function of red, aerobic muscle from both species were investigated using the work-loop technique across naturally occurring temperatures (8, 16, 24°C). Swordfish, like regionally endothermic fishes, seem more responsive to temperature increases, with an enhanced ability to augment swimming speed at warmer temperatures relative to bigeye thresher sharks. Specifically, swordfish acquired faster cycle frequencies, namely 1-2Hz, at 16 and 24°C, while bigeye thresher sharks were unable to augment frequency above 0.5 Hz, even at 24°C. Regional endothermy could thus be advantageous to swordfish by allowing faster tail-beat frequencies during foraging in surface waters and initial dive periods. The increased thermal sensitivity of swordfish relative to bigeye thresher sharks, coupled with previous findings for endothermic fishes suggests a relationship between red muscle morphology, thermal strategy (e.g. endothermy versus ectothermy), and thermal sensitivity with more complicated endothermic morphologies exhibiting greater dependence on heat conservation for locomotion.
