The red muscle morphology of the thresher sharks (family Alopiidae)

A more medial and anterior position of the red aerobic myotomal muscle (RM) and the presence of a vascular counter-current heat exchange system provide the functional elements that facilitate regional RM endothermy in tunas, lamnid sharks and the common thresher shark (Alopias vulpinus). The convergent RM morphology among all species capable of RM endothermy suggests that RM position is a strong predictor of fish endothermic capacity. The present study investigated the comparative RM morphology of the other two thresher shark species (bigeye thresher, Alopias superciliosus, and the pelagic thresher, Alopias pelagicus), for which there is no information regarding their capacity for RM endothermy, and compared these data with published works on A. vulpinus. The digitization of transverse sections along the body of A. superciliosus and A. pelagicus enabled quantification of the relative amount of RM and the position and placement of the RM along the body. The RM in both A. superciliosus and A. pelagicus is positioned subcutaneously, along the lateral edges of the myotomes, and is distributed relatively evenly over the trunk of the body. The position of maximum RM area is at 50% fork length (FL) for A. superciliosus and at 75% FL for A. pelagicus. The amount of RM (mean +/- S.E.M.) is 2.31 +/- 0.11% and 3.01 +/- 0.10% in A. superciliosus and A. pelagicus, respectively. When compared with A. vulpinus, all three alopiid sharks have a similar amount of RM. However, A. superciliosus and A. pelagicus differ from A. vulpinus in that they do not possess the medial and anterior RM arrangement that would likely facilitate metabolic heat conservation (RNI endothermy).