In the fish world, it's normal to be cold-bodied and take on the temperature of your surroundings. But there are exceptions to every rule;tunas and lamnid sharks are endothermic, managing to keep their bodies warmer than the waters they live in. The common thresher shark, a large alopiid shark with a formidable tail that roams temperate and deep tropical waters, is also warm-bodied. Since all tunas and all lamnid sharks are endothermic, it seemed fair to suppose that the two other known thresher shark species, bigeye and pelagic threshers, are also warm-bodied. But Chugey Sepulveda, Diego Bernal and their colleagues reveal that bigeye and pelagic thresher sharks are nothing like their warm-bodied common cousins(p. 4255).
Most fish keep the red muscle that's used for swimming just beneath their skin and can't retain the heat their muscles produce, but tunas, lamnid sharks and the common thresher shark all keep their red muscles nestled deep inside their bodies, insulating their internal `furnace' from the cold water. If bigeye and pelagic thresher sharks also bury their red muscle deep inside their bodies, Sepulveda and Bernal reasoned, then it's likely that these sharks are also warm-bodied.
Sepulveda and Bernal teamed up with Nick Wegner and Jeff Graham to find out if all threshers have the same red muscle distributions. Since live thresher sharks are very difficult to catch, the team bought some dead thresher sharks from commercial fisheries. They froze the sharks, sliced them into thin transverse sections and took a digital image of each slice. Reconstructing a 3D computer model of each species, the team visualized red muscle distribution all along the length of the sharks' bodies. But as soon as they cut open the first bigeye and pelagic threshers, the team spotted that these sharks are clearly nothing like their common cousin. The red muscle of both bigeye and pelagic threshers runs just beneath the skin and is mainly distributed towards the tail end of the body, while in the common thresher shark the red muscle runs along the backbone and is concentrated at the head end of the body. `We didn't expect to see such a big difference', Bernal recalls, `so our first thought was that the fisheries had sold us the wrong species.' But a DNA analysis soon confirmed that they really were inspecting the right species.
The team also found another clear difference among the three thresher species. Tunas, lamnid and common thresher sharks retain the heat produced by their red muscles using vascular counter-current heat exchange systems that short-circuit heat loss to the environment. But when the team examined the bigeye and pelagic thresher sharks' vascular systems to look for these modifications, they found no evidence of such heat exchangers. This finding,together with the team's observation that bigeye and pelagic thresher sharks don't keep their red muscle near their body core, means that it's very unlikely that these two species are warm-bodied.
The team concludes that the common thresher shark is the only thresher species with the anatomical specializations for red muscle endothermy. Why and how these specializations evolved in common threshers but apparently not in bigeye and pelagic threshers is a mystery that the team is now delving into.