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These fishes diverged millions of years ago, but selection pressures have brought them very similar biomechanical schemes for movement
In the early 1800s, the British physician John Davy provided the first insight that tunas were different from other fish. He made measurements showing that tunas were not cold-blooded, as all fishes were assumed to be at that time. In a paper read to the Royal Society of London in 1835, he reported the remarkable fact that deep muscle temperature in a skipjack tuna (Katsuwonis pelamis) was 10 degrees Celsius higher than the warm tropical water in which the fish was captured. Since then, many investigators have documented warm temperatures in muscle and viscera of other tunas.
Sharks of the Lamnidae family, such as makos and whites, are now also known to have regional endothermy. On an expedition to Alaska in 2004, my colleagues Diego Bernai, Jeanine M. Donley and Douglas Syme and I made measurements on the salmon shark (Lamna ditropis), another Lamnidae member, and we recorded deep-muscle temperatures of 26 degrees in fish taken from 6-degree sub-arctic water.
This parallel between tunas and the sharks of this family, called lamnid sharks, is just one of the many anatomical and physiological specializations shared by these two groups of highly active pelagic predators. The evolutionary convergence between them is so striking that in many ways these distantly related groups resemble each other more than they resemble their own close ectothermic relatives. Such similarity is more remarkable considering that these features evolved independently, long after the ancestors of bony and cartilagenous fishes diverged more than 400 million years ago. The shared characteristics in these distantly related groups, which distinguish them from virtually all other fishes, probably arose roughly 40 million to 60 million years ago from similar selection pressures for fast and continuous locomotion.
My colleagues and I have recently made significant advances in quantifying some of these characteristics, particularly the nigh-identical mechanisms of internal biomechanics that evolution has bestowed upon these two groups of fishes. Although tunas and lamnid sharks are not exactly the same anatomically, they have developed systems-to create a hydrodynamic body shape, keep warm and transmit muscle force to the tail-that perform in precisely the same fashion. Our studies have confirmed how...