Scombroids are a collection of primarily predatory fish that include mackerels, tunas, bonitos and billfishes, found worldwide in tropical and temperate waters. Most scombroids are speedy swimmers, with a streamlined body shape as well as muscular, metabolic and cardiovascular features that allow them to swim continuously. They also have enhanced aerobic capabilities to power their athletic swimming. In addition, some scombroids, such as tuna and billfishes (e.g. marlin and swordfish) have a remarkable physiological adaptation known as regional endothermy, where they maintain regions of their body at an elevated temperature relative to their surroundings, allowing them to expand their geographical range. These unique characteristics suggest that tuna and billfishes share a close evolutionary history; however, the phylogenetic relationship between scombroids remains unresolved. Numerous morphological and molecular studies have been conducted to determine the relationships between scombroid family members, producing conflicting results. Chris Moyes and his colleagues from Queen's University in Canada sought to resolve the classification of scombroid fishes using DNA sequencing.
The team collected muscle samples from numerous species of billfishes (e.g. swordfish, striped marlin, blue marlin) and tuna (e.g. bigeye tuna, yellowfin tuna, bullet tuna) near the Hawaiian Islands in the Pacific Ocean. Next, they sequenced the DNA from nine mitochondrial loci and three nuclear loci from each muscle sample. Finally, they used statistical analysis to compare their results with sequences from additional species of fishes available on GenBank (publically available DNA sequences) in order to generate the best pylogeny for scombroid fishes.
Remarkably, the team found that tuna and billfish are only distantly related. This finding suggests that regional endothermy and continuous swimming has arisen independently in these two groups of fishes. What's more, the authors found that billfishes are closely related to flatfishes (Pleuronectiformes) and jacks (Carangidae). This is an astonishing result considering the differences in lifestyle, physiology and morphology observed among these fishes. While billfishes are extremely athletic, and have regional endothermy and elongated bills, flatfish primarily live on the sea floor and are usually asymmetrical with their two eyes located on the same side of their head.
The evolutionary relationships found in this study clearly demonstrate that shared selective pressures can lead to similar adaptations in distantly related species. Equally, it also shows how disparate selective pressures can lead to divergent adaptations in closely related species. These findings have important implications for scientists studying scombroids and their relatives and also provide a dynamic system to study the evolution of endothermy in fishes. At the very least, this study should remind us that you can't judge a book by its cover!
