http://www.fao.org/fishery/topic/16082/en#Roberts/1978
As one learned in elementary biology, all fish are “cold-blooded” (ectotherms), but this is not the case as there are exceptions. Many in the Thunnus genus is actually able to thermally regulate their bodies and keep themselves quite warm, ranging from 25 to 30 degrees Celsius. Especially those that do long-distance swimming in the open ocean. In fact, tuna have a higher proportion of red muscle present in comparison to other bony fish, which is advantageous for long distance swimming as red muscles are made for longer lasting activities. With these muscles found in tuna they have quite the unique ability! They use an efficient counter-current heat exchange system within their muscles to maintain high body temperatures. As blood vessels overlap heat diffuses to the recently cooled side, making it warm again by the time it reaches the more internal parts of the organism. Heat is generated by their hard working red muscle, and through this system they are able to retain 70-99% of that heat! This feature is quite useful as they don’t have to worry about their muscles cooling down, making them unable to swim long distances. Instead it stays an optimal temperature for muscle stimulation and higher reaction rates.
Such a unique instance of endothermy within a group (bony fish) that is primarily known for ectothermy was seen as interesting from an animal biology perspective. Surely, this trait was a point of homoplasy. This means it was not a derived trait from ancestors, but it was evolved independently. This was likely an adaptation that met their needs of long-distance swimming within the vast open ocean as they hunted for their prey. This overall could show how physiological features found within animals can be from decades of selective pressure based on environmental and community interactions. So if you look at the physiology of an organism, it is very likely there for a reason! Animal biologists may not know everything about all animal functions yet, but with these little bits of information it can lead to the reveal of much more.
Citations:
National Geographic. (2015). “Atlantic Bluefin Tuna.” Retrieved from https://www.nationalgeographic.com/animals/fish/a/atlantic-bluefin-tuna/.
Hill, R. W.; Wyse, G. A.; Anderson, M. (2004). Animal Physiology. Sinauer Associates, Inc.
Block, B. A.; Dewar, H; Blackwell, S. B.; Williams, T. D.; Prince, E. D.; Farwell, C. J.; Boustany, A; Teo, S. L.; Seitz, A; Walli, A; Fudge, D (2001). “Migratory Movements, Depth Preferences, and Thermal Biology of Atlantic Bluefin Tuna”(PDF). Science. 293 (5533): 1310–4.
Goujon, M. and Majkowski, J. (2000). FAO Fisheries & Aquaculture – Tunas & tuna-like species group. [online] Fao.org. Available at: http://www.fao.org/fishery/topic/16082/en#Roberts/1978 [Accessed 23 Oct. 2019].
