There are certain things about the animal kingdom that we are taught are absolutes. Birds fly. Reptiles have scales. Fish are cold-blooded. But, if you're anything like me, it's the exceptions to these rules that you'll find the most fascinating. Which is why I'd like to take some time to talk about endothermy, where heat is generated and maintained from within, in fishes.
The vast majority of fish species are ectothermic, which means their body temperature is largely controlled by the environment around them. As a result, their internal temperature can vary greatly and they don't have much physiological control over this. But, despite how it sounds, it's actually an advantage.
Water acts as a heat sink and tends to drain heat from anything it is in contact with, including living organisms. Due to a fish's gills, its blood is constantly in near direct contact with this heat-draining environment. With that in mind, it makes sense that a fish would not have the means to use its metabolism to warm up internally. It would be a massive waste of the fish's energy. It is far more efficient to rely on external factors instead.
But that's not the case for all fishes. Some have physiological mechanisms that enable them to be at least partially endothermic.
Tunas, swordfishes, and certain species of shark, such as the salmon shark (Lamna ditropis), are able to maintain body temperatures warmer than the surrounding water. They generate heat with their powerful swimming muscles, which they can then use to stay warm. For example, bluefin tuna (Thunnus thynnus) is able to maintain muscle temperatures of between 27 and 32°C (82 to 91°F) in waters of around 7 to 25°C (45 to 80°F).
Large, predatory billfishes, such as marlins, swordfishes, sailfishes, and spearfishes, are able to use the heat that they generate from swimming to warm their brain and eyes. This serves them well when they are hunting as it allows them to function efficiently and effectively in the deeper waters where they catch their prey.
Many sharks, including the great white (Carcharodon carcharias) and shortfin mako (Isurus oxyrinchus), also have a large vein that helps to warm the central nervous system (CNS). It does so by transporting blood from the swimming muscles to the spinal chord, keeping the nervous system in good working order at a functional temperature. This, again, serves them well when hunting in deeper waters where the temperature is lower.
The only truly endothermic fish that we currently know of, as in one that is fully warm-blooded, is the opah (of the family Lampridae), of which there are two living species. It can maintain its body temperature above the surrounding water temperature. They are found in tropical to temperate waters and swim at depths between 50 and 500 m. They are able to generate heat by constantly flapping their pectoral fins and retain this heat within their layers of fat, which acts as insulation.
However, in 2015, it was discovered that opah also possess a rete mirabile, a specialised system of arteries and veins, within their gill tissue. This rete mirabile is found in several warm-blooded vertebrates and it allows for countercurrent heat exchange, where warm blood is carried from the heart and, in the case of the opah, warms the veins in the gills, which are carrying colder blood. In the reverse, the gills are also able to be cooled by contact with the surrounding water.
So, as you can see, what we think of as fact doesn't always line up with what is actually true. Not all fish are cold-blooded, and those that defy the norm each have their own way of warming the blood. Each method has been developed over time to suit the individual species, their lifestyle, and the habitat they live in. And, though each method is different, they are all effective in their own right.
The diversity of fishes is truly remarkable, as is the diversity of the animal kingdom as a whole. Life is so much stranger than we could ever possibly imagine. I will be covering more Weird and Wonderful creatures on the blog in the future, focusing on nature's oddballs and all the animals that dare to be different!
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