The view that many people have of fishes as “cold-blooded “is not accurate. Most fishes are about the same temperature as the surrounding water, which may be cold or warm depending on the habitat. That temperature can change, but usually any change is slow due to the thermal stability of water. Animals that rely primarily on external heat sources are referred to as ectotherms, and include most invertebrates, fishes, amphibians, and reptiles. Animals that generate their own heat and generally maintain stable body temperatures, such as birds and mammals, are endotherms.
Most fishes are ectothermic because they lack any mechanism for heat production and retention. In addition, whenblood flows through the gills it becomes the same temperature as the surrounding water due to the thin gill membranes, before then flowing to the rest of the fish’s body. There are, however, interesting exceptions of heat production or conservation in some fishes, a condition often referred to as either heterothermy or regionalendothermy.