Final answer:
Fishes, amphibians, and reptiles are considered ectothermic because they can't generate body heat metabolically and must rely on the environment to regulate their body temperature.
Step-by-step explanation:
Fishes, amphibians, and reptiles (other than birds) don't have a mechanism to metabolically generate body heat and thus are considered ectothermic.
Ectothermic animals rely on environmental heat sources, and their body temperature changes with the ambient environment. Unlike endotherms, such as mammals and birds, ectothermic creatures do not have the internal physiological mechanisms to maintain a stable body temperature through metabolism.
Reptiles, for example, must use behavioral adaptations like basking in the sun or seeking shade to regulate their body temperature. This lower need for metabolically generated heat allows ectotherms to require less food energy than endothermic animals.