Kalad on kõigusoojased loomad, kelle füsioloogilised funktsioonid aeglustuvad, kui ümbritsev veetemperatuur langeb oluliselt allapoole optimaalset taset. Sama toimub ka kalade organismi kaitsemehhanismide aktiivsusega: see väheneb.
Cold-blooded animals, unlike warm-blooded organisms, do not maintain thermal homeostasis; that is, they do not keep their core body temperature at a nearly constant level regardless of the temperature of the surrounding environment. Rather, cold-blooded animals have a variable body temperature, which reflects the environmental temperature. Invertebrates, fish, amphibians, and reptiles are considered to be cold-blooded, while birds and mammals are defined as warm-blooded animals.
