Environmental temperature and body temperature in fish
 
Fish, amphibians and reptiles belong to a group called ectotherms meaning that these animals do not produce heat to maintaining a constant and normally high body temperature (as is the case for birds and mammals). Instead they rely on the environment and their own behaviour to control their temperature. For fish, this means that body temperature varies directly with water temperature as seen in the diagram below. Ectotherms can be further divided into poikilotherms and homoetherms. In the poikilotherm group, the body temperature varies over time with environmental temperature. In the homeotherms, the environmental temperature is stable which means that the body temperature is also stable. In fish, most species are poikiolothermic ectotherms, but Antarctic and deep sea living species are homeothermic ectotherms.