 A fish is any member of a paraphyletic group of organisms that consist of all gill @-@ bearing aquatic craniate animals that lack limbs with digits . Included in this definition are the living hagfish , lampreys , and cartilaginous and bony fish , as well as various extinct related groups . Most fish are ectothermic ( " cold @-@ blooded " ) , allowing their body temperatures to vary as ambient conditions change , though some of the large active swimmers like white sharks and tuna can be hyperthermic . Many fish species are able to regulate their own body temperature , and some are able to tolerate living in both fresh and salt water .
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