Fish
Fish are cold-blooded, aquatic vertebrates. They have fins and a body covered with scales. They usually breath with their gills, but some species, such as the lungfish, have lungs of some sort.
Their internal temperatue varies with their environment (they are thus said to be ectothermic). Recent studies have shown that some predatory species, like sharks and tuna, are capable of maintaining the temperature of their red muscles a few degrees higher than the water temperature, which allows them to swim faster over a longer period of time.
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| Koi (Cyprinus carpio) |