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SESK: Stuff Everyone Should Know · sesk.info/fish-cold-blooded

Why does water temperature matter so much to a fish's body?

Fish are cold-blooded: their body temperature follows the water.

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  1. Fish are cold-blooded ✔
  2. Fish are warm-blooded like us
  3. Fish sweat to cool off

Why: Fish are ectothermic, or cold-blooded. They do not hold a steady body temperature, so the water sets it. That is why the right temperature for each species is so important.

Huh, didn't know that: Fish can see colors, and many have excellent vision.

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What did the fish say when it saw the thermometer? Uh oh, I'm in hot water now.
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References

Answer: Fish are cold-blooded. Fish are ectothermic, or cold-blooded. They do not hold a steady body temperature, so the water sets it. That is why the right temperature for each species is so important.

  1. Merck Veterinary Manual (pet owners): Description and Physical Characteristics of Fish www.merckvetmanual.com/all-other-pets/fish/description-and-physical-characteristics-of-fish
    Unlike mammals, fish are cold-blooded (ectothermic).
    Checked 2026-10-11.

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