Fish need oxygen just like humans do, but instead of using lungs, they use gills. Gills are usually on both sides of a fishes head. These contain many blood vessels called capillaries. When the fish opens its mouth, water goes over the gills, and blood in the capillaries get oxygen thats dissolved in the water. Then, similar to humans, the oxygen goes up the blood to wherever it needs to go. All fish also have operculum, which is a bone that opens and closes to protect the gills. Mollusks and crustaceans also have gills, that also pull oxygen out of water. Some animals like the nudibranch however, have their gills outside their body. Breathing with gills is harder than with lungs, as water has less parts of oxygen per million.
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