When we watch goldfish gulping water, rhythmically opening and closing its mouth, we see it breathe. Fish take in oxygen from water rather than air. Instead of drowning in water, they require it for life.
From the mouth, water passes over the fish’s gills, which are fleshy filaments with blood vessels lying close to the surface. In the gills carbon dioxide, which has been carried away from body cells by the blood, is exchanged for oxygen dissolved in the water. Gills function so efficiently that they can take up to 75% of the available oxygen.
How much oxygen is in the water influences which variety lives there. Because species of freshwater fish need large amounts of oxygen, they inhabit cold water, which holds more oxygen than the warm water does. In contrast, catfish thrive where the oxygen content is low – in sluggish streams for example.
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