Here's why:
* Gas Exchange: Gills are the primary organs for respiration in fish. They have a vast network of thin, delicate blood vessels called capillaries that run very close to the surface.
* Oxygen Uptake: As water flows over the gills, oxygen from the water diffuses into the capillaries. The capillaries carry this oxygenated blood throughout the fish's body.
* Hemoglobin: The red color of blood comes from hemoglobin, a protein in red blood cells that binds to oxygen. Because the capillaries in gills are so dense and filled with oxygenated blood, they appear red.
In summary, the red color of fish gills is a direct result of the high concentration of oxygen-rich blood needed for efficient gas exchange.