Why the gills of fishes are red?

Fish gills are red due to the presence of a protein called haemoglobin, which contains iron. Haemoglobin binds to oxygen molecules and transports them from the water to the fish's bloodstream. The oxygenated blood is then pumped throughout the fish's body, delivering oxygen to the cells.

The amount of haemoglobin in the gills of a fish varies depending on the species. Some fish have a lot of haemoglobin and their gills are a deep red colour, while others have less haemoglobin and their gills are a lighter pink colour.

The colour of a fish's gills can also be affected by the environment in which they live. Fish that live in cold water tend to have more haemoglobin in their gills than fish that live in warm water. This is because the oxygen levels in cold water are lower than the oxygen levels in warm water, so the fish need more haemoglobin to bind to the oxygen molecules.