Counter-illumination
Counter-illumination is a camouflage technique used by some fish to avoid being seen by predators or prey. It involves the fish having a light-colored belly and a dark-colored back. This helps the fish to blend in with the light from above and the darkness from below, making it difficult for predators or prey to see them.
Ventral bioluminescence
Ventral bioluminescence is the production of light by the belly of a fish. This is typically used by deep-sea fish to attract prey or mates. The light is produced by specialized cells called photophores, which are located on the fish's belly.
In addition to these two main reasons, there are a number of other reasons why some fish have white bellies. For example, some species of fish have a white belly because it helps them to reflect heat. This is important for fish that live in warm climates, as it helps them to avoid overheating. Other species of fish have a white belly because it helps them to communicate with each other. For example, some species of fish use their white bellies to flash signals to each other during mating season.