Countless fish fill the seas and almost all other waters on the blue planet, some even visit or invade bordering land. Evolution created a huge variety in size, shape, defense means etc., fit for varies ways of life in all kinds of waters, allowing camouflage, shelter etc. They occupy various positions in aquatic and related food chains, eaten by and/or eating other fish and vertebrates, crustaceans etc. Other relationships are parasitic or symbiotic. Some know role-reversal, with males caring for eggs and/or hatchlings. Many species live in huge schools, often for safety.
