Fish have a common cloaca. The common cloaca is a chamber into which faeces, urine, sperm and eggs are all passed to be removed from the body. A common cloaca is not a problem for aquatic animals. Sexual reproduction is where gametes, sperm and eggs, join to form a zygote which develops into a new organism. Sexual reproduction, unlike asexual reproduction, enables variation. Variation is a good thing because it allows organisms to adapt to changing environments. Hence most organisms use sexual reproduction. Sexual reproduction is very easy in water. The female fish deposits her eggs on the river or sea bed. The male fish swims over the eggs and squirts sperm over them. The sperm swim through the water to fertilise the eggs. Easy!! This is the result of sex: