The vast majority of fishes reproduce themselves by means of eggs that are both laid in the water and fertilized there.
Their eggs are usually spherical, less than one-eighth of an inch in diameter, and may sink or float. A few kinds of fishes give birth to living young, and a still smaller number lay eggs that have already been fertilized while inside the female. Although most fishes show no concern toward their eggs, quite a number make nests for them, guard them, and otherwise care for them until they hatch.
A few of these also watch over the fry when they hatch from the eggs. With few exceptions, it is only the male fish who makes the nest and performs the “nursery” duties; in certain species both parents cooperate, and in others the female alone cares for the eggs and young.