Bony fish have a variety of reproductive strategies, but in most species two separate sexes release sperm and eggs into the water, and fertilization occurs externally. Parental care is quite rare, but seahorses and a few other species do care for their young. Some bony fish change sexes depending on conditions, while others fertilize eggs internally or even birth live young.
Bony fish are a diverse group of animals, which does not include sharks and jawless fish. Their most frequent method of reproduction is spawning, where unfertilized gametes are released in proximity of a fish of the opposite sex to meet by chance. In such species, the likelihood of any particular egg or sperm released reaching adulthood is low, so the number of gametes produced is often quite high. The ocean sunfish, for instance, lays up to 28 million eggs at a time. Species that mate and bear live young, such as guppies, often have as few as 25 young at a time.
Fish have many strategies between these extremes. Some fish have internal fertilization and then lay eggs. Others keep fertilized eggs, supported only by yolks, inside until they hatch. A few others provide direct nutrient support to their young until they are ready to be born.