There are three main types of growth processes observed in fish offspring. Though all fish fit into one of these categories, it should be noted that there can be significant differences between species in the same group, in terms of parental care, length of developmental periods, and nesting or "brooding" habits.
In oviparous fish, eggs grow and develop outside the mother's body. Eggs are generally fertilized outside of the mother's body as well, with the exception of oviparous sharks and rays. Eggs hatch relatively quickly; in goldfish, it takes only 48 to 72 hours.
After hatching, the young enter a larval state. They are largely unformed, sometimes resembling tadpoles, and get their nutrition from a yolk sac that they carry. When this is used up, they begin eating zooplankton, microscopic organisms that live in the water. The larval stage only lasts a few weeks at the most, and the the hatchlings go through a metamorphosis that causes them to look much more like the adult fish of their species. There is speculation that the growth period is short in order to minimize cannibalism by adult fish. About 97 percent of fish are oviparous.
In ovoviviparous fish, the eggs develop inside the mother's body. Each embryo develops its own egg and yolk, from which it receives nutrients. When born, the offspring are past the larval stage, in a juvenile state and capable of feeding on their own. Guppies and angel sharks are both ovoviviparous.
Viviparous fish are unique in that the mother directly provides nourishment to her young. Eggs are internally fertilized, and offspring are fed through uterine "milk" or through an organ similar to a placenta. There are some stranger modes of nourishment, both observed in shark species: oophagy, where the mother produces eggs solely to feed the embryos, and intrauterine cannibalism, where bigger embryos consume their smaller siblings. As in ovoviviparous fish, the young are in a juvenile state when born, as opposed to larval.
- Shubunkin goldfish, U.S. NOAA