In general, five major developmental periods are recognized in fish: embryonic, larval, juvenile, adult, and senescent. Fish development is known for its confounding terminology, so there are many gray areas within these major categories, and, as with many other animals, many species tend to defy classification into discrete categories. For instance, species in several teleostean families bear live young (viviparous) – Poeciliidae, Scorpaenidae, and Embiotocidae (to name a few), and the young in some families (Salmonidae) seem to emerge as juveniles after hatching (externally) from the egg.

There are two important developmental characteristics that separate fish from most vertebrates: indeterminate growth (growing throughout life) and a larval stage. The fact that most fish (although there are always exceptions) are always growing means they constantly change in terms of anatomy, ecological requirements, and reproduction (i.e. larger size means larger clutches, more mates, better defense, etc. in most species). Increased age is also associated with better survivability, As physiological tolerances and sensitivity improve, familiarity with the local environment accrues, and behavior continues to develop. The larval stage is usually associated with a period of dispersal from the parental habitat. Also, the disappearance of the yolk sac (the beginning of the larval stage according to most researchers) marks a critical period in which most individuals die from starvation or predation.

Recently, researchers of coral reef fishes (mostly of the order Perciformes) have made significant advances concerning the life history of larvae. Nearly all bony coral reef fishes produce pelagic young (meaning they live in the water column for a period of time before settling on reefs), and the length of the stage is highly variable, from only a week in some damselfishes to greater than 64 weeks in some porcupine fishes . Initially, researchers made relatively simplistic assumptions about the pelagic phase, "portray[ing] larvae as little more than passive tracers of water movement that 'go with the flow,' doing nothing much until they bump into a reef by chance and settle at once" (Lies and McCormick 2002:171). Actually, the larvae of most coral reef fishes are endowed with good swimming abilities, good sensory systems, and sophisticated behavior that is quite flexible. And, while mortality rates are quite high at this stage (as with many other actinopterygian larvae), many larvae are able to detect predators at a considerable distance, and they are often transparent (usually larvae) or cryptically colored (many juveniles).

It is important to note that the young of reef fishes develop quite differently from most temperate fishes that have been studied. While the eggs of most temperate fishes hatch from 3 to 20 days after laying, the eggs of most coral reef species hatch within only a day. Also, at any given size, the larvae of reef fishes are more developed than most temperate, non-perciform fish: they have "more complete fins, develop scales at smaller size, [have] seemingly better sensory apparatus at any size, and are morphologically equipped for effective feeding within a few days of hatching" (173). Finally, the settling habitat for reef fishes (coral reefs) tends to be relatively fragmented and, therefore, much more difficult to locate, unlike the habitat of temperate fishes, which tends to have large expanses suitable for settling. This brief glimpse into the pelagic stage of reef fishes reveals the diversity and complexity of development in actinopterygians.

Development - Life Cycle: neotenic/paedomorphic; metamorphosis ; temperature sex determination; indeterminate growth