Fishes have a great diversity of reproductive strategies and associated traits, as well as complex life-cycles with multiple stages. This chapter briefly introduces different, commonly found, reproductive strategies and focus mainly on the early stages of fish life cycles, in particular eggs and larvae. Most fishes are iteroparous, spawning several times during their life, but some species, semelparous, adopt an extreme strategy, spawning only once in their life cycle.
There is a great diversity of fish egg types and adaptations, and they can be classified as either pelagic or demersal, depending on where they occur in the environment. The embryonic development depends on the species, and also on abiotic factors such as temperature, oxygen and salinity. The ontogenetic development that starts during the embryonic stage continues during the larval life. During this phase, fundamental structural and functional changes will occur in a short period, increasing the larval abilities to interact with the environment. Depending on the species reproductive strategy and life history, newly hatched larvae can vary from very small and poorly developed, to larvae that hatch larger with developed sensorial and functional capabilities; some larvae can even resemble the adults at hatching.