 Teleosts have adopted a range of reproductive strategies . Most use external fertilisation : the female lays a batch of eggs , the male fertilises them and the larvae develop without any further parental involvement . A fair proportion of teleosts are hermaphrodites , starting life as females and transitioning to males at some stage , with a few species reversing this process . A small percentage of teleosts are viviparous and some provide parental care with typically the male fish guarding a nest and fanning the eggs to keep them well @-@ oxygenated . 
