 Most teleost species are oviparous , having external fertilisation with both eggs and sperm being released into the water for fertilisation . Internal fertilisation occurs in 500 to 600 species of teleosts but is more typical for Chondrichthyes and many tetrapods . This involves the male inseminating the female with an intromittent organ . Fewer than one in a million of externally fertilised eggs survives to develop into a mature fish , but there is a much better chance of survival among the offspring of members of about a dozen families which are viviparous . In these , the eggs are fertilised internally and retained in the female during development . Some of these species , like the live @-@ bearing aquarium fish in the <unk> family , are ovoviviparous ; each egg has a yolk sac which nourishes the developing embryo , and when this is exhausted , the egg hatches and the larva is expelled into the water column . Other species , like the <unk> in the family <unk> , are fully viviparous , with the developing embryo nurtured from the maternal blood supply via a placenta @-@ like structure that develops in the uterus . <unk> is practised by a few species , such as Nomorhamphus ebrardtii ; the mother lays unfertilised eggs on which the developing larvae feed in the uterus , and intrauterine cannibalism has been reported in some halfbeaks . 
