"Internal bearers may have similarities to pouch brooders, although fertilization is internal and females always carry the embryos and/or young. Internal bearers typically produce only a small number of large, active young, which is a strategy characteristic of all sharks and rays and a few bony fish. There seems to be no strong correlation between internal bearing families and the elaborateness of courtship behaviour. In guppies, mosquitofish and other livebearing poeciliids. the most important trait of the courtship behaviour of males that successfully copulate with females seems to be persistence (although subtle interactions among competing males and other courtship-related behaviour do exist). In contrast, some surfperches have fairly elaborate courtship behaviour, with males establishing breeding territories to attract passing females."
Facultative internal bearers: The beginning of the evolutionary process of livebearing starts with facultative (optional) internal bearing. The process occurs in several species of oviparous (egg-laying) killifishes which spawn in the normal way on the substrate, but in the process accidentally fertilize eggs which the female retains and does not spawn. These eggs are spawned later, usually without allowing much time for embryonic development.
Obligate internal bearers: The next step in the evolution of livebearing is obligate (by necessity) internal bearing, where the female retains all the embryos. "The only source of nutrition for these embryos, however, is the egg yolk, as in externally spawned eggs. This situation, also referred to as ovoviviparity, is characteristic of marine rock fishes and the Lake Baikal sculpins. This strategy allows these fish to have fecundities approaching those of pelagic fish with external fertilization, but it also enables them to protect the young during their most vulnerable stage of development. By contrast, sharks and rays using this strategy produce a relatively small number of embryos and retain them for a few weeks to 16 months or longer. The shorter times spans are characteristic of species that eventually deposit their embryos in the environment, surrounded by a horny capsule; whereas the longer periods are characteristic of sharks that retain the embryos until they are ready to emerge as actively swimming young."
Viviparous fish: "Given the advantages of retaining the embryos internally, it is not surprising that a number of fishes have developed means to provide additional nutrition for their young while the female is carrying them and then give birth to large, active young (viviparity). Viviparous fishes provide nutrition for their young in various ways. The greatest variety occurs in sharks, but the teleosts also show a considerable range of methods. In the superperches, the young develop in the mother's ovary and obtain nutrients through the close contact of the extra-large fins with the ovarian wall. Many poeciliid embryos have highly vascular pericardial tissue that loops around the neck and is in close contact with the ovarian wall of the mother, through which nutrients are exchanged."
However, some fish do not fit these categories. The livebearing largespring gambusia (Gambusia geiseri) was thought to be ovoviviparous until it was shown in 2001 that the embryos received nutrients from the mother.
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