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Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union.〔 Although the term often refers to placental mammals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g. Consistent with this definition, there are several types of pregnancy in fish. In ovoviviparity, there is internal fertilization and the young are born live but there is no placental connection or significant trophic (feeding) interaction; the mother's body maintains gas exchange but the unborn young are nourished by egg yolk. There are two types of viviparity in fish. In histotrophic viviparity, the zygotes develop in the female’s oviducts, but she provides no direct nutrition; the embryos survive by eating her eggs or their unborn siblings. In hemotrophic viviparity, the zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta. In seahorses and pipefish, it is the male that becomes pregnant which is termed male pregnancy. ==Types of reproduction and pregnancy== Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union. Despite strong similarities between viviparity in mammals, researchers have historically been reluctant to use the term "pregnancy" for non-mammals because of the highly developed form of viviparity in eutherians. Recent research into physiological, morphological and genetic changes associated with fish reproduction provide evidence that incubation in some species is a highly specialized form of reproduction similar to other forms of viviparity. Although the term "pregnancy" often refers to eutherian animals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g.〔〔〔〔 Five modes of reproduction can be differentiated in fish based on relations between the zygote(s) and parents: * Ovuliparity: Fertilization of eggs is external; zygotes develop externally. * Oviparity: Fertilization of eggs is internal; zygotes develop externally as eggs with large vitellus. *Ovoviviparity: Fertilization is internal; zygotes are retained in the female (or male) but without major trophic (feeding) interactions between zygote and parents (there may be minor interactions, such as maintenance of water and oxygen levels). The embryos depend upon their yolk for survival. There are two types of viviparity among fish. *Histotrophic ("tissue eating") viviparity: The zygotes develop in the female’s oviducts, but she provides no direct nutrition. The embryos survive by eating her eggs or their unborn siblings. *Hemotrophic ("blood eating") viviparity: The zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pregnancy in fish」の詳細全文を読む スポンサード リンク
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