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The mating of gastropods is a vast and varied topic, because the taxonomic class Gastropoda is very large and diverse, a group comprising sea snails and sea slugs, freshwater snails and land snails and slugs. Gastropods are second only to the class Insecta in terms of total number of species. Some gastropods have separate sexes, others are hermaphroditic. Some hermaphroditic groups have simultaneous hermaphroditism, whereas some sequential hermaphroditism. In addition, numerous very different mating strategies are used within different taxa. This article currently focuses primarily on the mating habits of air-breathing terrestrial slugs. Land slugs can be thought of as land snails that over evolutionary time have either lost the shell completely, have a very reduced external shell, or have retained only internal remnants of a shell. Land slugs are a highly polyphyletic group, which means that many land slug families are not at all closely related to one another. The majority of land slugs are simultaneous hermaphrodites, meaning they possess both male and female reproductive organs that are functional at the same time. Some species regularly self-fertilise. Uniparental reproduction may also occur by apomixis, an asexual process.〔Leonard JL, Westfall JA, Pearse JS. Phally polymorphism and reproductive biology in Ariolimax (Ariolimax) buttoni (Pilsbry and Vanatta, 1896) (Stylommatophora: Arionidae) *. ''American Malacological Bulletin''. 2007;23(1):121-35.〕 For the most part, however, land slugs do mate: they find partners, and engage in elaborate courtship rituals before actual sperm transfer takes place. It is common for slugs to mate in a simultaneous reciprocal manner, as occurs in the monophyletic groups Limacoidea and Philomycidae.〔Davison A, Wade CM, Mordan PB, Chiba S. Sex and darts in slugs and snails (Mollusca: Gastropoda: Stylommatophora). ''Journal of Zoology''. 2005;267(04):329-38.〕 Limacoidea comprises the family Limacidae (the keelback slugs) and the largest genus of slugs, ''Deroceras'', which contains over 100 known species.〔Reise H. A review of mating behavior in slugs of the genus ''Deroceras'' (Pulmonata: Agriolimacidae) *. ''American Malacological Bulletin''. 2007;23(1):137-56.〕 Sperm transfer may be external (as in ''Deroceras'') or internal (as in ''Ariolimax''). ==Gender strategy== When it comes to the mating behaviour of simultaneous hermaphrodites such as pulmonate land snails and pulmonate land slugs, as well as opisthobranch sea snails and opisthobranch sea slugs, there is the question of which sexual role or roles an individual will adopt in a mating encounter.〔Anthes N, Putz A, Michiels NK. Sex role preferences, gender conflict and sperm trading in simultaneous hermaphrodites: a new framework. ''Animal Behaviour''. 2006;72(1):1-12.〕 If the individuals involved both have a preference for the same gender role, then their mating interests are inherently incompatible. Further conflicts arise because a hermaphroditic individual may not necessarily use received sperm; it may use another partner’s, or self-fertilise. According to risk-averse models,when this is the case, it would be more prudent for an individual to invest in eggs.〔Leonard JL. Modern Portfolio Theory and the prudent hermaphrodite. ''Invertebrate Reproduction & Development''. 1999;36(1-3):129-35.〕 However, alternative views state that the male role is preferred, as males benefit more from multiple matings than females do.〔Michiels NK, Raven-Yoo-Heufes A, Kleine Brockmann K. Sperm trading and sex roles in the hermaphroditic opisthobranch sea slug ''Navanax inermis'': eager females or opportunistic males? ''Biological Journal of the Linnean Society''. 2003;78(1):105-16.〕 Neither strategy is evolutionarily stable, as both gender roles are required for successful reproduction. Because sperm digestion in land slugs reduces the likelihood of a given set of sperm fertilising the partner’s eggs, increased sperm investment is selected for. This can lead to a co-evolutionary cycle in which the amount of sperm digested and male allocation both increase until eventually, male and female gametes are equally invested in.〔Greeff JM, Michiels NK. Sperm digestion and reciprocal sperm transfer can drive hermaphrodite sex allocation to equality. ''Am Nat''. 1999;153(4):421-30.〕 The larger and more costly ejaculates become, the more reciprocal sperm transfer is favoured, as each slug receives compensation for its investment. Reciprocity, either simultaneous or serial, appears to alleviate the gender conflict, and is a feature of most slug matings. 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「Mating of gastropods」の詳細全文を読む スポンサード リンク
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