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Thorson's rule (named after Gunnar Thorson by S. A. Mileikovsky in 1971) 〔Mileikovsky, S. A. 1971. ''Types of larval development in marine bottom invertebrates, their distribution and ecological significance: a reevaluation.'' Marine Biology 19: 193-213〕 states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to pelagic (often planktotrophic == Groups involved == The rule was originally established for marine bottom invertebrates, but it also applies to a group of parasitic flatworms, monogenean ectoparasites on the gills of marine fish.〔Rohde, K. 1985. Increased viviparity of marine parasites at high latitudes. Hydrobiologia 127: 197-201.〕 Most low-latitude species of Monogenea produce large numbers of ciliated larvae. However, at high latitudes, species of the entirely viviparous family Gyrodactylidae, which produce few nonciliated offspring and are very rare at low latitudes, represent the majority of gill Monogenea, i.e., about 80–90% of all species at high northern latitudes, and about one third of all species in Antarctic and sub-Antarctic waters, against less than 1% in tropical waters. Data compiled by A.V. Gusev in 1978 indicates that Gyrodactylidae may also be more common in cold than tropical freshwater systems, suggesting that Thorson's rule may apply to freshwater invertebrates.〔Gusev, A.V. 1978. Monogenoidea of freshwater fish. Principles of systematics, analysis of the world fauna and its evolution. Parasitologicheskij Sbornik 28: 96-198 (in Russian).〕 There are exceptions to the rule, such as ascoglossan snails: tropical ascoglossans have a higher incidence of lecithotrophy and direct development than temperate species.〔Krug, P.J. 1998. Poecilogony in an estuarine opisthobranch: planktotrophy, lecithotrophy, and mixed clutches in a population of the ascoglossan Alderia modesta. Marine Biology 132:483-494.〕 A study in 2001 indicated that two factors are important for Thorson's rule to be valid for marine gastropods: 1) the habitat must include rocky substrates, because soft-bottom habitats appear to favour non-pelagic development; and 2) a diverse assemblage of taxa need to be compared to avoid the problem of phyletic constraints, which could limit the evolution of different developmental modes.〔Gallardo, C.S. and Penchaszadeh, P.E. 2001. Hatching mode and latitude in marine gastropods: revisiting Thorson's paradigm in the southern hemisphere. Marine Biology 138 547-552〕 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「Thorson's rule」の詳細全文を読む スポンサード リンク
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