Thorson's rule (named after Gunnar Thorson by S. A. Mileikovsky in 1971) [1] is an ecogeographical rule which states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to pelagic (often planktotrophic [plankton-feeding]) and widely dispersing larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic (yolk-feeding) eggs and larger offspring, often by viviparity or ovoviviparity, which are often brooded.
[3] Most low-latitude species of Monogenea produce large numbers of ciliated larvae.
[5] 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.
The conclusion is that the most likely explanation for the Monogenea (and by implication for other groups) is that small larvae cannot locate suitable habitats at low temperatures, where physiological including sensory processes are slowed, and/or that low temperatures prevent the production of sufficient numbers of pelagic larvae, which would be necessary to find suitable habitats in the vast oceanic spaces.
[3] Rapoport's rule states that latitudinal ranges of species are generally smaller at low than at high latitudes.