Thorson's rule

Ecogeographical rule describing reproductive strategies of marine invertebrates
Thorson's rule (named after Gunnar Thorson by S. A. Mileikovsky in 1971)
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.

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. 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. There are exceptions to the rule, such as ascoglossan snails: tropical ascoglossans have a higher incidence of lecithotrophy and direct development than temperate species. 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 temperature gradient from warm surface waters to the deep sea is similar to that along latitudinal gradients. A gradient as described by Thorson's rule may therefore be expected. However, evidence for such a gradient is ambiguous;