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. [2]
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. [3] 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. [4]
There are exceptions to the rule, such as ascoglossan snails: tropical ascoglossans have a higher incidence of lecithotrophy and direct development than temperate species. [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. [6]
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; [1] Gyrodactylidae have not yet been found in the deep sea. [3]
Several explanations of the rule have been given. They include:
Most of these explanations can be excluded for the Monogenea, whose larvae are never planktotrophic (therefore eliminating explanations 1 and 2), their larvae are always short-lived (3), Gyrodactylidae are most common not only close to melting ice but in cold seas generally (5). Explanation 6 is unlikely, because small organisms are common in cold seas, Gyrodactylidae are among the smallest Monogenea (7), and Monogenea do not possess calcareous skeletons (8). 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. Thorson's rule contradicts this rule, because species disperse more widely at low than at high latitudes, supplementing much evidence against the generality of Rapoport's rule and for the fact that tropical species often have wider geographical ranges than high latitude species. [7] [8]
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The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word pelagic is derived from Ancient Greek πέλαγος (pélagos) 'open sea'. The pelagic zone can be thought of as an imaginary cylinder or water column between the surface of the sea and the bottom. Conditions in the water column change with depth: pressure increases; temperature and light decrease; salinity, oxygen, micronutrients all change. Somewhat analogous to stratification in the Earth's atmosphere, but depending on how deep the water is, the water column can be divided vertically into up to five different layers.
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Rapoport's rule is an ecogeographical rule that states that latitudinal ranges of plants and animals are generally smaller at lower latitudes than at higher latitudes.
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Marine larval ecology is the study of the factors influencing dispersing larvae, which many marine invertebrates and fishes have. Marine animals with a larva typically release many larvae into the water column, where the larvae develop before metamorphosing into adults.
Gunnar Axel Wright Thorson was a Danish marine zoologist and ecologist, who studied at the University of Copenhagen under the professors C.G. Johannes Petersen, August Krogh, Theodor Mortensen, Ragnar Spärck and Carl Wesenberg-Lund. In 1957, Thorson was appointed professor of marine biology at the University of Copenhagen.
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Klaus Rohde is a German biologist at the University of New England (UNE), Australia, known particularly for his work on marine parasitology, evolutionary ecology/zoogeography, and phylogeny/ultrastructure of lower invertebrates.
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