Benthos (adj. benthic): fauna and flora in the lowest level of a water body (e.g., ocean or lake) including the sediment surface and layers just below the surface.
Brooding: incubating eggs by a parent.
Lecithotrophic: yolk feeding.
Ovoviviparity (adj. ovoviviparous): development within eggs that remain within the mother’s body until hatching or just before hatching.
Plankton (adj. planktonic): flora and fauna floating in the open water columns of oceans, lakes etc., i.e., in the pelagic zone.
Planktotrophic: plankton feeding.
Viviparity (adj. viviparous): development of the embryo inside the mother.
Ovoparity: development from an egg outside the mother
Definition and groups involved
Thorson’s rule states that marine benthic 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 they tend to produce fewer and larger lecithotrophic (yolk-feeding) eggs and large offspring, often by viviparity or ovoviviparity, which is often brooded (e.g., Thorson 1950,1957; Mileikovsky 1971; Aenaud 1977; Simpson 1977; Picken 1980; Jablonski and Lutz 1983; Rohde 1985; Pearse 1994; Krug 1998; Stanwell-Smith et. al. 1999; Gallardo and Penchaszadeh 2001; Laptikhovsky 2006). Rohde (1985) has shown that the phenomenon is not restricted to benthic invertebrates, i.e, it 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 Subantarctic/Antarctic waters, against less than 1% in tropical waters. 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 1998; see also Pearse 1994 for echinoderms). The studies of Gallardo and Penchaszadeh (2001) indicate that two factors are important for Thorson’s rule to be valid for marine gastropods, i.e.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 data in Gusev (1978) indicate that Gyrodactylidae may also be more common in cold than tropical freshwater systems, suggesting that Thorson’s rule may apply to freshwater invertebrates.
Does Thorson’s rule apply to deepsea species?
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 (Mileikovsky1971); Gyrodactylidae have not yet been found in the deepsea (Rohde 1985).
Explanations of the phenomenon
Several explanations of the rule have been given (for a discussion and further references see Rohde 1985). They include 1) Because of the reduced speed of development at low temperatures, most species cannot complete development during the short time of phytoplankton bloom, on which planktotrophic species depend; 2) Most species cannot synchronize hatching with the phytoplankton bloom; 3) Slower development increases the risk of predation on pelagic larvae; 4) Non-pelagic larvae can settle close to the parent, i.e. in a favourable environment; 5) Small pelagic larvae may have osmotic difficulties in Arctic and Antarctic summers, due to the melting ice; 6) Small larvae may not be able to survive at very low temperatures; 7) Cold temperature may select for large size at the beginning of development, resulting in non-pelagic larvae;8) In cold waters it is more difficult to precipitate dissolved calcium, which results in reduced body size of animals supported by calcium skeletons, leading to viviparity.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). Rohde (1985) concluded 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 down, 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.
Implications for Rapoport’s rule
Rapoports’s rule states that latitudinal ranges of species are generally smaller at low than at high latitudes (Stevens 1989). Thorson’s rule contradicts this rule, because low-latitude 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 (e.g., Rohde et al. 1993; Rohde 1999).
Laptikhovsky (2006) pointed out that the inverse relationship between egg size in marine animals and water temperature was simultaneously with Thorson discovered by Rass. However, whereas Thorson’s rule describes ecological processes caused by natural selection on a larger scale, i.e., at the level of species and higher taxa, Rass’s rule describes physiological processes within populations and species, and between closely related species; populations produce larger eggs in colder environments because of phenotypic plasticity. Therefore, it is incorrect to refer to a Thorson – Rass rule.Copyright NoteThis is a largely revised version of my article originally published in Wikipedia (link here). The section on Rass’s rule and some references have been added. The article contains no substantial changes by others.
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