Thorson's rule
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Thorson's rule
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.
Most low-latitude species of Monogenea
produce large numbers of ciliated larva
e. However, at high latitudes, species of the entirely viviparous family Gyrodactylidae, which produce few nonciliated offspring and are very rare at low latitude
s, 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 snail
s: 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 substrate
s, 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.
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;
Gyrodactylidae have not yet been found in the deepsea.
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 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.
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.
states that benthic marine invertebrates
Marine invertebrates
Marine invertebrates are animals that inhabit a marine environment and are invertebrates, lacking a vertebral column. In order to protect themselves, they may have evolved a shell or a hard exoskeleton, but this is not always the case....
at low latitudes tend to produce large numbers of eggs developing to pelagic (often planktotrophic
Ovoviviparity
Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...
, which are often brooded.
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.Most low-latitude species of Monogenea
Monogenea
Monogenea are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.-Characteristics:Monogenea are very small parasitic flatworms mainly found on skin or gills of fish....
produce large numbers of ciliated larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e. However, at high latitudes, species of the entirely viviparous family Gyrodactylidae, which produce few nonciliated offspring and are very rare at low latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s, represent the majority of gill Monogenea
Monogenea
Monogenea are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.-Characteristics:Monogenea are very small parasitic flatworms mainly found on skin or gills of fish....
, 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 snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s: 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 substrate
Substrate (marine biology)
Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....
s, 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.
Does Thorson's rule apply to deepsea species?
The temperature gradientTemperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...
from warm surface waters to the deep sea
Deep sea
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...
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;
Gyrodactylidae have not yet been found in the deepsea.
Explanations of the phenomenon
Several explanations of the rule have been given. They include:- Because of the reduced speed of development at low temperatures, most species cannot complete development during the short time of phytoplanktonPhytoplanktonPhytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
bloom, on which planktotrophic species depend; - Most species cannot synchronize hatching with the phytoplankton bloom;
- Slower development increases the risk of predation on pelagic larvae;
- Non-pelagic larvae can settle close to the parent, i.e. in a favourable environment;
- Small pelagic larvae may have osmotic difficulties in Arctic and Antarctic summers, due to the melting ice;
- Small larvae may not be able to survive at very low temperatures;
- Cold temperature may select for large size at the beginning of development, resulting in non-pelagic larvae; and
- 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). 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 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
Rapoport's ruleRapoport's rule
Rapoport’s rule is an ecological hypothesis that states that latitudinal ranges of plants and animals are generally smaller at lower than at high latitudes.-Background:...
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.
See also
- Latitudinal gradients in species diversityLatitudinal gradients in species diversityThe increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal diversity gradient , is one of the most widely recognized patterns in ecology. Put another way, in the present day localities at lower latitudes generally have more...
- EcologyEcologyEcology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
.