Coloniality

Coloniality, in the context of invertebrate zoology, refers primarily to the general morphological plan in which the genetic individual (or genotype) is composed of physiologically connected units or modules (called polyps, thecas, zoids, or zooids, depending on the taxonomic group). Coloniality usually develops through some process of repeated budding in which the daughter modules remain attached to the parent modules. If individual modules separate, these are collectively and individually considered to be a clone and clones, respectively. Because connected sets of modules can also separate from other sets to become clones of subcolonies, however, the terminological distinction between colonial and clonal is rather flexible, and all colonies are potential clones (though all clonal taxa are not colonial). Found in protozoans, sponges, cnidarians, bryozoans, ento-procts, urochordates, and hemichordates, coloniality is widespread within Metazoa (that is, multicellular animals) and important ecologically in many communities. The concept of coloniality is also closely analogous to that of plants as "populations" of node and internode units. Nonclonal organisms, such as most vertebrates, are known as unitary organisms.

Phenotypic plasticity and modular polymorphism are two morphological characteristics that are closely associated with colo-niality. Phenotypic plasticity, in this context, refers to the ability of a single genotype to alter the expression of its phenotype across a range of environments (the phenotype is the appearance of a genotype in a particular environment). Although all organisms modify their phenotypic expressions of traits to various extents, the overall morphology of colonial individuals is often especially "plastic" in this way. For example, a branching coral colony growing in an extremely wave-exposed site will tend to grow with its polyps close together in wider branches, thereby forming denser, stouter branches. If one cloned this colony by carefully collecting a branch from this colony and transplanting it to a site with much calmer water, the regenerating coral would likely grow with less dense polyps arranged along thinner, more stretched out branches. Hence, the arrangements of the modules (that is, polyps) that collectively make up the shape of the colony is phenotypically plastic across these two environmental conditions.

Modular polymorphism refers to the ability of colonial organisms to produce modules of different types during their growth. For example, some colonial species produce one type of module to function primarily in the capturing of food, whereas other types are produced for other functions, such as defense against predators or reproduction. Such polymorphisms are actually a specific type of phenotypic plasticity, since within a colony the phenotype of the modules changes while the genotype remains the same. Scientists believe that both general phenotypic plasticity and modular polymorphism provide mechanisms for colonial organisms, many of which remain attached to the benthos throughout their lives, to adapt to their local environmental conditions as much as possible.

Note that an alternative usage of colonial-ity from the behavioral sciences refers to the close and cooperative living association of unitary (that is, nonclonal) individuals. Such associations are found in kin groups in many species of vertebrates, as well as in social insects and other arthropods.

—Daniel R. Brumbaugh

See also: Bryozoa; Cnidarians (Sea Anemones, Corals, and Jellyfish); Sponges

Bibliography

Hughes, Roger N. 1989. A Functional Biology of Clonal Animals. New York: Chapman and Hall; Jackson, Jeremy B. C., Leo W. Buss, and Robert E. Cook, eds. 1985. Population Biology and Evolution of Clonal Organisms. New Haven: Yale University Press.

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