Variation in Species
In considering what species are it is helpful to compare corals with plants. Like most plants, corals have morphological characteristics that vary according to the environment in which they occur, their geographic location, and genetic links among component populations.
Most colonial corals show morphological variation within the one colony. Colonies of some species have variable growth forms which are at least partly genetically regulated independently of environment. However, most species exhibit variations that are clearly associated with different parts of the colony having different micro-environments. Differences among micro-environments include growing space, light availability, sedimentation and fish grazing. The morphological variations that result affect both growth form and corallite structure.
Different colonies (or individuals) of all coral species vary along environmental gradients. Divers can readily see this as they descend down a reef slope. This variation is also seen if a coral is transplanted from one habitat to another. It will usually change its appearance to reflect the conditions of the new habitat. Most plants do likewise; these responses are not under direct genetic control, although particular genotypes may be associated with particular morphologies.
The most important environmental factors controlling growth form are exposure to wave action, levels of illumination, sediment load and exposure to currents. The different morphologies that result must be accommodated by taxonomic descriptions and understood by persons identifying corals.
There are two distinct categories of geographic variation in corals: those which are the outcome of environmental factors, and those which are genetic.
Geographic variation which is the outcome of environment is best seen in high latitude coral communities, where low temperature and non-reef habitats result in colonies which are distinct from their tropical reef-dwelling counterparts. Likewise, there are whole geographic regions where the water is almost always clear (such as the Red Sea or Bahamas) and other regions where the water almost always contains sediment from the land (such as the south-east Asian coast). Corals from these regions commonly have distinct points of morphological detail which are probably due to environmental differences.
Geographic variation which is genetically based affects almost all species in some way or other. Again like most plants, the appearance of a single species changes (gradually or abruptly) from one region to the next. These changes may not concern the individual who is interested in the species of a single country, but they will definitely concern anybody who tries to answer the question ‘what are species’? Of more practical importance, it will also concern anybody who tries to identify a species in one country from information about that species (including photographs) from another country. The taxonomic issues that arise are explained here.
Genetic links among populations
It is common for a series of adjacent colonies of the same species growing in a uniform environment to display a wide range of colours and to have a variety of morphological differences. In such cases the presence or absence of morphological continuities among colonies, or populations, can be used to distinguish groups of species (commonly called ‘sibling species’) from a single variable species (commonly called ‘polymorphic species’). Although the differences seen are widespread, intermediate colonies are sometimes found. For this reason it is considered to be a single species.
Genetic bridges and barriers generate reticulate patterns in time and space, patterns which maintain the species’ genetic heterogeneity. The taxonomic issues that arise are of endless complexity. They inevitably lead to the conclusion that there are no fundamental differences between species and subspecies taxonomic levels.