Before the advent of taxonomic studies on reefs, many coral taxonomists thought that a specimen with a different growth form implied a different species. Thus, thousands of supposedly ‘new species’ were described from specimens collected on expeditions and brought back to museums and universities for study. Scuba diving was largely responsible for changing this. It allowed taxonomists to observe variation in corals more or less as botanists had long observed variation in trees. It allowed differences between species that grow together to be studied in great detail and it also allowed environmental variation within the same species, such as occurs down a reef slope, to be observed directly.
Geographic variation is much less simple to study as knowledge of it must be accumulated from separate studies in different countries: it cannot be directly observed. The main taxonomic issues are: (a) species become progressively less recognisable as single units with increasing geographic range, (b) taxonomists are forced to make arbitrary decisions, (c) synonymies vary geographically. In each case, the more detailed a taxonomic study is, the greater the problem becomes.
Taxonomic certainty and geographic range
With corals as with most plants, most species do exist as more-or-less definable units in single geographic regions, such as the Red Sea, the Indonesian/Philippines archipelago, the Great Barrier Reef or the Caribbean. However, widespread species commonly show sufficient geographic variation that they could reasonably be divided into several separate ‘sibling species’ were it not for the fact that these smaller units form continua. Thus, for example, the majority of species of the Red Sea also occur in the central Indo-Pacific, but many are sufficiently different in the Red Sea that they would be considered to be distinct species if they were transplanted to the central Indo-Pacific.
In theory, taxonomy should accommodate biogeographic patterns. In practice, doing this creates issues like those created when a flat map is projected onto a sphere. The bigger the area of the map, the greater the distortion that results. It is possible to modify the flat map using different types of projections, but this only changes the nature of the distortion.
In traditional taxonomy, geographic variation within a species is accommodated by creating divisions within the species, such as varieties, races or geographic subspecies. In reality, geographic variation repeatedly overrides the morphological boundaries of individual species. In other words, natural continua go beyond the taxonomic or morphological boundaries of individual species. This cannot be accommodated by creating divisions within species. The problem remains if the species unit is ‘split’ into smaller units or ‘lumped’ into larger units and it is not solvable by further or more detailed study. Ultimately, the only unit in Nature that is real is the syngameon, described here.
The reasons why syngameons are not used in operational taxonomy are (a) they can only be determined with any degree of certainty through exhaustive cross fertilisation studies in all geographic regions where their component species occur, (b) they are not likely to have distinguishing morphological characteristics and (c) they would include so many morphological species that they would need to be re-divided into sub-units of some kind in order to be useful.
This issue will always force taxonomists to make an arbitrary decision as to what a particular species is. Some groups of species may be distinctive in some geographic regions and not in others. The outcome of detailed studies of these species may either be a single ‘species complex’ or a group of similar species. In either case, species descriptions and distribution maps artificially simplify the reality of the complex.
Geographic patterns and synonymies
One of the principle outcomes of any taxonomic revision is the formulation of ‘synonymies’. A synonymy is usually a list of names that have been applied to a particular species together with the identity of the authors who used the names. With corals, synonymies typically include names used by different authors working in different countries at different times. They primarily focus on subspecies levels. These vary geographically just as species do, only more so. Therefore, the more comprehensive a synonymy is intended to be, the more likely it will be that it will include ‘fuzzy’ species boundaries and thus the more likely it will be that it includes arbitrary decisions.
General principles about species over large geographic ranges are: (1) their geographic boundaries interact with other species, (2) their morphological boundaries interact with other species, (3) their synonymy interacts with other species, (4) there are no definable distinctions between species and subspecies taxonomic levels.