Corals defy species classification

Classifying corals in terms of species is a risky business. Biologist Onno Diekmann from the University of Groningen has discovered that four species of stone corals differ so little in terms of their genetic material that they can scarcely be termed separate species.

Corals are formed by a collection of identical coral polyps which together form a coral colony. Onno Diekmann compared the genetic material from six different species of coral from the Madracis genus, which are found in the coral reefs around Curaçao. The coral exists in many different physical forms. There are knobby, branched and crust-forming colonies. The corals grow at depths varying from 2 to 70 metres. The external appearance is partly determined by the environmental conditions, such as temperature, water movements and the amount of available light. Therefore, it is difficult to determine if two coral colonies belong to the same species, if only the external appearance is used.

Two forms of Madracis were found to be clearly distinct species. Yet four other species exhibited a considerable overlap in the genetic variation. Therefore, which of the four species these corals belong to cannot be determined with any certainty. The spectrum of intermediate forms indicates that these four species can interbreed. However, the four species do differ in their physical appearance. In addition to the colony form there are also smaller characteristics where differences might be exhibited. Yet none of the individual microcharacteristics can be used to unequivocally determine which species an individual coral belongs to. For this several characteristics need to be analysed at the same time.

It is difficult to apply the term ’species’ to corals. Perhaps this is because they are found in the ocean where physical barriers to reproduction between different species are not or are scarcely present. The ocean currents determine the direction in which a species can be moved. Due to sea level changes the ocean current patterns are highly variable as a result of which the mixing of various coral ’species’ can continually occur.

For corals where fertilisation and development of the larvae takes place in water, it was already known that differences between species can be sufficiently small to allow interbreeding to take place with the production of fertile offspring. This research on Madracis has demonstrated that corals which reproduce by internal fertilisation and the hatching of offspring can also interbreed.

For further information please contact Onno Diekmann (Department of Marine Biology, University of Groningen), tel +31 (0)50 3632226, fax +31 (0)50 3632261, e-mail: o.e.diekmann@biol.rug.nl. The defence of the doctoral thesis will take place on 27 February 2003. Mr Diekman’s supervisor is Prof. R.P.M. Bak.

Image at www.nwo.nl/nieuws

The research was funded by the Netherlands Organisation for Scientific Research (NWO).