Differentiation in the Coral Genus Madracis

M. Vermeij 1,2, J. Kaandorp 2, R. Bak 2,3 1.

1. Caribbean Institute for Management and Research of Biodiversity,
Piscaderabaai z/n, POBox 2090, Willemstad, Curac,ao,
Netherlands Antilles

2. University of Amsterdam,
Kruislaan 318,
1098 SM Amsterdam,
The Netherlands

3. Netherlands Institute for Sea Research (NIOZ),
P.O. Box 59, 1790 AB Den Burg, Texel,
The Netherlands

We used three-dimensional scanning techniques to study morphological variation in the Caribbean coral genus Madracis. We want to know if these techniques (1) allow us to obtain discrete morphological characterizations of the different Madracis species, (2) are useful to describe morphological intermediates between these species that could be indicative of hybridization and (3) allow us to compare simulated growth forms with actual 3D-forms. We will determine the interspecific morphological range for the different Madracis species and the effect of depth on colony morphology by 3D-CTD measurements. By comparing the environmental component and the species (genetic) component we want to gain insight in the species boundaries based on morphological characteristics on colony level. The coral genus Madracis currently comprises five species in the Caribbean, which are dominating on Caribbean reefs. They survive in a heterogeneous environment by a wide array of ecological strategies and morphological adaptations. We found that the classification of species within the genus is unreliable because of the absence of discrete morphological species-characteristics. Morphological and ecological variation has traditionally been explained as different genetic responses to a variable environment. In a heterogeneous environment a single genotype can therefore develop different phenotypes in different environments. The resulting phenotypic plasticity affects both natural selection on phenotypes and the genetic response to selection. Combined morphological, ecological and genetical information will provide insight in the species characterization in stony corals (Scleractinia), their mutual inter-relationships (e.g. hybridization) and their evolutionary pathways. Distinction of species using morphological characteristics is often impossible or subjective because of poor morphological terminology and the great variability connecting distinct morphological traits by intermediate forms. We found this variability to be great on corallite and colony level.

Three -dimensional morphological analysis will be useful in the characterisation of (morphological) species, but also provides insight in the occurrence of hybridisation. The ability of corals to hybridise influences the formation of new species and the genetic variation within gene pools. Synchronised spawnings occur for many sympatric corals providing an ideal environment for hybridisation. Madracis species produce gametes in the same period making hybridisation a possible process to occur. Other studies have shown that hybridisation occurs between coral-species and that fertilisation success was highest for morphologically related species. This makes clear distinctions between (morpho-)species impossible if the hybrids (i.e. morphological intermediates) survive. Again, the lack of detailed objective morphological criteria would hinder the recognition of such morphological hybrids. The morphological or phenetic species concept, traditionally well established in coral taxonomy at the corallite but also at colony-level appears therefore questionable.