HISTOLOGICAL DATA IN A COMBINED PHYLOGENETIC ANALYSIS OF SCLERACTINIAN REEF CORALS

Robust phylogenetic reconstructions are critical to understanding the history of life. Cnidarians are of particular interest as they make up a large component of tropical coral reefs, which are in need of conservation. A combination of molecular and morphological data sources are necessary to resolve this phylum at both higher and lower taxonomic ranks.

Traditionally, studies of the stony coral order Scleractinia have focused on morphological characters from the hard skeleton. While useful, this alone has not yielded enough characters to differentiate all taxa and makes comparison to exclusively soft tissue orders (e.g. Actiniaria) more complicated. For this study, eight Pacific species of scleractinian corals, from the families Lobophylliidae and Merulinidae, were investigated for new characters within the soft tissue. Specimens were ordered from retail stores and housed in home aquaria, decalcified, embedded in paraffin, cut into thin sections and stained with H&E; similar to medical histopathological studies. A total of 20 characters focusing on stinging cell density, diversity and relative tissue thickness were coded. These 20 new histomorphological characters were added to data sets of 46 skeletal characters and 1383 molecular characters (cox1 and cob genes). This combined data set was analyzed with both parsimony (3 MPTs of length 203) and Bayesian methods, which yielded similar topologies. Character maps were also produced to show that families Lobophylliidae and Merulinidae are monophyletic, and have distinct stinging cell densities and diversity.

Despite adding only a small number of new characters, in comparison to much larger skeletal and molecular character data sets, increased support can be achieved by the addition of histomophological characters to a combined analysis. This suggests that histological characters are a valid taxonomic tool to use for reconstructing lineage histories of extant corals. Thus, with a larger number of characters to work with, phylogeneticists can better resolve phylogenies and use these data to provide conservation managers information about their phylogenetic diversity.