A Brain Explosion

Elliptical star coral Favia speciosa, photographed by Utsunomiya.
For today's random taxon, I drew the Faviinae. This is a subfamily within the Faviidae, commonly known as 'brain corals' and recognised as an important family among the tropical reef-builders. Families of corals have generally been distinguished by the arrangement and morphology of skeletal structures within the coral cup: Faviidae were characterised by having trabeculae (the calcareous fibres forming the basis of the skeletal septa) arranged in one or two fans, with more or less regular marginal teeth at the top of the septum. The Faviidae were divided into two subfamilies, the Faviinae and Montastreinae, based on whether the budding of polyps takes place inside (Faviinae) or outside (Montastreinae) the individual cups in a colony. Several genera recognised within the Faviinae were mostly distinguished by their colony form and how the individual polyps are arranged (Budd & Stolarski 2011).

Leptoria phrygia, photographed by Neville Coleman.
And if any of you were wondering about the use of the past tense in the last paragraph, that is because more recent studies have been pretty unanimous in indicating that the system just described is in need of a significant shake-up. As noted here in an earlier post, molecular studies have indicated that coral taxa distinguished by septal characters are widely problematic. A broad phylogenetic study of corals by Fukami et al. (2008) found that, of sixteen recognised families tested, eleven were polyphyletic. Faviidae, in particular, were scattered between no less than seven of the twelve supported clades identified by Fukami et al. within the broader 'robust clade'. Some of the larger genera, such as the type genus Favia, were also polyphyletic and dispersed between multiple clades. The level of discordance between morphological classification and molecular phylogeny is reflected by the fact that Fukami et al.'s large clade XVII, containing members of the families Faviidae, Merulinidae and Pectiniidae, has since been informally dubbed the 'Bigmessidae' (Huang et al. 2011). The 'Bigmessidae' also includes the genus Trachyphyllia, a morphologically very variable coral that is generally found free-living, either solitary or colonial, among the sand at the very base of coral reefs (Best & Hoeksema 1987). Trachyphyllia has been treated by some authors as its own family, or regarded by others as an unusual member of the Faviinae.

Maze brain coral Goniastrea australensis, photographed by David Witherall.
As yet, no formal reclassification of the corals has been proposed, but when it eventually is, it is likely that there will be no Faviidae at all. The type species of the family, Favia fragum, is closely related to the type species of another family, Mussidae, and the latter name is the one with priority. Interestingly, both these taxa are found in the Atlantic Ocean, and the molecular phylogenies do support a separation of the Atlantic faviids from the Pacific species. Also, it is still possible that morphological characters will play their part in the coming coral reclassification: even though the broader scale features of the septa and colony form have proven vulnerable to convergence, smaller scale features of the skeletal microstructure promise to be less discordant with molecular phylogenies (Budd & Stolarski 2011).

REFERENCES

Best, M. B., & B. W. Hoeksema. 1987. New observations on scleractinian corals from Indonesia: 1. Free-living species belonging to the Faviina. Zoologische Mededelingen 61 (27): 387-403.

Budd, A. F., & J. Stolarski. 2011. Corallite wall and septal microstructure in scleractinian reef corals: comparison of molecular clades within the family Faviidae. Journal of Morphology 272: 66-88.

Fukami, H., C. A. Chen, A. F. Budd, A. Collins, C. Wallace, Y.-Y. Chuang, C. Chen, C.-F. Dai, K. Iwao, C. Sheppard & N. Knowlton. 2008. Mitochondrial and nuclear genes suggest that stony corals are monophyletic but most families of stony corals are not (order Scleractinia, class Anthozoa, phylum Cnidaria). PLoS ONE 3 (9): e3222.

Huang, D., W. Y. Licuanan, A. H. Baird & H. Fukami. 2011. Cleaning up the ‘Bigmessidae’: molecular phylogeny of scleractinian corals from Faviidae, Merulinidae, Pectiniidae and Trachyphylliidae. BMC Evolutionary Biology 11: 37.