Cnidarian classification can be a terrible thing. Like many other groups of soft-bodied animals, useful characters for distinguishing taxa can be few and far between, and those few characters that are available may be difficult to identify and readily subject to evolutionary change. Corals are no exception. The current generally-used classification of living corals divides them between seven suborders, distinguished from each other primarily by the structure of their septa, the ribs of skeletal material within each polyp. The Caryophylliina, for instance, have simple septa, with little in the way of internal ornamentation. Members of this suborder have been found from as long ago as the early Jurassic, not too long after the modern Scleractinia corals originated in the mid-Triassic. Caryophylliines remain a successful group - nearly five hundred species have been described, and they include both shallow-water and deep-sea forms. The majority of caryophylliines do not contain zooxanthellae (symbiotic dinoflagellates) and while there are some colonial forms, the majority are solitary.
Which sounds all very fine and dandy, but is not anywhere near as informative as one might think. It doesn't take much reading between the lines to note that the 'Caryophylliina' with their 'simple septa' are essentially united by the absence of the features characterising other suborders. As such, it is hardly surprising that they should be nearly as old as the Scleractinia as a whole, because they are quite possibly phylogenetically equivalent to the Scleractinia as a whole. A caryophylliine is simply a coral that doesn't put on airs. This possibility is bourne out by molecular analysis (Le Goff-Vitry et al., 2004), which divides scleractinians between two major clades that have been dubbed the 'robust' and 'complex' clades (or 'Robusta' and 'Complexa' by Kerr, 2005)*. Though not corresponding to earlier morphological divisions, the two clades are not without morphological support. 'Robust' corals have solid, heavily calcified skeletons, while 'complex' corals have lighter, more porous skeletons. Caryophylliines, it turns out, are distributed between both clades, and multiple subclades within those clades. Even the type family of the 'suborder', the Caryophylliidae, is not monophyletic, with a suggested division between no less than five clades scattered among the Robusta and Complexa (Kerr, 2005).
The necessary changes to scleractinian classification could yet be even more radical. Medina et al. (2006) found that the Complexa were more closely related to the soft-bodied, skeletonless Corallimorpharia than they were to the Robusta. It remains an open question whether the calcified skeleton evolved independently in the two clades, or whether the corallimorpharians represent a secondary loss of the skeleton, but my one suspicions lean towards the latter, especially since it has been demonstrated that the loss of the ability to secrete a skeleton does not constitute a death sentence for a coral (Fine & Chernov, 2007)*. Skeletal construction may yet play a role in coral taxonomy, as researchers identify more reliable ultrastructural characters (Stolarski & Roniewicz, 2001), but I think we can safely say that the 'Caryophylliina' as we have hitherto known it is, well, dead in the water.
REFERENCES
Fine, M., & D. Tchernov. 2007. Scleractinian coral species survive and recover from decalcification. Science 315 (5820): 1811.
Kerr, A. M. 2005. Molecular and morphological supertree of stony corals (Anthozoa: Scleractinia) using matrix representation parsimony. Biological Reviews 80: 543-558.
Le Goff-Vitry, M. L., A. D. Rogers & D. Baglow. 2004. A deep-sea slant on the molecular phylogeny of the Scleractinia. Molecular Phylogenetics and Evolution 30: 167-177.
Medina, M., A. G. Collins, T. L. Takaoka, J. V. Kuehl & J. L. Boone. 2006. Naked corals: skeleton loss in Scleractinia. Proceedings of the National Academy of Sciences of the USA 103 (24): 9096-9100.
Stolarski, J., & E. Roniewicz. 2001. Towards a new synthesis of evolutionary relationships and classification of Scleractinia. Journal of Paleontology 75 (6): 1090-1108.
Was there supposed to be footnotes to go with the asterisks?
ReplyDeleteHuh. I was going to include some rampant speculation about the potential implications of coral survival sans skeleton for the hitherto unpopular idea that scleractinian corals may be descended from Palaeozoic rugose corals, part of the problem with which is supposed to be that rugose corals disappear at the end of the Permian but scleractinians don't appear until mid-way through the Triassic, leaving a noticeable gap. But then I looked into things further and found that there appears to be something of a preservation gap at the beginning of the Triassic anyway, which would rather shut down the ability to say anything productive about what was and wasn't around then anyway, so I withdrew the speculation. Guess I forgot to remove the asterisk.
ReplyDeleteI wrote a song called Solitary Coral last year! I used the same pic as you too lol.
ReplyDeletehttp://web.mac.com/kzelnio/Site/Song_Vault/Entries/2007/7/27_Solitary_Coral.html