Ocean acidification and scleractinian corals [2]

In their Brevia “Scleractinian coral species survive and recover from decalcification” (30 March, p. 1811), M. Fine and D. Tchernov present an exciting experimental approach documenting how coral skeletons dissolve as a physiological response to increased atmospheric CO2, a subject currently at the height of public concern (1, 2). The fact that these authors demonstrated that five species of living scleractinian corals could lose their aragonitic skeletons, in response to elevated CO2, and then continue to exist perfectly well as soft-bodied polyps is a confirmation of the ephemeral or “naked coral” hypothesis (3, 4). This physiological response assists our understanding of the survival potential of corals after mass extinctions such as after the devastating one at the end-Permian (5). In addition, it explains the previously unexplained, geologically “sudden” appearance of order Scleractinia in Middle Triassic time, when geochemically perturbed oceans returned to normal. Before this time, corals and reefs disappeared from the fossil record for millions of years but may have continued to exist as “naked corals,” thus remaining paleontologically “hidden” from our view. An important implication of this work is the arbitrary and artificial nature of the current phylogenetic classification of scleractinian corals and some anemones. Why should Order Scleractinia be recognized on the basis of a calcified skeleton when essentially identical soft-bodied, anemone-like forms such as the Corallimorpharia belong to another group? Fine and Tchernov's decalcification experiments may not be representative of all varieties of corals. I suspect that zooxanthellate reef-building species would have responded very differently to the experiment because of the complex nature of their photosymbiosis (6). Readers should not misconstrue this paper as tacit validation that we need not be as concerned about the growing problem of ocean acidification with regard to corals and reefs.