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Smithsonian National Museum of Natural History
Naked Coral

Oral view of the “naked coral” Ricordea florida (Hexacorallia, Corallimorpharia). This species inhabits shallow tropical waters in the Caribbean Sea.  This image was taken by Mónica Medina and David Keys.

Naked Corals: Did They Lose Their Skeletons?

Corals structure the most diverse ecosystems in the oceans: the tropical reef environments. Today corals face tremendous pressures stemming from human activities and are under the threat of extinction. If all corals were to go extinct, the repercussions would likely affect all life on earth. Understanding these basic life forms is, therefore, essential to our future.

All corals in the sea, particularly the familiar kinds that form reefs, have hard external skeletons. But some close relatives of corals, the anemones, do not have external skeletons at all. Inside their bodies, however, the two groups are similar. For example, corals and anemones have mouths through which food enters into their stomachs, which, in turn, have tissue partitions aiding digestion. It has long been thought that corals and anemones are related. But how are they related? Could anemones lacking skeletons be descendants of coral ancestors? In other words, could some corals simply have lost their skeletons long ago through evolutionary processes, giving rise to anemones?

Allen Collins

Allen Collins preserving tissue samples

In a new article by Allen Collins and his colleagues, published in the prestigious Proceedings of the National Academy of Sciences USA (Medina M., A.G. Collins, T.L. Takaoka, J.V. Kuehl and J.L. Boore. 2006. Naked corals: skeleton loss in Scleractinia. Proc. Natl. Acad. Sci. USA 103: 9096-9100), the authors hypothesize the loss of the coral skeleton during the Cretaceous Period (110-132 million years ago), creating “naked corals.” Stony corals (Scleractinia), as both their formal and common names imply, have hard external skeletons. Evolutionary discussion of stony corals has often raised the possibility that various anemone groups lacking skeletons might have descended from coral ancestors, an idea that has been dubbed the “naked coral” hypothesis. But there has never been any strong evidence to support the idea.

An affiliated National Museum of Natural History scientist with the National Systematics Laboratory of the National Oceanic and Atmospheric Association’s National Marine Fisheries Service, Allen Collins, along with Mónica Medina, generated molecular data (complete mitochondrial genomes from both stony corals and anemones) and conducted phylogenetic analyses that provided the first clear evidence that one group of stony corals is more closely related to an anemone group (Corallimorpharia) than it is to a second coral group. With this strong evidence, the scientists concluded that while some corals continued their evolution as stony corals with skeletons, another lineage (line of ancestry) of corals lost its skeleton giving rise to the “naked corals” still in existence today.

The team used a molecular clock analysis (using genetic distances and rates of divergence) to date the origin of stony corals to between 240 and 288 million years ago, much more closely matching the fossil record of corals than earlier estimates. The scientists also estimated that the skeleton-lacking Corallimorpharia (naked corals) originated more recently, between 110 and 132 million years ago.

Most interesting was the finding that during this time period in Earth’s history, when corals most likely lost their skeletons, the oceans had high CO2 levels, which would have negatively impacted their ability to grow their skeletons. The authors surmise that naked corals may have escaped extinction by adapting to these conditions. They also raised the question of what will be the fate of the thousands of today’s stony coral species facing pressures of increasing CO2 in the ocean brought about by human activity. Since this rising CO2 interferes with the stony corals’ ability to secrete their skeletons, could these beautiful corals lose their skeletons? Will any go extinct and what would be the impact of losing such key reef-building animals?

Clade Tree

Evolutionary relationships among sampled hexacorallians (the larger group to which corals belong). Support values (Bayesian posterior probabilities and maximum parsimony bootstrap indices) are shown at each node of the tree. A single 100 indicates that the node has maximal support for both values. Ranges of estimated divergence dates are shown for nodes indicated by open circles. Fixed divergence dates based on earliest fossil appearances of different groups are shown at nodes indicated by filled circles.




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