There has been recent interest in the corallimorpharians by marine biologists, due to genetic studies that demonstrate that some hard corals (Scleractinia) are actually more closely related to corallimorphs than to other members of Scleractinia. This has resulted in several papers that suggest that corallimorphs once had a Scleractinian ancestor with a calcium carbonate skeleton. Prior to these genetic studies, it had been suggested (through taxonomic studies) that corallimorphs were in fact more closely related to sea anemones (Actiniaria).
To explain why the loss of this skeleton may have occurred, researchers have been suggesting that increased levels of carbon dioxide during the Triassic period (from about 200-240 million years ago), would have caused a reduction in oceanic pH, which resulted in a decreased ability by hard corals to build a calcium carbonate skeleton. These conditions would therefore select for corals that could adapt to live without their skeletons. And voila!, the corallimorpharians were born. Unfortunately, the fossil record of corallimorpharians is minimal due to their lack of calcium carbonate skeleton, making this claim hard to prove. It is certainly a very seductive argument, especially now that atmospheric levels of CO2 are once again rising to levels that may soon have a detrimental impact on hard corals and their ability to continue reef building. This has some scientists positing that in the future as hard corals start dying off, soft bodied anthozoans will once again have an adaptive advantage, resulting in their proliferation.
An interesting poster entitled “Mechanisms of Microhabitat Segregation among Corallimorpharians”, examined the ability of two common Red Sea corallimorphs to handle UV light stress. One species Rhodactis rhodostoma, is commonly found in shallow-water reef flats where light intensity is very high. The other, Discosoma unguja, is common at deeper depths in shaded areas. The researchers exposed both species to identical conditions of light with several variable treatments (High light, shaded, etc).
To understand what the researchers were looking for, as far as “light stress” is concerned, one must understand what happens within the corals’ tissues and zooxanthellae during photosynthesis. To summarize, one byproduct of zooxanthellae photosynthesis is the formation of highly reactive and potentially damaging oxygen ions. The coral must deactivate these harmful byproducts in order to prevent tissue damage. Fortunately, corals have evolved certain enzymes that are capable of deactivating these oxygen radicals. This is the reason why it is very easy to “light shock” your corals by rapidly increasing the amount of light they receive. The coral is simply unprepared to deactivate all of the additional harmful byproducts of the increased photosynthetic activity. A typical emergency reaction by the coral is to expel it’s zooxanthellae (bleaching) so that no further damage can occur.
Anyway, back to the study at hand… When the Discosoma unguja were placed under full strength light, they showed a decided inability to handle the affects of increased photosynthetic activity. Instead of an enzymatic ability to deal with high light, they expressed an ability to move away from the light source. Crafty little creatures.
Rhodactis rhodostoma, on the other hand, proved to be resilient to high levels of light. Tissue analysis showed much higher levels of the enzymes necessary for deactivating harmful oxygen ions than in D. unguja.
What the study did not mention however, was whether the Discosoma unguja could be acclimated to higher light over a long period of time. And if so, would they be capable of gradually increasing their enzymatic abilities? If the Discosoma were taken from a shaded location and immediately exposed to high light, one could only expect a rapid stress response. But as we aquarists have learned, corals can be highly adaptive to unnatural conditions so long as the conditions do not rapidly fluctuate.
This study seems to reinforce the hypothesis that as climactic and atmospheric conditions become more extreme, the corallimorphs are poised to be able to adapt (some might even say morph…) swimmingly to changing water chemistry, nutrient and temperature increases, even as hard corals decline.