The anemone-like Corallimorphus profundus. Credit: Reimann-Zürneck & Iken 2003
Mushroom Corals: Order Corallimorpharia There are only about fifty described species of corallimorpharians, but, despite this limited diversity, they are well-known amongst aquarists. The genera commonly exported are fairly homogenous in resembling anemones that have nearly lost their tentacles, but there are some very anemone-like exceptions—like the “orange ball anemone” Pseudocorynactis. In truth, this group is actually far more closely related to the stony corals, which at first glance seem quite different. How then do we differentiate these three groups? An important distinction must be made concerning the placement of the tentacles in anemones and corallimorphs. In many anemones, the tentacles are located only around the margin of the oral disk, whereas in corallimorphs these tentacles extend towards the mouth to fill most of the disk. This is not universally true, as some anemones have secondarily evolved a tentacle-filled oral disk. In fact, nearly all of the common aquarium anemone species violate this rule—Stichodactyla, Radianthus, Entamacaea, Cryptodendrum, Heterodactyla. This even caused confusion amongst taxonomists, who at one time considered Ricordea as belonging amongst these anemones. The differences that exist involve subtle nuances of the nematocysts and body musculature, but a good rule of thumb is that corallimorphs lack the ability to rapidly move themselves in the manner of an anemone. The anatomy of a corallimorph is essentially identical to the stony corals, save for the lack of a secreted skeleton. But, as discussed earlier, stony corals were not always stony, and this begs the question: Are corallimorphs actually a distinct evolutionary lineage, or are these merely stony corals that lost their skeleton? This latter scenario is known as the “naked coral” hypothesis. Definitive answers to this mystery have not been forthcoming, but recent studies analyzing the complete mitochondrial genomes of numerous genera have allowed researchers to make some headway. The results so far have provided strong evidence that corallimorphs are indeed a monophyletic group sister to the stony corals, though until studies of nuclear DNA are completed there is still some room for uncertainty. The species at the base of the corallimorph tree is Corallimorphus profundus, a large, anemone-like species known from the deep ocean floors and shallow waters of Antarctica. Its mitochondrial genome is ordered in much the same manner as in scleractinians, but the genetic code contained in it shows it to belong with the Corallimorpharia.… More:
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