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Corallimorpharians, also known as "mushrooms," are undoubtedly among the most interesting corals in nature. They are soft-bodied and lack external calcareous skeletons, despite being closely related to stony corals, the Scleractinia. They are oddballs, much as Heliopora coerulea (blue coral) and Tubipora musica (pipe organ coral) are soft corals that do produce an external calcareous skeleton. They are found in three or four families, and perhaps 13 genera, comprising an unknown number of species. Even genus-level identification is tenuous in this group.

Corallimorpharians are found in all oceans, from tropical to polar and shallow to deep waters. The most common types found in the aquarium trade are believed to belong to one of six genera: Actinodiscus, Rhodactis, Discosoma, Amplexidiscus, Psuedocorynactis and Ricordea. The beautiful Psuedocorynactis are not often available, but occasionally are found as hitchhikers on live rock, and individuals are commonly known as the orange ball corallimorph.

The single-polyped corallimorpharians are more recent in evolutionary history than the stony corals. This is interesting, because logically it would seem that selection would act to have corals produce skeletons as an adaptive measure. This probably did happen, prior to the Scleractinia, but at some point, a group of stony corals evolved that could survive well without a skeleton. How and why this happened is uncertain, but I would offer some possible explanations.

There are several taxonomic groups of corallimorpharians, none of which are clearly defined. Although most closely related to stony corals, they are also closely related to the actinians, or sea anemones. It is perhaps not surprising to know that many of their modes of asexual reproduction are also known in the anemones, as well as a common "purse-string" type prey envelopment. Some scleractinians are also very close in digestive behavior and cnidae development to the corallimorpharians, including the Caribbean Mycetophyllia species. Some Indo-Pacific corals are able to bail out of their skeleton and attach to substrate using adhesive filaments. This behavior illustrates the potential of coral polyps to exist without a skeleton.

These animals are minimally colonial, although often gregarious, forming clusters of polyps from a few to many hundreds of individuals. They tend to be most abundant in rubble areas in shallow turbid water that are unsuitable for stony corals. On reefs, corallimorpharians tend to consist of small clumps of isolated individuals, and they are often found under overhangs or in recesses of the reef framework. Others, such as the anemone-mimic giant corallimorph, Actinodiscus fenestrafer, are found in more exposed locations in shallow to mid-depth water. I have found many of the more common species in the trade in deeper water in marginal reef environments. To this day, I have never seen the common "Actinodiscus" red, blue, and striped-type mushrooms in the wild.

It is perhaps ironic that aquarists are so familiar with these animals since they have been common in the trade for such a long time, because they are among the most unstudied and poorly understood tropical corals. Little is known of their reproduction, behavior, ecology, and biology. At least some species are broadcast spawning.

Perhaps one of the most interesting observations about these animals that has occurred from their long-term husbandry is their competitive ability. They are resistant to disease, and extremely competitively dominant. Nothing seems to settle near corallimorpharians, and I have never seen or heard of anything able to take over space occupied by corallimorpharians. Co-existence with other species is possible, but the corallimorpharians never seem to "lose a battle." They are the only animals which I have found able to resist nearby settlement and competition by Aiptasia anemones. For this trait alone, they must be admired!

Aquarists probably know more about these animals than any other group of people. It is important for us to maintain living stocks of these animals, for so little is known of their taxonomy and distribution. Some may be exceedingly rare and never described. I would urge everyone to work diligently at maintaining widespread propagated stock of corallimorpharians since some of them may never be found again. This should not be difficult as they survive and reproduce well in aquaria, and are tolerant of most any conditions that exist in aquaria. For husbandry requirements and more information about these animals, please refer to the chapter on these fascinating animals in my book, Aquarium Corals.

Text by Eric Borneman.
Photos by Reef Central members.
A special thanks goes out to Dave Bayne (Nanook)
for his assistance on this project.