News / Feature Articles / A Guide To Aquarium Mushroom Corals (07/10/18)

A Guide To Aquarium Mushroom Corals

A Guide To Aquarium Mushroom Corals
Mushroom Corals are a small group with an outsized influence in the aquarium hobby. For novice reef aquarists, these are oftentimes the first coral to be recommended, and rightfully so. They are colorful, easy to care for, pose little danger to other organisms, and are more often than not quite affordable. But despite how common they are, even experienced aquarists (and scientists) have trouble identifying them.

A quick science lesson: Mushroom Corals belong to their own order among the anthozoans known as the Corallimorpharia. These creatures are easily confused with the true anemones (Actiniaria), but lack the muscular foot which enables that group to move about (along with differences in their stinging nematocysts). And, of course, these are very distinct from both the Soft Corals (Alcyonacea) and the Stony Corals (Scleractinia), though this latter group is actually the closest relative of the corallimorphs.

Internally, stony corals and corallimorphs are essentially indistinguishable, sharing the same arrangement of their mesenteries and a similar array of nematocysts. Their differences are more general, with corallimorphs being largely solitary and never possessing a skeleton. Researchers have at times wondered if the corallimorphs were in fact members of the stony coral lineage which had simply lost their skeleton, but recent genetic study has finally confirmed that they are indeed two distinct lineages that happen to be each other's closest relatives.

With corallimorphs, there are a few major types to get familiar with. In the deep sea, we find some very anemone-like groups, like Corallimorphus, Nectactis, and Sideractis (obviously, none of these ever find their way into aquariums). There are also the anemone-like species in Corynactis and Pseudocorynactis, the latter of which regularly finds its way into reef tanks as a hitchhiker on live rock. These are the mostly transparent anemone-like creatures that tend to have white or orange tips to the tentacles. Temperate reefs often abound with dense colonies of the colorful Corynactis, which youll see in many public aquariums. Paracornyactis hoplites is a gigantic version of this type that occasionally makes its way into aquariums; it preys on sea stars in the wild!

The remainder of this order consists of species that are flattened, usually with little in the way of tentacle development, and all of these genera show up in aquarium exports to varying degree. These are the Mushroom Corals, and their classification has been mired in confusion for pretty much ever. The deeper one looks into the scientific literature on this group, the more unclear it becomes what the correct name is for many of these common corals. But lets take a look at these species and see how to identify them.

Mushroom Coral (Discosoma sp.)
The most common and variable species of corallimorph on Indo-Pacific reefs is here identified as Discosoma, with no attempt being made at identifying what the correct species name is. This species can be identified in that it is mostly lacking in discal tentacles, with these being reduced to a few scattered bumps. Coloration is highly variable, with bright red and green being some of the more characteristic phenotypes. Another interesting variety is a green and brown pinwheel motif that is said to mimic Plate Corals. With all these differences, it might suggest that multiple species are present, but a recent genetic study done in Singapore found that all of these are essentially indistinguishable.

The name used most readily in the aquarium trade has been D. nummiforme, which is currently the type species for this genus, but when the actual specimens were examined by J.C. den Hartog in his 1980 study of this group, he found that the discal tentacles were dendritic, which means they couldnt have been Discosoma as we know this group today. A lot more work is needed here. Note that the synonym Actinodiscus is rampant in aquarium references, but has been treated as invalid for nearly forty years!

Aquarium care is simpler than the confused taxonomy, thankfully. Specimens need moderate to high light to truly flourish, though this might depend on the particular phenotype being kept. Do red mushrooms need more or less light than green or blue mushrooms? Little is known beyond empty anecdote. Feeding is typically necessary and most foods are simply ignored.

Umbrella Mushroom (Discosoma neglecta)
This Caribbean species is easily identified thanks to the peculiar lappets that extend around its perimeter. Its colors are generally more muted and camouflaged than the Indo-Pacific species just discussed, varying from a mottled green to a mottled grey, often with some concentric or radial patterning present. Colonies are typically small, and aquarium specimens are most frequently seen as single polyps.

Hairy Mushroom (Rhodactis rhodostoma)
Hairy Mushrooms grow much larger in diameter (up to six inches) than the aforementioned Discosoma species and feature more prominent tentacles on the oral disk, along with a row of short marginal tentacles that extend laterally from the perimeter. Its these marginal tentacles that help this corallimorph to compete for space on the reef, potently stinging many encrusting stony corals that it compete with for real estate. In many areas, this corallimorph is a pest, overtaking large patches of the reef as a monoculture that very effectively deters other corals.

There are two main phenotypes, which may or may not be the same species. One is bright green with mauve tentacles, while the other is mostly mauve, often with green tentacles. The correct name of this furry species is up for debate. The oldest, and likely correct, option is what weve used here. In 1897, an early Australian researcher described a virtually identical coral as R. howesii, differentiating it by stating that there were unbranched tentacles surrounding the mouth (versus branched in R. rhodostoma), but, given how variable the tentacles can be in the Hairy Mushroom, there isnt much reason to believe this is a valid species. Theres also R. indosinensis, named in 1943, but it too is probably a synonym of R. rhodostoma.

Oddly, this species was traditionally a food source for Pacific islanders, but, if eaten raw, it causes a paralytic condition that is usually fatal. Please, resist the urge to eat your Hairy Mushrooms!

Bullseye Mushroom (Rhodactis sp.)
This species has the same marginal ring of tentacles as the previous, but those of the disc are flat and dendritic (rather than branched and fingerlike). These polyps usually grow considerably smaller, more like a Discosoma than the bulky Hairy Mushroom. Colors are all over the board, with the most vibrant specimens being true collectors items. In recent years, a phenotype with inflated, balloon-like tentacles has become one of the hottest of corals, going by the trade name Bounce Mushroom.

Again, the correct name of this species is unclear. Aquarium references favor Rhodactis inchoata, named in 1943, but the correct option might actually be the species described in 1828 as Discosoma nummiforme, which is said to have dendritic tentacles (something that no other known species has). If this is true, it will potentially create all sorts of confusing name changes, but we wont know until a proper revision of this group is published.

Giant Cup Mushroom (Amplexidiscus fenestrafer)
The largest and most impressive of the corallimorphs is the Giant Cup Mushroom. This is a close relative of Rhodactis and the only member of its genus. This Indo-Pacific species typically occurs in shallow, turbid environments, either singly or in small (or not so small) groups. Mature specimens average a foot in diameter, with colors that range from green to brown, but never the reds or oranges common to other genera. The tentacles are simple and fingerlike, with a very distinctive ring near to the perimeter that they are absent from. Another useful trait to look for is the ragged appearance around the edge of the polyp, similar to the lappets of Discosoma neglecta.

Amplexidiscus is famous for the neat trick it does to capture prey. Unsuspecting fishes will often swim into the center of the oral disk, especially at night, at which point the massive polyp furls its edge upwards into a cuplike shape. It eventually completely closes off the opening and slowly begins the process of stinging and suffocating the unlucky fish caught inside. This can happen to all sorts of fishes in an aquarium, with wrasses and surgeonfishes often falling prey. Even anemonefishes are not immune, though they will frequently host in this corallimorph without problem until one day they are suddenly gone. Beware of this deceitful beast.

Ricordeas (Ricordea yuma & Ricordea florida)
These two species are very similar to one another, but quite distinct from others in the Corallimorpharia. In fact, for a while these were grouped in the same family as Carpet Anemones (Stichodactyla), but they were eventually returned to their rightful home among the Mushroom Corals when the internal morphology and nematocyst structure was investigated. Still, they are treated as a separate family, the Ricordeidae, compared to those species weve discussed thus far, all of which are in the Discosomidae. The main differences relate to the stinging organelles, the nematocysts, which in Ricordea consists of a variety called a spirocyst (which also can be found among the Corallimorphidae) and lacks the dense nematocysts (the acrospheres) of the Discosomidae.

These two species are morphologically, ecologically, and biogeographically quite different from one another, though they still tend to get confused. The Yuma Mushrooom (often simply referred to as Yumas) is an Indo-Pacific species that grows a bit larger in diameter (two inches or so) and is usually solitary. It comes in a range of colorsred, orange, greenoften with a radial patterning. Sometimes, a single white line of tentacles will extend from the central mouth to the outer edge of the polyp, in sharp contrast to the rest. Most important to note is that the raised oral cone in the middle of the disk is covered in tentacles.

The Florida Ricordea (R. florida) is, as the name suggest, a West Atlantic species. It is smaller in comparison to a Yuma, usually not much larger than a Discosoma. In the wild, it often grows in small, dense colonies, but regulations require that there is as little rock as possible attached to collected specimens, thus those exported for the aquarium trade should, if legally harvested, be sold individually (unless they have been aquacultured). This species can be found in yellow, orange, green, and blue, often with a contrasting ring of tentacles around the perimeter. The mouth of this species is largely barren of tentacles, though this can be hard to discern if the polyp isnt fully expanded.

Both Ricordea species do well in captivity, though R. yuma is a bit more sensitive to the stresses of collection and shipping. Yumas are said to come from deeper habitats, and specimens do best under a combination of low light and low flow. Its Floridian counterpart is generally quite hearty and can tolerate brighter conditions just fine. Feeding with meaty foods or pellets will greatly increase the growth rate in these corallimorphs, especially for the colonial R. florida. A single specimen or a small colony can quickly grow to cover the bottom of an aquarium when well cared for.