Within the order corallimorpharia, there are a number of species of so-called “mushrooms” that make ideal tropical marine reef aquarium inhabitants, and those from the genus Rhodactis are amongst the best. Alternatively called disc anemones, mushroom polyps, and mushroom coral, these cnidarians are fairly common in the wild and easy-to-care-for in the aquarium. They are also well-suited to aquaculture, and many are easily propagated in captivity. Most common in the North American marine aquarium hobby are Ricordea and those species sold as Discosoma species and, the subject of this article, Rhodactis species. It should be noted that there is fair amount of disagreement over the distinction between the last two genera.
To many, corallimorphs resemble both coral and anemones, although they are technically neither. Lacking a skeleton, corallimorphs possess an oral disc elevated on a short column and attached to the substrate with a pedal disc. Internally, they resemble the stony corals, and many are considered photosynthetic, as they host symbiotic zooxanthellae. Despite this fact, many species do well in lower light situations, although some do prefer brightly lit settings. In addition to the nutrition provided by zooxanthellae, corallimorphs rely on dissolved nutrients and detritus that may settle on the oral disc. Corallimorphs usually lack elongated, retractable feeding tentacles. Instead, most corallimorphs have small, almost bump-like tentacles that are generally inefficient when it comes to aggressive food-capture. Many do possess stinging cells, and some can defend themselves against the intrusion of a neighboring animal, and some are fully capable of capturing and consuming small fishes.
The corallimorphs from the genus Rhodactis are relatively large mushrooms (some have an oral disc with a diameter of more than 30 cm (12 inches) or more) that inhabit a wide range of habitats. Most are found in the Indo-Pacific, although at least one species lives in the Caribbean (some believe this Caribbean corallimorph more properly belongs to the genus Discosoma). Growing both horizontally and vertically, Rhodactis species may be found as solitary, deepwater specimens or in relatively large colonies in shallow water. Most are extremely tolerant of a wide range of lighting schemes, and they generally do quite well in the nutrient-rich backwaters and other areas with greater turbidity. These mushrooms are frequently called hairy mushrooms, and many do have a hairy appearance, although the so-called hairs are actually short, radially-arranged tentacles or pseudotentacles.
Species from the genus Rhodactis commonly have a thinner oral disc than species from the genus Discosoma, although this is not always the case (e.g., R.inchoate, a.k.a. the purple Tonga mushroom). Many Rhodactis species are also more adept at feeding on zooplankton and meaty foods than those from the genus Discosoma, and they can be observed folding their disc margins around larger food items. Some of the larger species in the genus are even capable of capturing and consuming small fishes. It is possible for some individual animals to possess multiple mouths.
While Rhodactis species are frequently imported from throughout the Indo-Pacific for the marine aquarium trade in North America, Blue Zoo Collector’s Choice Livestock Manager Kris Wray, really likes some of the ones currently being imported from Vietnam. “In general,” says Wray, “some of the mushrooms collected from regions in Vietnam are particularly sought after for both their color and size.”
Rhodactis Aquarium Husbandry
It is generally agreed upon that Rhodactis species are amongst the hardiest coralmorphs commonly available to the marine aquarist. Many require no supplemental feedings at all, and most are tolerant of varying levels of illumination from bright, reef-ready lighting (so long as they are properly light acclimated) to lower light situations (e.g., high output fluorescent bulbs). Note that the color of the animal is often related to the light level, and the more dramatic colored Rhodactis species often prefer higher lighting regiments. It s advisable to begin by placing the animal near the bottom of the aquarium until it opens fully, and then slowly move it up to its final position.
Rhodactis species do best with low to moderate flow (if the animal does not open fully, the flow may be too strong), and they tolerate higher levels of nutrients than most commonly kept cnidarians. Nonetheless, general reef-type filtration is advisable.
Unlike some other coralmorphs, Rhodactis species are considered semi-aggressive and may damage adjacent cnidarians that are not from the same genus. Allow room for growth and expansion, as a growing Rhodactis colony may overtake adjacent sessile invertebrates in the aquarium. While these animals are, as noted above, photosynthetic, they appreciate supplemental feedings (e.g., phytoplankton and commercially available reef foods such as our Blue Zoo Mix or other small meaty foods. Many aquarist recommend using an iodine supplement.
“Although I would not recommend Rhodactis mushrooms as a very first coral introduced into a first time tank,” says Mark Martin, director of marine ornamental research at Blue Zoo, “they can be considered a beginner coral.” While most Rhodactis specimens are quite hardy and tolerant of varying light regiments andnutrient rich water, they are more sensitive than some other mushroom species (e.g. Discosoma/Actinodiscus). “Discosoma will weather just about anything,” says Martin. “Rhodactis will react more like anemones to environmental stress and are known to expel their symbiotic algae when stressed.” Nonetheless, Eric Borneman, author of Aquarium Corals, writes that "many Rhodactis species are among the hardiest of the corallimorphs.”
So what’s the bottom line? Are these animals appropriate for the beginning reef aquarist? “If you have Discosoma sitting next to Rhodactis,” says Martin, “and something goes wrong in the tank, the Rhodactis will go down first. In other words,” Martin concludes, “as a group, mushrooms are bulletproof, but the Rhodactis are slightly less bulletproof.”