One of the favorite reef keeper corals is ‘zoanthids,’ which come in a variety of sizes and colors, besides being very hardy and easy to maintain. Their common names include Sea Mat, False Coral, Button Polyps, or Colonial Anemones.
These so-called soft corals resemble miniature anemones, and can be solitary, connected by runners, or connected with soft tissue called ‘coenenchyme.’ Most often they form small encrusting mats or clusters of polyps and encrust hard substrates, living sponges or other sessile invertebrates.
Actually, they are technically not soft corals, as zoanthids are relatives of stony corals and anemones! In fact, they are part of the Phylum Cnidaria, which includes the Order Scleractinia (Stony Corals) and Order Actiniaria (Sea Anemones). Nevertheless, their appearance is that of soft coral, therefore continue to be described by most aquarists and non-scientists as such. They occur in the Order Zoanthiniaria, which is further divided into two suborders; Brachycnemina containing the Families Neozoanthidae and Zoanthidae, and Macrocnemina with the Families Epizoanthidae and Parazoanthidae.
Most available species are reasonably priced, except the latest so-called ‘blue’ zoanthids, which are proving to be quite expensive, as I’ve seen them priced by the polyp for as much as 10 dollars each!
Only four of the genera in these two suborders/four families, i.e., Zoanthus, Parazoanthus, Palythoa, and Protopalythoa, contain species commonly seen in the trade. However, some species in the genera Acrozoanthus and Isaurus occasionally show up in the trade. And recently, I’ve seen some species believed to be in the genera Neozoanthus and Epizoanthus, but that’s still guesswork on my part. The genera Gerardia, Isozoanthus, Sphenopus, and Thoracactis are not purposely collected for the trade.
The most commonly seen genera are seen in the Suborder Brachycnemina, and the Family Zoanthidae;
The most popular of all zoanthids, both described and yet undescribed, are found in this genus as they exhibit many different color forms, including occasional fluorescent colors such as blue, pink, orange, green, and yellow. And after many decades in the hobby, I continue to see new differently colored specimens. Sea Mat, False Coral, and Button Polyps are their common names, however, Sea Mat appears to be the most often accepted name. Despite that, the common name game in local shops is only limited by one’s imagination because of the wide array of colors or color combinations now available.
They usually have dense encrusting colonies consisting of small tightly packed polyps, about 1 cm or less across such as Zoanthus pacificus and expand their area of growth by budding off their spreading tissue base. Some species form branched growths and do not exhibit offensive tactics, except that they can overgrow defenseless neighbors.
Their polyps are open during the day, and mostly closed during the night. They are found in both the Tropical Pacific and Atlantic Oceans where many live in intertidal zones where they are sometimes exposed to air, extreme temperature changes and/or freshwater from rain showers.
Even though quite hardy, they fair better under excellent lighting and where there is good water movement. Yet, as with many other corals, are sensitive to stings from corals and anemones that exhibit sweeper tentacles. Keep in mind the snail, Heliacus areola, (Box Snail) feeds upon zoanthid colonies. It is usually more visible at night when polyps are closed, and should be discarded when seen.
Nutritionally, most of their needs are provided by their zooxanthellae, however, some capture suspended organic substances. Larger polyped species will take meaty foods such as black worms, brine or mysis shrimp, or other tiny bits of marine flesh-type foods.
These differ quite broadly from the genus above, as they prefer low to moderate water currents, and moderate to bright light, such as the species Protopalythoa grandis. They are also much larger and can be fed meaty foods, including zooplankton type additives
Common names include Sea Mat or Button Polyp Coral, with Button Polyps the generally accepted name. Their polyps usually have more tentacles than Palythoa specimens and can live in colonies with or without stolons or a coenenchyme between the polyps. Range includes both the Tropical Pacific and Atlantic Oceans. Larger polyped specimens usually come from deeper water.
Specimens from the Palythoa genus are probably the least available of the four more common genera in the trade. Their common names are the same as mentioned above for those in the Protopalythoa genus. They usually have a pale brown or yellow color, however, fluorescent green colonies appear in the trade occasionally. They form thick encrusting mats containing the embedded polyps, which sometimes retract almost completely into the base tissue,such as Palythoa tuberculosa. Often, the coenenchyme contains sand grains.
They are generally found on reef tops and slopes, and/or in lagoons in both the Tropical Pacific and Atlantic Oceans where they form large colonies, sometimes many meters across. Their growing conditions also differ from other species in this family, as they require turbulent water movement and excellent lighting. And they can be quite aggressive therefore do not let them come in contact with stony corals. They also can occasionally be fed with meaty type foods, e.g., fortified live adult brine, mysis shrimp and/or other zooplankton-type foods.
As with all Palythoa/Protopalythoa species, they produce a toxin called palytoxin (Mebs, 1989) that is found in its mucus coating. It is used to protect itself from predators and also clear the path of other corals so as to make space for expanding its colony. Keep in mind that if this toxic mucus enters your eye, mouth, or an open wound, it can cause various disorders including numbness, hallucinations, or even death. Therefore, always wash your hands after handling these creatures, and never handle them if you have open wounds on your hands! Wearing gloves is a good idea when handling these animals! And if it becomes necessary to divide their colonies, do so outside the aquarium, as the toxin released by ripping them apart or cutting the colony in the aquarium can affect other corals throughout the aquarium.
The remaining two infrequently seen genera in this family have species that have yet to established their hardiness in my opinion, as I’ve tried them, but have never attained long term success with them.
The occasionally seen species in this genus are called 'Stick Polyps' (Acrozoanthus australiae) because their brown polyps, which have a growth format similar to Parazoanthus gracilis and may be slightly larger, are growing on polychaete or fan worm parchment tubes. Collectors simply cut off the hollow tube above the sand, leaving the worm below in the remaining tube to rebuild the tube above the sand. Once the tube structure is rebuilt in these areas, this species reestablishes itself on the exposed tubes.
Their polyps harbor zooxanthellae, therefore they need to be placed in well-lit environments, along with gentle water movement. Feeding meaty type foods seems to encourage their growth, and if so, they can spread to nearby rocks.
There is some thought they can draw some nourishment from the substrate of the tube they are attached to. And once it disintegrates (usually caused by natural decay bacteria since the worm is no longer there to maintain it), they also dissipate. More needs to be learned about these creatures.
The other infrequently seen family member have various species called ‘Snake Polyps’ or ‘Tube Polyps.’ There are three valid species in the Pacific, - Isaurus tuberculatus, I. cliftoni, and I. maculatus, and three in the Atlantic, - I. duchassaingi, I. spongiosus, and I. tubercuulatus. Only one, I. tuberculatus, is distributed circumtropically, and occasionally seen in the trade.
These species form elongated tubular structures with tubercles (bumps) along their structure. During the day, their polyp tentacles are usually retracted, and extend during evening hours to feed upon zooplankton. Nevertheless, the column tissue contains zooxanthellae and if provided a well-lit environment, feedings of meaty foodstuffs may not be required. ‘May not’ are the key words here, as I doubt from previous experiences its zooxanthellae content can provide adequate nourishment, as they seem to waste away without dedicated nighttime feedings. More research is needed on this species.
In the Suborder Macrocnemina, Family Parazoanthidae, we have an extremely popular species in the Genus Parazoanthus:
Only one common species is regularly seen in the trade, and that is Parazoanthus gracillis, and its common name is Yellow Polyps or Yellow Colonial Anemones. Scientists consider its scientific name incorrect, yet a new name has yet to be decided upon.
Even though only one species commonly shows up in the trade, there are many in this genus that are described as ‘Yellow Polyps,’ and they occur in both the Tropical Pacific and Atlantic Oceans, and the Mediterranean. They can be found on the surfaces of rock, branches of dead coral, hydroids, and sponges. Their encrusting anemone-like polyps are connected at the base via small canals called stolons, forming groups of individuals. Most of the species in this genus do not harbor zooxanthellae, therefore they depend upon their long tentacles for capturing drifting organic matter, as does P. gracilis.
P. gracilis is a beautiful and a somewhat hardy species if fed various meaty foodstuffs, e.g., Cyclop-eeze, enriched brine/mysis shrimp (live or defrosted), and will even accept some flake foods! Placement in the aquarium should be in moderately lit areas having moderate water flow. Keep in mind that if placed near other corals and the specimen is fed, it can rapidly spread onto nearby corals and harm them.
Let me leave you with one very important ‘Caution’ concerning most of the above named species – besides the well known toxicity associated with the Palythoa and Protopalythoa species, as noted above, the toxin has now been found in ‘various’ other Zoanthus species (Fosså and Nilsen, 1998). If the need arises to divide these species, do so outside of the aquarium and wash your hands with soap thoroughly after handling ‘any’ of the above named species, or even better, use gloves when handling them!