Zoanthids are often called colonial polyps, sea mats, or button polyps. They are from the Anthoza Class, in the Zoanthidae family. There are over 60 species, with a few hundred more in limbo taxonomically. They can be either colonial, or solitary polyps, depending on the species.

Like many coral-like animals, they can show variances in color and the shape of the colony arrangement. This is thought to be because they have more than one species of Zooxanthellae living inside them usually, and this gives the coral its color. Certain conditions dealing with water movement, water quality, lighting, and nutrients will cause one species to outcompete another, changing the coral’s color. This makes identification even harder. Furthermore, their nematocysts do not allow for positive identification like they do in most corals.

Many Zoanthids reproduce by sprouting out from their stolons. They are very prolific generally, and will spread across a wide variety of substrates, from dead animal tissue, to glass. The polyps are generally attached to a “mat,” called the Coenenchyme. This mat may or may not have sediment incorporated into it as a structural element. The tentacles are always in multiples of 6. Zoanthids obtain most of their energy from their symbiotic Zooxanthellae, although they still need to filter feed fine particles, like bacteria and plankton, from the water in order to supplement this. They do not require a whole lot of light, nor very specific conditions concerning water quality or water current, and so are found in varying locations in the Coral Reefs. Many can also survive extended periods entirely out of water, as many corals can.

While Zoanthids are usually considered to be a soft hexacoral, they are actually classified as Stoloniferans, because they have pieces of tissue connecting their stolons. Being some polyps are solitary, they really can’t be classified as true corals using most definitions of the word. Those solitary species may actually be more closely related to the Sea Anemones. Their nematocysts are not overly powerful, and they are not an overly aggressive coral-like animal, although they do have the ability to protect themselves. Their communication (word used loosely) is thought to be accomplished by means of the stolons. In many cases, one polyp that is stressed will cause the whole colony to close, sometimes needlessly.

Zoanthids, not having a strong sting, nor a thick calcareous skeleton, are preyed upon by many animals. Triggerfish, Dancing (Camel) Shrimp, small Hermit Crabs, Lobsters, Filefish, Angelfish, Butterflyfish, certain Nudibranches, et cetera, can all consume them. Some corals are able to overtake them in some ways as well. Acropora, Pocillopora, and other Small Polyped Stony Corals, for example, frequently grow over them, shading them of light. Their main anti-predation defense is found in the genre Zoanthus and Palythoid, and is a toxin called palytoxin. Certain species of Nudibranches seem immune to this, and as they consume the Zoanthid, are able to store and use that chemical for themselves. It is a very strong chemical, however, not nearly as toxic as say, Sarcophin. It is also known to be toxic to specific corals, such as ones of the Xeniaidae family.

Zoanthids are very simple, yet beautiful creatures. It would be hard to go SCUBA diving without seeing at least a few species in almost every area of reef. Their maintenance in captivity could not be easier. Providing a nutrient source and enough light for their Zooxanthellae are the two main requirements. Giving them a little water flow to carry wastes away from the polyps, and nutrients to them, is also important. Water quality is not too important, as long as Nitrates remain well under ten parts per million, and Phosphate is kept low, as that interferes with calcification. The addition of Calcium, Iodide, and Strontium is essential. Other than that, they are very fast growing hardy animals that make an incredible addition to any reef tank.

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