News / Species Spotlight / Revisited: Zoanthids (01/29/14)

Revisited: Zoanthids

by Charles J. Hanley III


Scientific Name: Too numerous to list
Common Name(s): Zoanthids, Button Polyps, Sea Mat, Zoos, Zoas, Pallys, Snake Polyps, Yellow Polyps


Domain: Eukarya
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa Order: Zoanthiniaria (= Zoantharia, = Zoanthidea)
Families: Abyssoanthidae, Epizoanthidae, Neozoanthidae, Parazoanthidae, Zoanthidae
Genera: Paranzoanthus, Palythoa, Protopalythoa, Epizoanthus, Isozoanthus, Savalia (Gerardia), Sphenopus, Acrozoanthus, Isaurus, Abyssozoanthus

There may not be a more popular, or prolific, invertebrate aquarium animal than the zoanthid. Zoos are an absolute staple in reef tanks everywhere. The hardy polyps are small, psychedelically colored, and reproduce readily. Colony frags are usually inexpensive and are generally considered good for beginning reef-keepers. Just about anyone with experience in this field has kept their fair share of zoanthids. They are one of my personal favorites, and Ill take a nice colony over just about any other coral choice.

Genetically, zoanthids land somewhere in between anemones and corals, though in truth, they are closer to anemones. They are not true corals (order Scleractinia). In fact, they are not even soft corals (order Alcyonacea). Instead, zoanthids belong their own order entirely. Though some debate about the true name of the order exists, Zoanthiniaria (= Zoantharia, = Zoanthidea) is the most agreed upon.

Visually, zoanthids bear superficial resemblances to anemones and corals alike. Their exterior appearance varies somewhat according to genus, but most have a narrow stalk, or stolon, supporting a relatively wider oral disc. Many species form colonies of polyp connected at the base by a thin layer of tissue, called the coenenchyme. The oral disc is flattened, with ridges radiating out to one or two rows of surrounding tentacles. Typically, a colony of zoanthids with open polyps will form a continuous swath with no space between the tentacular crowns. When closed, polyps look like tightly packed round balls or rounded columns. A new colony quickly spreads outward and carpets the surrounding substrate in polyps. This behavior gives rise to one of the common zoanthids monikersSea Mat.

Some varieties exhibit a singular color, while others my have two or more contrasting pigments. The colors vary from deep purple, burgundy red, and bright orange, to bright green, yellow, and blue. Some, like button polyps, are golden brown overlayed with fluorescent green. Many of the common aquarium species fluoresce brilliantly beneath actinic, metal halide, and LED lighting. In fact, zoanthid fluorescence is one of the most impressive displays of its kind. That aspect alone is worth the investment in these creatures.

Natural Habitat and Ecology:

Polyps of the order Zonathiniaria range far and wide. In fact, zoanthids are much more diverse in their habitat choices than what can be reproduced in an aquarium environment. They are found on both sides of the Pacific basin, as well as the Caribbean sea. They are found in the deep sea, as well as the shallows. Intertidal species sometimes even endure exposure at low-tide. Others live on pristine fore-reef slopes, or the nutrient-laden waters of harbors and estuarine river mouths. There exist both zooxanthellate and azooxanthellate species, though they are all ahermatypic (non-reefbuilding). Most of the important aquarium species are zooxanthellate. Members of the small and colorful genus Zoanthus are one such group, and are probably the most quintessential zoanthids in the trade today. Other important aquarium genera are Palythoa, Protopalythoa, Parazoanthus, and Isosaurus.

One of the most significant aspects of zoanthid ecology is production of palytoxin, one of the most potent of natural poisons in the world. Palytoxin is typically associated with Palythoa and Protopalythoa species, but it has recently been found in Zoanthus polyps as well. The toxin is produced by symbiotic bacteria and subsequently absorbed by the polyps. Palytoxin is an effective defense mechanism which prevents corals from overgrowing the zoanthid colony. Interestingly, many differing zoanthid species can coexist even when they touch each other, giving rise inter-woven colonies with many mixed colors.

Another way zoanthids prevent other corals from overrunning their colony is by rapidly reproducing. Though sexual reproduction occurs in the wild as well in captivity, asexual budding is the more common mechanism. New polyps bud off from the base of larger stolons, and soon form their own oral discs. The new polyps quickly develop into full-sized polyps, and then begin budding their own clones. In this manner, a zoanthid colony grows exponentially, until it runs out of room. In some cases, a species is relegated to a particular living host, such as a specific sponge or coral species.

Aquarium Care:

Aquarium care is relatively easy for zoanthids. Considering that some types are known to survive tidal exposure, while others are found in murky, estuarine environments, it is not surprising that these animals are hardy. Their toughness is precisely why zoanthids are so suitable for beginning marine aquarists. They can handle minor fluctuations in water quality, which also makes them perfect for nano-tank setups. Most do well with moderate to high light, but each colony will be different. Expect a colony to change colors slightly when first introduced into your home aquarium. In fact zoanthids are infamous for doing so, but I have found that experimenting with a colonys location will often reveal a spot where their growth and coloration are maximized. Most enjoy moderate water flow rates, which helps them snare passing plankton and flushes away from detritus and toxins. It is not necessary to feed Zoanthus species polyps, but they will accept weekly feeding of phytoplankton. Other zoanthids, like Palythoa, Protopalythoa, and Parazoanthus species, will accept small meaty foods. Non-photosynthetic Parazoanthus polyps must be fed regularly.

Keeping zoanthids does have several unfortunate drawbacks. First, they can be toxic to a number of SPS, and even some soft corals. They can also be stung by LPS sweeper tentacles or mushrooms (corallimorphs). Incompatible tank mates such as these should be kept well away from zoanthids. Prevailing water currents should also flow in a way that minimizes the exposure of other corals to the palytoxin. Strong protein skimming can help to scrub this toxin from the water, but a mixed reef tank is also well-served by being large.

Space requirements notwithstanding, the remaining water quality needs of the zoanthid are fairly straightforward. Standard reef tank parameters will suit them just fine:

Temperature: 72 to 78 degrees Fahrenheit
pH: 8.3 to 8.4
Specific Gravity: 1.023 to 1.025
Alkalinity: 7.0 to 10.0 dKH
Calcium: 412.0 to 450.0 mg/L
Magnesium: 1288.0 to 1320.0 mg/L
Ammonia: 0.00 mg/L (immeasurable)
Nitrite: 0.00 mg/L (immeasurable)
Nitrate: <10.0 mg/L

You will know they are happy when they open fully everyday and exhibit brilliant colors. They should reproduce quickly, and may even have to be pruned back if they begin to encroach upon other corals. A zoanthid colony should be given plenty of room to grow.

Care must be taken when handling zoanthids, especially to fragment the colony. The palytoxin they produce is a extremely potent, and exposure must be prevented. Rubber gloves and a face shield are must when handling or propagating a zoanthid colony. Fortunately, the rest of the process is easy. Zoanthid polyps can be snipped from coenenchyme with a pair of sharp scissors or a clean razor blade. It is best to include at least 3 connected polyps together with each new colony. Using the flat edge of the blade, scrape off the substrate under the new colony so that a thin layer of rock or coralline algae remains attached to the polyps. By doing so, you can use a cyanoacrylate gel glue to attach the colony to a new plug, shell, or rock.

If possible do your fragging/pruning outside of your tank, in a separate tub. Using tank water, fill the tub with enough tank water to cover the entire colony when you take it out. By working in this separate container, you can avoid polluting your main tank with the pollutants loosed by the fragging process. It is good idea to use a second tub in which to dip the newly glued colonies. By dipping the glued colony immediately, the cyanoacrylate glue cools and forms a skin on itself. Quickly removing the colony will allow the glue to solidify in the air. Leaving the polyps exposed to air for several minutes while the glue dries will not hurt them, and it will prevent them from coming loose when they go back into the tank. It is not uncommon for polyps to remain closed for a couple of days after fragging, but do not be surprised if some begin to open almost immediately.

Quick Notes:

  • Zoanthids are low maintenance, high reward animals that are good for all levels of aquarist.
  • Most zoanthids need moderate to high lighting and moderate water flow.
  • Zoanthids can be propagated easily, but care must be taken to avoid Palytoxin exposure.
  • Some zoanthids need feeding, all will benefit from feeding.

Works Cited:

Brightwell, C.R.Marine Chemistry Neptune City: T.F.H Publications, 2007.

Brightwell, C.R.The Nanoreef Handbook Neptune City: T.F.H Publications, 2006.

Fautin, D.G., S.L. Romano, and W.A. Oliver, Jr. Zoantharia, Sea Anemones and Corals. Version 04 October 2000. Tree Of Life Web Project. URL:

Goldstein, Robert J. Marine Reef Aquarium Handbook Hauppauge: Barrons, 1997.

Jennings, Greg. The New Encyclopedia of the Saltwater Aquarium. Buffalo : Firefly books Ltd., 2007.

Shimek, Ronald L. Marine Invertebrates. Neptune City: T.F.H Publications, 2004.

Reimer, J.D., F. Sinniger, C.P. Hickman. Zoanthid Diversity (Anthozoa: Hexacorallia) in the Galapagos Islands: a Molecular Examination. Coral Reefs 27:3. 2008.

Sprung, Julian. Aquarium Invertebrates Zoanthids: Polyps as Cute as a Button. Advanced Aquarists Online Magazine. 2003. URL:

Staff. Advanced Zoanthaid Fragging. Reef Hobbyist Magazine. Fourth Quarter 2008, Vol. 2.

Tullock, John H. Natural Reef Aquariums; Simplified Approaches to Creating Living Saltwater Microcosms Neptune City: T.F.H Publications, 2001.