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Zoanthids by Eric Borneman Aquarium.Net January 1998

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Coral? Anemone? Both? Neither?

by Eric Borneman

Class: Anthozoa

Subclass: Zoantharia

Order Zoanthidea

Family: Zoanthidae

Genus: Zoanthus, Palythoa, Protopalythoa, Isaurus, Sphenopus, Parazoanthus, Epizoanthus, Acrozoanthus

common species: Z. sociatus, pulchellus, pacificus, mantoni. vietnamensis, coppingeri, and others

P. grandis, caribaeorum, complanata, variabilis, grandiflora, mammillosa, toxica, and others P. mutuki, and others P. swiftii, parasiticus, axinellae, gracilis, dichroicus, and others I. tuberculatus, and others S. marsupialis, and others

Common names: sea mats, button polyps, colonial polyps, colonial anemones

What exactly is a zoanthid? They are not a stony coral, though they are a hexacoral. They are not an anemone, though they are soft bodied anthozoans.

They are not a soft coral (octocoral) as they do not have eight tentacled polyps. They grow in vast legions in a matter most unbefitting any respectable predatory coral or anemone. In fact, being predatory seems almost below them...a lower class sort of thing in terms of invertebrate etiquette. Colorfully basking in the sun like so many bodies at South Beach, these interesting and hardy mats of polyps share more than a few similar traits.


Although Wilkens (1990) claims that there are over 300 species of zoanthids, more recent literature asserts there are likely 60 or so true species, many of which are taxonomically indecise. There is a revolution going on in the land of the Zoanthidae, and it is is not altogether a pretty one. Not only are reclassifications of species likely, but there may even be reclassifications of genera. Delbeek and Sprung (1997) make note that information they had asserted that many, if not most of the species of Palythoa will soon be reclassified into the genus Protopalythoa. However, there is even more recent evidence questioning the validity of Protopalythoa as a genus at all, with Palythoa likely to emerge as the victor of the taxonomic battleground (Burnett, 1997). Furthermore, much of the current question of genera seems to reside around the differences between Zoanthus and Protopalythoa, and not Palythoa at all. Indeed, to classify a zoanthid, one must look at the arrangements of their mesenteries (no small task), and not by examining nematocysts as many recent attempts had done (Haddon and Shackleton, 1891). Being soft bodied animals, fossil records are of little use. There is a large degree of morphological variation within the taxa, and this merely confounds any attempts at classification that does not reach into a genetic level involving high specific testing that is of little use or importance to the home aquarist.

Nonetheless, there are certain features which can be used to make a somewhat assured classification to the home zoanthid caretaker as provided in a key to the species, by Burnett (1997).

In general (and pending any further, albeit likely, inevitable changes):

Palythoa species are recognized by having a coenenchyme that contains bits of debris and reef elements that help to support the polyp. The colonies may form large tracts of individual colonies that often grow in a curved convex shape and usually remain less than a foot across. They have polyps with wide flattened oral disks and variously shaped tentacles surrounding the outer margin of the oral disk. Their colors are typically more muted shades of brown with some fluorescent elements to the tentacles and oral disks common. They may also have a marbled or striated pattern on the oral disk. Palythoa are normally voracious feeders, accepting food willingly. The overall need for feeding is somewhat questionable, although most zoanthids do not produce enough energy from photosynthesis to meet their daily needs. They reproduce by budding from their stolons, remaining attached and connected by the coenenchyme.

Protopalythoa is different from Palythoa in that the polyps are not immersed in the coenenchyme. They are undoubtedly closely packed together, but they remain individual. These animals can form small or even solitary colonies, though they are far more commonly found in huge landscapes that encrust the reef substrate. Their polyps are most frequently brown, with usually shorter and blunted tentacles surrounding the oral disk. The tentacles and oral disk (often marbled of striated) are often of contrasting color to the polyp body. These species have prolific mesenteries, often numbering more than sixty.

Zoanthus are normally much smaller polyps and they do not incorporate sediment into their bases. A divided sphincter muscle is the heretofore base of their classification. They reproduce by budding from the base of the parent colony to form very large mats of stoloniferous polyps which encrust the reef substrate. They are not normally seen to feed on large prey. Zoanthus are normally brightly colored animals, usually with oral disks 1/2" or less across their diameter.

Sphenopus are very similar to Protopalythoa without sand encrusted polyps. They are found on sandy bottoms, small pieces of rubble, or attached to sea grass blades They are not found on the reef itself. Sphenopus are solitary polyps and never colonial.

Isaurus are larger, tubular zoanthids found alone or in very small upright colonies with individual polyps attached to each other at the base. Their body wall has large tubercles and the tube-like polyp body is quite distinct. They are normally gray to brown in color and can be found on reef crests or in littoral zones. Their tentacles are quite reduced and their bodies may remain buried in sediments for those found in such locales.

Parazoanthus are solitary, though usually colonial, polyps that do not incorporate sand into their bases. Many of them are found commensally attached to other living or dead organisms. A membranous mat may or may not be present. They have normally fluted polyp bodies with long thin tentacles that feed readily. Parazoanthus can be muted or brightly colored, occasionally speckled, and normally contrast to their commensal organism. Reproduction is by budding followed by separation from the parent polyp.


One of the most distinctive features of the zoanthids is that they are so uniquely not a coral and not an anemone. In fact, they are technically stoloni ferans. A stoloniferan is characterized by the presence of a runner that connects adjacent polyps, called a stolon. These appear as little bits of tissue that spread across the substrate like a root or a grass runner. Some zoanthids use their stolons to bud new polyps, but they are mostly a device for anchoring colonies to the substrate and for encroaching on new territory.

Some zoanthids also have a thick gelatinous mat, made of fibro-protein, and called the coenenchyme. Some authors and references also call the coenenchyme the mesoglea, since it is this middle psuedo-tissue layer from which the mat is formed. The coenenchyme is used to support the soft polyps that are imbedded into it. Palythoa species may incorporate bits of rubble, sand and debris into the coenenchyme to help stiffen it, and give added support.

Almost without exception, zoanthids have smooth, flat, broad oral disks with tentacles that radiate outward from their margins. Tentacles are found in two nearby rows, termed the brachycnemic and macrocnemic arrangements. The tentacles always are in a number that is a multiple of six. Their mouth is grooved with a furrow called a siphonoglyph. This furrow is lined with thousands of beating cilia that aid in feeding and water exchange. The cuticl e is a leathery outer skin that affords these soft bodied animals some degree of external protection. Finally, most zoanthids secrete a heavy, non-viscous mucus coat that further aids in protection, feeding, and confers a degree of immunity. The body of the polyps, or "stalk", is known as the scapus. The flared "head" or cap of the polyp, containing the oral disk and tentacles, is known as the capitulum. For those unfamiliar with the etymology, capitulum means head...<thud>...what a concept!

In Nature and in the Aquarium

OK. We know what they are and what they look like, but who are the zoanthids and why do we want them in our company? Well, zoanthids are a very prolific group of corals that inhabit a pretty diverse range in nature. Vast areas of coral reef substrate may be covered in mats of brightly colored zoanthids, while other areas may have isolated colonies in odd locations. In fact, they are found in areas that can be exposed to some pretty harsh conditions that would not allow for the survival of many other corals. Some can be found on rocky shorelines, subjected to the incessant pounding of waves breaking on top of them. Others can be found in intertidal zones where they are exposed directly to the air for extended periods of time. Still other are found alone in dark or remote nooks and crannies of the reef. Perhaps most notable is that large colonies of zoanthids may be found near the saddeningly more common effluent discharge of municipalities and industries. Such areas have locally high nutrient areas that would stifle, if not kill, the growth of most other common forms of reef life. Yet the zoanthids (and Xenia) can be found thriving in such areas, suggesting their inherent adaptability, tolerance, and ability to thrive in what can only be considered conditions euphemistically described as not conducive to coral reef growth.

In fact, zoanthids are quite hardy in the aquarium. Not only are these mostly common animals, but they reproduce very quickly. They will tolerate wide fluctuations in water parameters in the captive environment and still flourish. This is duly illustrated in the experience of many hobbyists who find that, despite prolonged periods while live rock is out of water, and despite the often gag-reflex triggering curing cycle, zoanthids show up quickly on live rock once it has cycled. Once they show up, zoanthid growth and reproduction can be quite prolific. A piece of live rock in one of my own aquaria has four species of zoanthid from three separate genera, all indigenous to the rock itself. This is quite common. One small colony of Zoanthus consisting of a mere six or seven animals have covered roughly a square foot of the rock in a year and a half, and now number in the hundreds.

Zoanthids also have quite a number of "moods." In other words, when certain tank conditions deteriorate, they may show their displeasure in diverse ways that can be excellent indicators of any corrective action that might be needed. Degrees, of expansion or contraction, feeding behavior, coloration, and complete closure are all among their various reactions to changing water conditions.

What can be discovered about zoanthids from their appearance? Quite a bit, as it turns out. Zoanthids from shallow water, especially those of the genus Zoanthus, tend to be brightly colored. It is important to remember that some of the bright colorations/pigments may or may not be directly attributable to the higher light levels, but the fact remains. Colonies that are darker are usually from lower light levels. Water movement is a much better colony shaper. Zoanthids from areas of strong water movement have shortened stiff polyp bodies, shorter tentacles, and smaller oral disks. They form dense low growing mats, secrete less mucus, and are usually very colonial. Zoanthids from areas of lower water motion tend to be larger, with longer tentacles, flexible bodies and wider oral disks. They secrete more mucus and tend to form smaller colonies. One can try to emulate a reasonable facsimile of tank conditions or placement options based on the observation of the aforementioned characteristics.

Zoanthids are also quite variant in terms of their capacity for feeding.

There have been many articles documenting the relative amount of energy that is obtained through photosynthesis in zoanthids. While somewhat variable, in no case does light alone supply anywhere near the total energy budget of any zoanthid. Furthermore, much of the carbon products produced by photosynthesis are used in the production of mucus, making external sources of carbon containing compounds, in addition to non-photosynthate material imperative to the health and survival of zoanthids. Zoanthids feed on particulate matter, organic macromolecules, bacteria, plankton, and even larger prey. To what degree each food item can theoretically comprise the natural dietary intake of zoanthids is anyone's guess, but it would appear that most tank conditions are able to supply the required external nutrition to a tolerable level based on the success of colonies in the aquarium.

Therefore, the relative proportions of each organic food source may not be critical for these animals. Some zoanthids may not be observed to feed at all, while others are capable of ingesting rather large pieces of food. One small colony of very large Palythoa individuals was observed swallowing an entire bay scallop. Of course, this is not a normal food item for a zoanthid, but when one foolishly attempts to balance a small plate of "reef foods" on the top edges of the corner of an aquarium, accidents may (and do) happen...thus, let this be one of those lessons....

when faced with the a choice between the distinct possibility of a disaster or "oops" caused by the inane and self deceptive behavior of the aquarist regarding the statistical probability of a circus performer-like balancing act, or the likelihood of forces of the physical world actually taking precedence over one's own hopes to the contrary that such natural laws can be "willed" into not happening, it is best to force oneself to remember the outcome of most previous such happenstances and opt for the method of actually using sane judgment and working in consort with the laws of the universe when attempting any such actions.

Thus, we can see that some zoanthids may enjoy feedings, some (especially certain azooxanthellate Acrozoanthus and Epizoanthus) may require food, and others may not even accept food that is offered to them...instead turning up their tentacles and stomping off in a huff at the audacity of such poor culinary choices. If a zoanthid is large and seems to rapidly consume food, one can assume that it will benefit from occasional feedings. Otherwise, they are likely getting enough from the immediate environment.

When Zoanthids Go Bad

What makes a zoanthid unhappy? As it turns out, not too much. Zoanthids have a somewhat unique complement of nematocysts that are fairly isolated in the feeding tentacles. They are also not terribly useful in terms of defense. Fortunately, zoanthids are fairly innocuous when it comes to "sting or be stung" responses. There are clearly events where it is obvious that a zoanthid has grown too close to a coral which is capable of dominance. The zoanthid will react by closing up, rather than "fighting back" and being damaged. Zoanthids can remain in such a closed state for surprisingly long periods of time...weeks may elapse before the polyp begins to deteriorate. Various corallimorphs, so often thought of as passive little reef creatures, seem to have a pronounced ability to ward zoanthids (and most other things) out of their territory.

Ya know, I am always quite perplexed as to why these corallimorphs should ever have been assigned the notion of being innocuous. Any coral with the "cajones" to have jumped out of its calcium armor suit so many millions of years ago certainly must have evolved some pretty darn good defense systems to just voluntarily give up that advantage. You'd think that would be self evident. I guess we don't see them brutally attacking and swallowing prey in a gnashingly carnivorous way so we assume, "hmmph, they seem like they they are nice animals." Chuh! Right!

Sorry, digressing again.

Just as zoanthids are capable of being stung, some of the larger Palythoids are capable of dominating over certain other neighbors in terms of the strength of the cnidocytes. Again, we see the exceptional nature of dominance hierarchies in these organisms. Corals which would ordinarily be considered highly passive grow in a and around zoanthids without being harmed, yet some corals ordinarily considered to be more aggressive seem distinctly unpleased to be in the company of even mild-mannered zoanthids.

Example: A large mat of Zoanthus sp. grows readily around branches of the normally passive Montipora digitata without overtaking them, yet encrusts and overtakes Pocillopora damicornis. The same colony allows the growth of certain indigenous sponges within its midst, yet avoids another indigenous sponge species like the plague. Thus, a case by case basis based on species is really the only feasible method of establishing dominance heirarchies.

One interesting aspect of at least some studied zoanthids is that many of them are unable to recognize other hexacorals or octocorals as allelic competitors or siblings, and thus are able to grow straight over the top of the competition.

In terms of disease, zoanthids are quite sturdy animals. They may succumb to jelly-type infections, especially if a polyp are colony area has been subjected to injury. There is also a common ailment that affects zoanthids that appears as a white cheesy growth usually seen on the outside of the polps stalk. This area is most likely a microorganism, though I have not seen any mention of what or if it has been identified. This should be a relatively easy process. The result of this infection is that the polyps becomes very soft and deteriorates entirely. It can continue to spread to nearby polyps, the entire colony, and even other colonies in the aquarium. This particular type infection seems specific to zoanthids, and neighboring colonies may react adversely when such a situation is occurring within a tank, even if they are not directly affected. The communication of individuals within a colony is sometimes eerie, as the stolons ( through either ring canal systems or other intercommunicative structures) act like rapid telephone transfer, alerting the entire colony that something is amiss. The result is a colony that remains totally or partially closed even when a few lone individuals are affected. This behavior is also seen when certain polyps are exposed to noxious stimuli or injury. Freshwater dips, brief iodine dips as outlined in numerous articles and resources, and the topical application of a fairly insoluble antibiotic paste such as Erythromycin may be the treatment of choice for such infections. It would be ideal to treat any bacterial infection with a stronger antibiotic topical paste, such as Neomycin or Kanamycin in a hospital tank. Removal of the potentially contagious infection from the tank is but an added benefit to the entire system.

Zoanthids are also susceptible to bleaching, and some Palythoids are the harbingers of mass bleachings in natural communities. Palythoa are often the first to bleach in extreme summer doldrums, though their hardiness usually allows for complete recovery pending the removal of temporary environmental stress. Partial bleaching or "paling" of colonies is common within specimens offered in the aquarium trade, as many have been subjected to fairly atrocious conditions in transit. Being hardy animals, often less care is given to them than more fragile animals. It is notable, though, that zoanthids usually can and do recover nicely once exposed to good aquarium conditions in the hobbyists' own tanks.

There are crustaceans and gastropodial predators of zoanthids, including the sundials (Heliacus sp) and Platypodiella spp. crabs. Certain shrimps and lobsters have been reported to pick on or even consume zoanthids, as have many butterflyfish, angelfish, triggerfish, filefish, and other normally corallivorous fish.

Growth of filamentous algaes and cyanobacteria on and among the mucus coated polyps is common. These areas are veritable petri dishes for algal and bacterial culture. Good water movement and well tended tank parameters are usually sufficient to prevent such proliferations, and the occasional use of a turkey baster or powerhead to dislodge excess debris within the colony is beneficial.

Interesting tidbits

While it would be nice to be able to provide a complete list of all commonly available species in terms of exact care and descriptions, I am afraid that would take a small book. Nonetheless, there are a few asides that are worth mentioning in Zoanthid World.

First, all members of Zoanthus and Palythoids contain the highly potent neurotoxin known as palytoxin. It is found in their mucus and in their mesenteries. This toxin does not seem to affect neighboring colonies in the same way as some of the other noxious chemical secretions of corals. Rather, it appears to serve as an anti-predation defense. Delbeek and Sprung note that several predators of zoanthids are not only unaffected by palytoxin, but actively store it in their bodies and shells. Notwithstanding this fairly common behavior of certain organisms to adapt highly specialized means to cope with their environment, palytoxin has been shown in grazing studies to be an effective anti-predation compound. Irrespective of its use to the zoanthid, palytoxin is a very dangerous substance, and anyone handling zoanthids of any species should be very careful not to allow the polyps to contact any area of broken skin. Handwashing to remove mucus after handling zoanthids is absolutely required in the interest of safety. Palytoxin is also denatured by heat, and hot water hand washes will further act in loosening and solubilizing and mucosal remnants.

The second tidbit of note involves the beautiful Palythoa grandis, or moon polyp. Although it is likely that a Pacific species, P. toxica, may indeed be the same animal, the collection of most P. grandis has always been from the Northern Gulf of Mexico and Florida. However, these animals may not be collected attached to live rock. It is illegal, and should not be supported.

Zoanthids are excellent choices for the home aquarium. From beautiful multi-colored floral-like arrrangements of Zoanthus, to the interesting shapes of Isaurus, these animals are hardy, fast growing, and fascinating creatures. Their inclusion in a reef aquarium will almost always be a natural part of any habitat specific or general display.

Eric Borneman


Borneman, Eric. 1997. In publication

Burnett, W.J., et. al. 1997. Zoanthids (Anthozoa, Hexacorallia) from the Great Barrier Reef and Torres Strait, Australia: systematics, evolution and a key to the species. Coral Reefs 16: 55-68.

Coll, J.C., and P.W. Sammarco. 1988. The role of secondary metabolites in the chemical ecology of marine invertebrates: a meeting ground for biologists and chemists. Proc. 6th Int'l Coral Reef Symp, Australia, 1: 167-73.

Delbeek, J.C. and Julian Sprung. 1997. The Reef Aquarium. Volume 2. Ricordea Publishing, Inc., 546 pp.

Koehl, M.A.R. 1977. Water flow and the morphology of zoanthid colonies. Proc. 3rd Int'l Coral Reef Symp. 437-44.

Sebens, Kenneth P. 1977. Autotrophic and heterotrophic nutrition of coral reef zoanthids. Proc. 3rd Int'l Coral Reef Symp. 397-404.

Steen, R. Grant and L. Muscatine. 1984. Daily budgets of photosynthetically fixed carbon in symbiotic zoanthids. Biologic Bulletin 167: 477-87.

Van Alstyne, K.L. and V.J. Paul. 1988. The role of secondary metabolites in marine ecological interactions. Proc. 6th Int'l Coral Reef Symp, Australia. 1: 175-85.

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