The Zoanthids: Sea Mats, Yellow Polyps, and Button Polyps
by James W. Fatherree, M.Sc.
Zooanthids are relatively small "soft corals" that can come in a wide range of attractive colors and patterns. They tend to be easy to care for and very hardy, and many can grow quickly, making various sorts great additions to a reef aquarium. So, this month we'll take a look at some basic zoanthid information, the common varieties you'll see for sale, the differences between them, and how to take care of them, etc.
To get started, let's look at some often confusing name game stuff. All of the types of zoanthids are members of the biological Order Zoanthidea, which is part of the Subclass Hexacorallia. These two names might not mean too much to you, but they do point out something odd about zoanthids if you know who their cousins are. Being hexacorallians means that zoanthids are much more closely related to stony corals than to the other soft corals, which actually belong to an entirely different group, the Subclass Octocorallia. So, technically the zoanthids aren't soft corals, but they're close enough for the name to stick anyway.
Then there's the commonly used name "colonial anemone", which is often generically used for any of these. Yes, they can look something like small sea anemones, but they aren't anemones either. Anemones are indeed hexacorallians, but they are in the Order Actiniaria, not Zoanthidea. But again, right or wrong, the name has stuck with them anyway...
The common name "xxxx polyp" is an odd one too, as the term "polyp" can apply to any soft or stony coral animal. We say a solitary coral is a single polyp, while a colonial coral is made of numerous polyps. Thus, it is also strange that names like button polyps and yellow polyps have been used forever by hobbyists for many varieties of zoanthids. But, I suppose there are LOTS of common names that don't really make great sense anyway, so this isn't too surprising. Just remember that if you are reading about corals and such, if the term "polyp" comes up, it doesn't necessarily mean the subject is a zoanthid.
Lastly, the Order Zoanthidea contains several genera, one of which is Zoanthus. Thus, the members of this particular genus are correctly called zoanthids, confusing folks from time to time. All the members of the Order Zoanthidea can correctly be called zoanthids, but then again, so can all the members of one sub-group of the order. This can make you wonder what someone is talking about - any type of zoanthid, or a specific type of zoanthid. Get it? Taking note of the context the term is used in is the only way to keep it straight really, unless you see it capitalized and in italics or underlined, which is always an indication that it refers to a genus.
Anyway, enough of that! Let's get on to the rest. All in all there are quite a few genera, but we'll stick with the four that you're most likely to find offered for sale, which are Zoanthus, Parazoanthus, Palythoa, and Protopalythoa. By common name these are the sea mats, yellow polyps, and button polyps, respectively. As you'll see, the palythoans and protopalythoans are both called button polyps.
Genus Zoanthus: the sea mats
Sea mats, which are the group of zoanthids most commonly called zoanthids are colonies of relatively small polyps that are usually something like a 1/2 inch or less in diameter. They are usually very short and tightly spaced (but not always), and they can form very large colonies, growing as encrusting mats from which the polyps emerge. They are all bound together by this mat, which is made of living tissue and called coenenchyme.
Each polyp in a colony is topped by several blunt tentacles and has an almost non-existent mouth, and they typically come in brown, green, blue, or gray with lighter-colored centers. There are also occasional specimens that may be bright red or orange, and a few especially nice specimens may have a half-dozen different colors mixed together, making these some of the most colorful things you can put in your tank. They are found encrusted over a wide-variety of surfaces, and may even grow over other corals at times, as well.
In an aquarium the sea mats are heavily dependent on good lighting and will need high-intensity illumination to thrive. They also do best when placed in an area where they are subjected to a moderate to strong current, which helps to prevent detritus from settling on a colony and collecting between the polyps. Keep in mind, though, that at times these little guys can over grow other corals. As a colony grows, it may come into contact with another coral and just keep on growing. This may have no adverse affect if they grow around the base of some corals, but in other cases it may indeed kill the overrun specimen. With that said, to the contrary, they may also be burned by the stings of more aggressive corals at times, which have no trouble defending their space.
Other than that, I should mention that sea mats aren't going to take any sorts of foods you may try to offer by hand, but they still may benefit from finer-sized plankton-type foods or a deep sand bed set-up that provides natural foods. Regardless, I've seen them multiply and watched colonies grow rapidly in some aquariums with no sort of food provided - when given sufficient lighting.
Genus Parazoanthus: the yellow polyps
There is more than one type of parazoanthan, but the only one you'll see with any regularity is the currently un-named species that used to be called Parazoanthus gracilis, better known as "yellow polyps". For whatever reason, it has been decided that the gracilis is wrong, yet no new name has been given to replace it (yet). Anyway, these are solitary zoanthids, although they do usually form colonies. What I mean is that the individual polyps don't neccessarily stay connected together like sea mat, as connecting tissues may or may not be present, but they typically live close together in groups anyway. Oftentimes they are only joined by thin strips of flesh called stolons, or are found in little patches with there bases growing together.
Yellow polyps have a taller body topped with much longer, thin tentacles that are indeed used for feeding. However, even though the polyps are taller than those of Zoanthus, they are typically smaller in diameter, usually being less than a 1/4 inch across. In addition to being found growing over hard substrates, some parazoanthans are also often found living on other organisms, such as sponges, too. And, while other varieties with other colors are occasionally offered, yellow polyps are, of course, yellow in color (or at least yellow-ish).
Yellow polyps can often thrive under a wider range of lighting conditions than the sea mats, but in general the brighter the lights the better. They are also "softer bodied" and will prefer a lower current, with low to moderate flow being best. They'll also greedily eat small meaty foods you may give by hand, and like so many other creatures, they will benefit from the use of plankton-type foods and/or a D.S.B. set-up, too. But, again, I've also seen them thrive in some aquariums when no food was provided at all.
As is the case with Zoanthus, you do need to be careful with these when it comes to where they are placed, though, as they can also kill other corals they come into contact with. They can multiply quickly under optimal conditions and will sometimes dispatch anything in their path as they spread, so watch out. But, then again, they are often susceptible to stings from more aggressive corals, too.
All in all, yellow polyps can be exceptionally hardy. However, many other parazoanthans you may see at some point may live in association with other organisms, and often rely on those organisms in some way. And unfortunately, these species of Parazoanthus will likely die if the organism they are living upon dies. In other words, if you jump on the opportunity to buy an unusual specimen that consists of some parazoanthans living in/on a sponge and let the sponge die - the polyps will follow suit. So, it’s better to stick with other specimens that are less demanding. In addition, note that there are some species of Parazoanthus that are azooxanthellate (can't use light to produce food) and will rely on food. Thus, they would require some form of food, provided regularly, and be much more difficult to care for than other species, as well. These are uncommon in the hobby though.
Genus Palythoa: the button polyps
Of the four types of zoanthids covered, the palythoans are by far the least common. Still, you're very likely to see these at some point or another, so here's some info. You can identify members of this genus fairly easily, as they form a thick, encrusting clump-like coenenchyme base, which the polyps are deeply embedded in. Not much of the polyp sticks out of the base, which is typically clearly visible. They also incorporate sand or bits of shells, etc. in the coenenchyme. Conversely, when looking at Zoanthus, the polyps often obscure the coenenchyme so that it cannot be seen, and there is no sand incorporated in it either.
The colors aren't as bright as those of Zoanthus either, as most are mixtures of brown, red, and/or cream. Their tentacles are so short they look more like a ring of bumps around their margins, as well. In addition, palythoans will take meaty foods, while Zoanthus won't.
However, despite these differences, they will certainly benefit from the use of plankton-type foods and/or a D.S.B. set-up, and have similar care requirements. They need good lighting and will thrive under high-intensity illumination, and should also be placed in an area where they are subjected to a moderate to strong current.
Lastly, these are also well-known for their ability to overgrow other corals, killing them in the process. So, be mindful of where you place a specimen in your aquarium. Of course, they too can be stung by more aggressive corals, though.
Genus Protopalythoa: the other button polyps
Members of the genus Protopalythoa are also most commonly called button polyps, but they look quite different, really. As is the difference between Zoanthus and Parazoanthus, palythoans form a thick, encrusting coenenchyme base, but protopalythoans don't. Indeed, the polyps can be connected together by a small base at times (which may or may not have some sand/debris in it), or may only be connected by thin bits of flesh, but they are sometimes not connected at all.
Protopalythoans also have longer tentacles than palythoans, and the tentacles are often seen to alternate in an up and down fashion around the rim of the polyp. The tentacles are usually rather thin and taper out to a point, as well. Protopalythoan polyps are much taller (in part because there is no coenenchyme) and are usually anywhere from 1/2 to 1 inch across, too. But, there are a few that are even bigger, sometimes being nearly two inches across, as well. In addition, the common colors are solid brown and red, but many are also mottled with cream, white, and/or green, and/or have cream or white stalks and bases, making them quite attractive.
As far as care goes, you can probably guess that they'll prefer lower currents than the palythoans, low to moderate being best. However, everything else is the same. Bright lights are best, although, moderate lighting is okay. Meaty foods given by hand will be appreciated, while plankton-type foods and/or a D.S.B. set-up can help, too. And yes, they can overgrow some other corals, but be stung by others, as well.
A last note: Be Careful!
So, they are all hardy and easy to care for, but there is one more thing you need to be particularly mindful of when it comes to zoanthids. Many types are very poisonous.
There are several types of commonly available zoanthids, including all of the palythoans, that can produce a deadly toxin (appropriately called "palytoxin"). It is found is the mucous coat that they cover themselves with, and if you get enough of it in an open wound, or your eye, mouth, etc. - it just might kill you. Many hobbyists have reported cases of numbness, sickness, and/or hallucinations, but the stuff is actually strong enough to kill, as well.
Handling them when you have a wound is an obvious no-no, but when you touch a colony and get the slime on your fingers (which is unavoidable with these things), it is imperative that you don't rub your eyes, suck your fingers, or even pick your nose until you have washed your hands thoroughly. Really, you should never handle these without wearing protective gloves. Some hobbyists (including me) have handled zoanthids without gloves many, many times in the past, but it is now well-known that things can go very wrong when this is done, even if you have no wounds you know of and plan on washing your hands immediately after touching a specimen.
Wear the gloves!
Borneman, E. 2001. Aquarium Corals - Selection, Husbandry, and Natural History. T.F.H. Publications, Neptune City, NJ. 464 pp.
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.
Delbeek, J. C. and J. Sprung. 1997. The Reef Aquarium: Volume Two. Ricordea Publishing, Coconut Grove, FL. 546 pp.