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.
