Keeping Zoanthids

Reef aquaria wouldn’t be reef aquaria without corals–plain and simple. But those extra little goodies (sea anemones, corallimorpharians, etc.) undoubtedly make reefkeeping a lot more interesting. This could certainly be said of the zoantharia. Indeed, as novel color morphs of popular species continue to be imported, aquacultured and distributed within the trade, zoanthids are increasingly becoming a hot commodity. Some aquarists amass sizable collections of these beautiful animals and often maintain them in dense, multicolored “zoa gardens.”

Any creature with bright colors has appeal in the aquarium hobby. But the enduring popularity of zoanthids owes not just to their color, but also their hardiness and relative ease of care. Think about all those saltwater aquarists who are in it mainly for the fishes. While they are most interested in and focused on their finned livestock, they nevertheless tend to desire an aquascape that looks “reefy” but spares them the rigorous husbandry demands of a conventional reef tank. Because zoanthids are generally less fussy than, say, stony corals when it comes to lighting and many water parameters (e.g. calcium concentration), they certainly are appropriate for these aquaria.

Of course, that’s not to say that you can simply toss any kind of zoanthid into any tank in any manner and be guaranteed that it will flourish without further ado. This article describes zoanthid biology and lists a few useful tips for keeping them healthy and gorgeous, plus some suggestions on avoiding contact with the dangerous toxins they produce.

Zoanthids Natural history

The order Zoantharia is assigned to the class Anthozoa, making zoanthids much closer relatives to sea anemones, corallimorpharians and stony corals than they are to soft corals, gorgonians, etc. Members of the order occur only in marine habitats, but are quite widespread in shallow or deep, tropical or temperate environments. 

Zoanthids are quite recognizable, as they are composed of a very “standard” flowery polyp shape with marginal tentacles. They universally lack a skeleton of any sort. They may occur as individuals, but even the polyps of colonial types are clearly distinct from one another. The only connection between polyps is a fleshy stolonal network at the bases of their long columns.

Those types most commonly kept in aquaria belong to the genera Zoanthus, Parazoanthus and Palythoa. Most of these tropical/subtropical shallow water species are (like many of the corals they share these environments with) zooxanthellate. Their reliance on zooxanthellae (i.e. symbiotic dinoflagellates such as Symbiodinium) necessarily restricts them to brightly illuminated sea bottoms.

Aquarium care

For sure, zoanthids are far more adaptable to “dirty” aquarium conditions than are most corals and sea anemones. Some say that they actually dislike “overly clean” systems. There may be something to this, as zoanthids do consume enriched particulate organic matter such as marine snow.

But as zooxanthellate animals, light availability/quality is of utmost importance. Lighting can be a somewhat controversial subject among zoanthid enthusiasts. Many claim that they do not like bright light, though it may be more correct to say that they do not like to be close to a bright light source. Is it that they prefer diffuse lighting? This might make sense, because they do indeed seem to appreciate fairly bright light, albeit way at the bottom of the tank. Their preference for more scattered light might make even more sense considering the many types that reportedly prefer bluish spectra, as these varieties likely originate from deeper waters.

The best placement for new zoanthids in your system is to start them at the very bottom; if you wish to keep them elsewhere, incrementally acclimate them to brighter/more direct light by moving them up the aquascape in a gradual manner (perhaps a couple inches every couple of weeks). 

While not very fond of powerful, direct, laminar flow, zoanthids of most types seem to appreciate strong water movement. Not only does this help to bring them foods (especially suspended particulate organic matter) but also to remove their slimy discharges and other waste products. Moreover, good water flow prevents sediments from accumulating around and fouling the stolonal mats of dense colonies.

Special precautions

Many zoanthids produce a poisonous compound known as palytoxin. While biologists are still unsure how much palytoxin is produced by each species, and whether or not environmental factors play a role in palytoxin production, it is clear that the substance is extremely dangerous. In some cases, contact (e.g. ingestion or inhalation) may be fatal. 

Normal handling without gloves can result in very uncomfortable skin irritation. Rougher handling (e.g. fragging) may encourage the secretion of these toxins and thereby potentially increases the severity of reactions. If one handles a zoanthid and then touches their eyes or mouth, much more serious reactions (including permanent blindness) may occur. Palytoxin can be transmitted through the air in steam; therefore, do not boil live rock that has, or recently has had, or may have recently had, zoanthids growing on it. Finally, it is extremely advisable to avoid handling (even briefly touching) these animals without proper protection, including goggles and water-proof gloves!