Zoanthids in the marine aquarium – Bartłomiej Stańczyk

Bartłomiej Stańczyk Marine biologist, graduate of the Department of Invertebrate Zoology and Hydrobiology at the University of Lodz. The author of Reefhub.pl, a popular blog about marine aquaristics. Since 2018 Tropical expert on products dedicated to marine tanks – Marine Power.

Although the term “soft” is more aquarium slang than a scientific classification, it reflects quite well the fact that these corals do not form a stony skeleton. However, despite the absence of a skeleton, Zoanthus form a compact and fairly rigid tissue of polyps unlike other popular soft corals – for example Xenia sp.

SCIENTIFIC CLASSIFICATION In taxonomy Zoanthus (Zoanthids) is a name reserved for a particular genus of the Zoanthidae family, but aquarists very often use it to describe all similar-looking corals in the Zoantharia order (including parazoanthus and palythoa). In the following text, I will use the same nomenclature, but please remember that it is a simplification. While we are at taxonomy, while preparing this material, I came across some statements suggesting that Zoanthids are not corals. I don’t want to go too far away from the main topic here, because I know that the taxonomic classification is always disputable. For example some organisms, once classified as plants, today are no longer plants. (I remember perfectly well when in primary school I had to write a hundred times “bacteria are the simplest plants”, because I wrote on a test that bacteria are animals...). Many taxonomic tables classify Zoantharia order as Cnidaria and Anthozoa corals. Maybe in a few years it will change, but at the moment I see no problem in calling Zoanthids corals. CHARACTERISTICS The vast majority of Zoas grow to form a colony in which each polyp is independent. In some species polyps may grow from a common tissue known as coenenchyme or, less commonly, stolons. Zoanthids usually have the form of a small cylindrical polyp – on the one end fixed to the ground and on the other ending in a clear and colourful oral disc, which is surrounded by a garland of arms. The appearance of a single polyp largely depends on environmental conditions. Two separate polyps of the same species may differ not only in size of the oral disc and the length of the polyp, but also in the intensity of coloration. A polyp that grows in a stronger current and is intensely lit will grow shorter and with a smaller head in comparison to the same polyp that grows in low light and low current. Since the vast majority of corals in the Zo- anthidae family contains symbiotic Zooxanthellae in their tissues, Zoanthids need light to survive.

EASY BREEDING In nature, these organisms have the ability to reproduce sexually and asexually. There is not much research on sexual reproduction in the Zoanthidae family, but aquarists are much more interested in asexual reproduction. In Zoanthids, it takes a form of budding, in which a new polyp usually grows not from the

tissue of another polyp, but from the tissue growing on the rock. Until a few years ago Zoanthids were considered to be one of the easiest corals to keep in a marine aquarium. Together with other soft corals, they were the standard stock of tanks set up by the beginners in the hobby before they learned enough about the secrets of the marine world to face more demanding and difficult to breed calcifying corals. This ease of Zoanthids breeding is due to several factors. First of all, they are quite resistant to bad water parameters, so they forgive mistakes made by beginner aquarists. Secondly, when they have the right conditions, they multiply easily and quickly cover the rock on which they grow. Thirdly, they are very easy to frag (i.e. make new cuttings for sale or exchange). Fourth, they are relatively resistant to disease and have few natural enemies. And finally, they do not require any special care or expensive equipment. A dozen or so years ago the offer of Zoanthids in marine aquarium shops was, to say the least, poor. Just a few simple colour variations. This was probably due, among other factors, to the fact that the aquarists themselves shared zoas with one another, and its popularity at one point was so great that they started to be treated like weeds. Internet forums were full of questions about fish species that could control the growing population of Zoanthids. This was accompanied by the problem of high nutrient levels, which caused these corals to take on brown shades and simply become unattractive.

THE RENAISSANCE OF SOFT CORALS Today, Zoanthids are experiencing a real renaissance in marine aquaristics. This has been happening for a few years now, because fabulously colourful varieties – often fluorescent ones – have begun to appear in shops. Salespeople quickly discovered that they could easily make a fortune on them, and the rarest colour varieties reached a price of even several thousand dollars per polyp. That’s how Zoanthids have come back on the table and many of the tanks dominated by their different colour varieties are simply breathtaking. The effect is all the more amazing as these corals do not require large tanks or extensive filtration systems (as SPS corals do) and with just a few different varieties focused on a small area and illuminated with additional UV LEDs, you can enjoy truly awesome view. Coral breeders who specialize in ”high-end” Zoanthus polyps varieties keep inventing catchy names such as Samurai Sunrise or Ice and Fire. These names, however, have nothing to do with scientific names, and it is possible that the same coral will be called differently by two different sellers. Unfortunately, it often turns out that rare things are rare not without a reason. And so many of these “ultra-coloured” varieties do not grow as fast as other more popular Zoanthids. But although they are still relatively easy to grow and nobody is calling them weeds anymore, obtaining vivid and durable colours requires more skill than in the case of ordinary Palythoa. So, how can we enjoy the colourful coral carpets? Let’s take a look at the requirements of these corals from the aquarium perspective.

WATER PARAMETERS SUITABLE FOR ZOAS As I mentioned earlier, the tissue of Zoanthids is quite thick and stiff, which protects them to some extent from fluctuations in water

parameters and limits the effects of chemical shock. However, this does not mean that these corals do not require stable water chemical parameters. On the contrary, it is the stable environment that allows them to grow and gain colours, and frequent changes in salinity and water composition will stress them and prevent from showing their natural beauty. Their specific requirements do not differ from standard aquarium water parameters. These corals will feel good in water with stable parameters – temperature: 25-27OC, salinity: 33-36 ppt, pH: 8-8.3. As far as the NO3 and PO4 nutrients are concerned, it is assumed that Zoanthids are not fond of zero levels. This is due to two facts. First of all, these corals largely base their nutrition on Zooxanthellae, and these, as photosynthetic organisms, need nitrates and phosphates. Secondly, having a large biomass and growing relatively fast compared to SPS corals, Zoanthids especially require phosphates to maintain many metabolic processes in the growing tissue. For this reason, both Zoanthids and other soft corals, for example Sinularia, Sarcophyton, Rhodactis etc., feel good in water with detectable nitrates (about 5-10 mg/l) and phosphates (0.1 g/l).

PLACE IN THE TANK Although Zoanthids require light to grow, they should not be placed directly under a strong light source, for example under LED panels or HQI lamps. Too much light, especially when given suddenly, for instance when we introduce new polyps into a bright aquarium without acclimatization, can cause oxidative stress and in extreme cases even tissue regression in the coral. If intensive lighting is used in the aquarium, for example because SPS corals make up a large part of the stocking density, zoas should be placed close to the bottom or in slightly shaded places (but not in dark places), for example under rock overhangs. In short: we must find an optimal location, because these corals will develop smaller heads in bright light and will lose their brightness when it’s too dark. Luckily, Zoanthids can be moved from one place to another without any problems, so it’s quite easy to find them the right place. As for the circulation, it should be moderate, but not too small. Zoanthids must under no circumstances be jerked off by the water current, but only steadily washed to remove any residual detritus from between the polyps. Zoanthus corals adapt very easily to the aquarium situation – as long as it is stable.

ARE ZOANTHIDS AGGRESSIVE / EXPANSIVE? Most of the Zoanthids examined have cnidocytes. However, they are not very active and efficient in paralyzing potential victims. Laboratory tests have shown that some species hardly ever deliver a sting, even when directly irritated by artemia larvae. If we add to this the fact that the Zoanthids’ arms are rather short and stiff, we come to the conclusion that the popular zoas are not very predatory corals. However, most Zoanthids are equipped with cnidocytes (nematocysts), which can potentially, although very rarely, endanger neighboring corals, but I have never heard that different colour varieties stinging one another. It would be a long time to debate which coral would win the battle. There is no research on this topic and practically all knowledge is based on different, often contradictory information

coming from the aquarists themselves. However, it can be assumed that even if Zoanthids lose, they can threaten other corals thanks to the relatively fast growth of the colony. Some Zoanthids varieties can be under optimal conditions quite expansive, so it seems reasonable to place them on separate rocks – so that they will not colonize other substrates. It is also a good idea to bring them all close together, which will increase and emphasize colour differences and create a spectacular view. However, one has to be aware that zoas, like many other corals, can, in extreme situations detach several polyps (this phenomenon is known as polyp bailout) and let them sail away in order to settle in a new place. This is usually a reaction to stress caused, for example, by being put in a place with too strong water current. I have observed such behavior many times, also when there was no room for new polyps on the rock.

FEEDING ZOANTHIDS Zoanthids are animals that need to get food by themselves. Many soft corals, due to their thick tissue, have plenty of space for Zooxanthellae. This is why these corals make excellent use of photosynthesis as a source of food, while at the same time making less use of heterotrophy. This phenomenon has become the cause of a myth among aquarists that soft corals do not need to be additionally fed. Indeed, many of them will survive in an aquarium without additional feeding, but this does not mean that they will not take any organic matter from the water. If we want to get the most beautiful colours out of corals, we have to feed them additionally, so that they won’t need an excess of Zooxanthellae and will get rid of some. Zoanthids are best fed during the day because many of them close polyps at night. Tropical Marine Power for SPS corals in powder or wetted food for LPS corals in mini granules will be a perfect food for them. Zoanthids don’t

actively catch sinking food so it’s a good idea to turn the circulation off for a moment and gently “blow” them with food using a pipette. Watch out not to do it too intensively, because irritated polyps close immediately. It is best to spray the food over the polyp colony and allow the powder to fall freely on the corals. After several minutes, the circulation will distribute the rest of the food, which will be eaten by other aquarium inhabitants.

TENDENCY TO DISEASES Like all other organisms, Zoanthids can suffer from some health problems. Apart from the stress caused by unfavorable conditions in the aquarium (e.g. bad salinity or lighting), Zoanthids usually have problems for two reasons. The first are the organisms that parasite on them, such as snails or spiders. They not only irritate the corals, but also bite their tissue, which ultimately leads to the slow death of the colony. The best way for unwanted guests is a bath in disinfectants. It is also good to check the polyp “roots” as the eggs of parasites can be often found there. The second is a bacterial infection known as Zoa Pox. The symptoms of this infection are closed polyps with signs of tissue regression and white “pimples” and discoloration. The best cure for all bacterial infections are always baths of new frags in a 3% solution of hydrogen peroxide or in an antibiotic. A very effective antibiotic is nitrofurazone (trade name Furacin). Unfortunately, in the European Union it is not available without a prescription and pet shops can’t sell it. The second effective antibiotic is chloramphenicol. It is quite easy to get it in the form of sachets in pigeon breeders’ shops. Another effective agent for diseases caused by bacteria is Chemiclean, well known to aquarists. Personally, I’ve never used it to treat Zoa Pox, but on various forums you can come across a lot of information confirming its effectiveness.

DO ZOANTHIDS CONTAIN PALYTOXIN? Palytoxin is one of the strongest non-protein poisons. And indeed, some corals contain a lot of it. It certainly occurs in three coral groups: Palythoa, Protopalythoa and literally a few species of Zoanthus1 . Fortunately the vast majority of colourful zoas are Zoanthus vitenamensis varieties and they contain no toxin. Unfortunately, there is no cheap and easy test for its presence, so every time we touch Zoanthids – especially outside the water – we should use safety glasses and gloves, and all fragging should be done in a well-ventilated room.

I hope I’ve shown you how beautiful Zoanthids can be. Their coloration is often more intense and varied than in other corals. That’s why I encourage all marine aquarium fans to introduce them to their tanks – especially if they plan mixed reefs. However, zoas-only-tanks where rocks and bottom are covered with variety of colourful polyps, will be definitely most impressive. I guarantee that if you devote some time and heart to them, the effect will be astonishing.

1A 2009 Toxicon Journal scientific study (which may no longer be up to date) reports the following species of Zoantharia containing palytoxin: Palythoa caribeaorum, Palythoa mammilosa, Palythoa tuberculosa, Palythoa toxica, Palythoa vestitus, Palythoa aff. margaritae, Zoanthus soanderi and Zoanthus sociatus.