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Zoanthids in the Marine Aquarium

Zoanthids are small soft corals that come in a wide range of colors and patterns. They are easy to care for and very resilient. Most of them can grow rapidly and make a delightful addition to the marine aquarium. Let's explore some basic information about zoanthids, the species you'll find in the trade, the differences between them, and how to care for them.


To start, let's look at the often confusing names. All types of zoanthids belong to the biological Order Zoanthidea, which is part of the Subclass Hexacorallia. These two names may not mean much to you, but they suggest something peculiar about zoanthids when you know who their relatives are. Being hexacorallia means that zoanthids are more closely related to stony corals than to soft corals, which belong to an entirely different group, namely the Subclass Octocorallia. So, in essence, zoanthids are not soft corals, but they closely resemble them, hence the name sticking.

Then there's the commonly used name "colonial anemone," which is used for all types. Yes, they often resemble small sea anemones, but they are not anemones. Anemones are hexacorallia too, but they belong to the Order Actiniaria, not Zoanthidea. But again, right or wrong, the name sticks.

The general name "XXX polyp" is also strange because the term "polyp" can be used for both soft and stony corals. We call a solitary coral a polyp, while a colonial coral is composed of multiple polyps. So, it's odd that names like "button polyps" and "Bali polyps" have been used for many varieties of zoanthids. But there are many common names that are not really logical, so this is not surprising. However, keep in mind that when you read about corals, and the term "polyp" comes up, it doesn't necessarily refer to a zoanthid.

Lastly, the Order Zoanthidea contains various genera, one of which is Zoanthus. The members of this genus are correctly called zoanthids, which can sometimes lead to confusion. All members of the Order Zoanthidea can be called zoanthids, but also all members of a subgroup of this order. This can make you wonder whether someone is talking about a type of zoanthid or a specific type of zoanthid. The only way to make this clear is through context unless you see it in uppercase, italicized, or underlined, which is always an indication that it refers to a genus.

But enough about that. On to the rest. Altogether, there are many genera, but we'll stick with the four most offered in the trade, which are Zoanthus, Parazoanthus, Palythoa, and Protopalythoa. The most commonly used names for them are respectively Mat Zoanthids, Bali Polyps, and Button Polyps. As you can see, Palythoa and Protopalythoa share the same name.

Genus Zoanthus: Mat Zoanthids

Mat Zoanthids consist of colonies of relatively small polyps, 1 centimeter or smaller in diameter. They are often very short and closely spaced. They form large colonies, growing like mats over the rocks. They are all connected by a mat, which consists of living tissue and is called coenenchyme.

Each polyp in a colony has many blunt tentacles and an almost non-existent mouth. They vary in color from brown, yellow, blue, or gray with lighter-colored centers, the heart. Sometimes there are polyps that are bright red or orange or have multiple colors mixed, making them a very nice addition to the aquarium. They grow almost anywhere and can even grow over entire corals.

In the aquarium, zoanthids depend on good lighting and need high intensity to survive. They are best placed in a spot with a lot of to moderate flow, which ensures that no debris and detritus remain between the polyps. Keep in mind that these little ones can grow over other corals. As a colony grows, it can come into contact with a coral and continue to grow. This can pose little danger if they grow around the base of the coral, but they can also grow over it and kill the coral. However, they can also fall victim to more aggressive corals that have no problem defending their space.

Zoanthids do not take in the food we dose in the aquarium, but they can benefit from small plankton-like food or a deep sand bed that provides natural nutrients. However, I have seen colonies thrive in aquariums where no food was given, as long as there is enough light.
Picture of Zoanthus

Genus Parazoanthus: Bali Polyps

There is more than one species of Parazoanthus, but the only one you will encounter in stores is the currently unnamed species Parazoanthus gracilis, better known as Bali polyp. For some reason, it has been decided that the name 'gracilis' was incorrect, but no new name has been proposed yet.
This is a solitary zoanthid, although it often forms colonies. By this, I mean that the individual polyp does not always stay attached to the mat, like the true zoanthids. But they do live in groups together. Often, they are only connected by thin strips of flesh, called stolons, or in small groups with their bases against each other.

Bali polyps have a longer body with much longer tentacles, which are actually used to catch food. Although the polyps are longer than zoanthids, they are often smaller in diameter, often no more than half a centimeter. They often live on hard surfaces but are also found on sponges. Bali polyps are always yellow or yellowish in color.

Bali polyps can tolerate a wider spectrum of light than zoanthids, but brighter is better. They also have a softer body and therefore require less flow to very low flow. They eagerly take in fleshy food like artemia. Plankton or a deep sand bed is also appreciated. But I have seen this species grow in aquariums where they were not fed.

Just like zoanthids, you should think carefully about the placement of these polyps. They can grow luxuriantly and overgrow other corals. However, they are also sensitive to strongly stinging corals.

All in all, Bali polyps can be very robust. Although there are other Parazoanthus that live in symbiosis with other organisms, such as sponges. Unfortunately, these zoanthids die when the sponge dies. In other words, if you ever come across this species and you think you have something special, and the sponge dies (which often happens), then the polyps will die shortly afterward. It is better to stick with the less demanding species. There are also some Parazoanthus that are azooxanthellate, meaning they cannot live on light. These require continuous feeding, making them less easy to keep. However, these species are rare in the aquarium hobby.
Picture of Parazoanthus

Genus Palythoa: Button Polyps

Of the four types of zoanthids we are discussing, Palythoa is the least commonly seen zoanthid. However, it is very likely that you will encounter this species somewhere. Members of this genus can be identified by a thick, overgrowing mass of coenenchyme, where the polyps are deeply embedded. Not much of the polyp protrudes from the base, which is clearly visible. They also incorporate sand and shells into the coenenchyme. In contrast to Zoanthus, where the polyps cover the coenenchyme and there is no sand.

The colors are not as vibrant as those of Zoanthus; most are brown, red, or cream. Their tentacles are more like a row of bumps around their center. Palythoa takes in fleshy food, unlike Zoanthus.

Despite the differences, they also benefit from plankton or a deep sand bed and require the same care as Zoanthus. They need a lot of light and moderate to high flow.

They are known for their ability to overgrow other corals. So, be thoughtful about where you place this species.
a picture of Palythoa 'Mind Tricks'

Genus Protopalythoa: Button Polyps

Members of the genus Protopalythoa are also called button polyps, but they look very different. Similar to the difference with Zoanthus and Parazoanthus, Palythoa also forms a thick overgrowing coenenchyme base, but Protopalythoa does not. The polyps can sometimes be connected by a narrow base (where sand or dirt may be present) or by a thin strip of flesh. Sometimes, they are individual polyps.

Protopalythoa also has longer tentacles than Palythoa, and the tentacles can often be observed when they describe an up-and-down movement. They are quite thin and taper to a point. Protopalythoa is larger (partly because there is no coenenchyme) and is usually between 1 and 2.5 centimeters in diameter. However, some can be larger, up to 5 centimeters. The most common colors are brown and red, often with spots, dots, or stripes, or green, cream, or white, making them attractive.

They require less flow than Palythoa, moderate to low. Everything else is the same. Lots of light and fleshy food, although plankton and a deep sandbed also help.
a picture of Protopalythoa

Finally: Be Careful!

Zoanthids are all strong and easy to care for, but there is one more thing that needs to be mentioned. Many species are highly toxic.

There are different types of common zoanthids, including palythoa, that can produce a deadly toxin (appropriately called palytoxin). It occurs in the mucus layer that covers them. If you get enough of it in an open wound, your eye, or your mouth, it can even kill you. Many aquarium keepers have reported cases of numbness, illness, and hallucinations, but the substance is potent enough to be fatal.

Handling them with bare hands is strongly discouraged. But when you touch a colony, you always get some slime on your fingers. It is crucial that you do not rub your eyes, put your fingers in your mouth, or even pick your nose until you have thoroughly washed your hands. Treat these polyps with respect and use gloves. Many have worked with zoanthids without gloves for many years and have never experienced any issues, but it can go wrong, and when it does, it goes very wrong.

Wear gloves!