Zoanthids belong in the reef aquarium and must be known to just about every marine aquarist. They are commonly known as button polyps.
These polyps could appear in ones or twos but more likely are displayed as what could be termed polyp boulders. These are of varying sizes but the ones often sold in local fish shops could be in the region of 4 to 8 inches in diameter. The whole surface apart from the underneath is usually densely packed with polyps.
‘Zoanthid’ covers a wide variety of proper names, the most common one found in local fish shops being, arguably, zoanthus. This name in itself covers numbers of individual proper names.
The polyps come in a variety of colours such as light and dark brown, light medium and dark green, bright green, with lesser numbers pale blue, bright blue, and red. It is possible for a group of polyps to be of different colours, this is usually because different types are intermixed. Some individual polyps themselves could be one colour, or have the disc one colour and the tentacles another. Some types have a stripe from the centre of the polyp to the outer edge.
Button polyps are known as a good starter for beginners. In marine terms generally they are hardy and should survive some mistakes a beginner could make. Use of the word ‘hardy’ does not mean that high quality seawater is not required, it is.
It has already been mentioned that their home should be a reef aquarium; this is because they require the correct lighting. Button polyps have a lot of zooxanthellae (single celled algae within the flesh) upon which they are highly dependent. It is possible that when a colony has been in an aquarium for a while the polyp colour changes to a degree; this is because of zooxanthellae adjustment to the available light. In an aquarium with powerful lighting the button polyp colony is often perfectly happy very low down, which is very helpful to the aquarist. After they have been placed, wherever that is in the aquarium, as with other corals they need to be observed as time passes to ensure they are thriving.
In addition to correct lighting, the colony could require medium or fast seawater flow. They are usually tolerant of medium or even slow flow whatever the type, though an indicator of likely flow requirement can perhaps be determined by the length of the polyp stalk and also the length of the tentacles.
Button polyps do not normally require special feeding as, as said, they have dense populations of zooxanthellae. In fact some types ignore the usual floating feeds intended for corals or fish anyway. Others will capture for example brine shrimp which have been put into the aquarium for the benefit of fish and also smaller foods, if used, intended for corals.
The link shows some general pictures of button polyps. Click on them to enlarge. The photograph shows a colony of polyps in my reef, these were grown from four polyps which were transplanted into suitable holes (for some reason they came loose from the mother colony, is this a way of spreading perhaps?).
A general caution should be given here. The Zoanthus type above does not, as far as I am aware, fall into this caution category but as the type is so similar to the types that do, Palythoa and Protopalythoa, the caution is relevant.
Palythoa and Protopalythoa types contain a neuromuscular toxin which is called palytoxin. The toxin is contained in the mucus. The toxin is potent and could be fatal to humans. Tribesmen in the relevant areas used to put the toxin on their spearheads in order to paralyze animals and enemies.*
Though it is very unusual to hear of any problems encountered by aquarists, it is clearly wise to be aware of the potential of the toxin and, particularly if there are cuts and/or abrasions on the hands, to take reasonable precautions such as wearing rubber protective gloves.
(* Reference: Eric H. Borneman. Aquarium Corals)