Foods and Feeding
One of the most satisfying aspects of caring for zoanthids is watching them feed. Because they have such large polyps it is easy to observe them capturing food and ingesting it. Using a turkey baster or similar device one can directly place food onto individual polyps, and they will slowly close over it. This is particularly true for species of Palythoa and with yellow polyps, but also for some species of Zoanthus.
There are Zoanthus spp. that don’t seem to take foods that one would expect them to eat, and these apparently depend more on their zooxanthellae for nutrition than on feeding. All zoanthids are able to feed on fine particulate and dissolved matter, which they can trap in their surface coating of mucus.
Zoanthids occur in a wide range of tropical and subtropical habitats, but they are typically found in reef flats, sandy hard bottoms, rocky shorelines and tide pools. Many species occur in the intertidal zone, where they are completely exposed to air at low tide.
Some species are also common on hard substrata, such as coralline algae or clam shells scattered among various types of seagrass. Although these shallow habitats are good places to find them, zoanthids can also be found on any hard substrate on a coral reef to considerable depth. Some species, the giant-polyped Palythoa grandis for example, are most common in deep water, below 80 feet.
Predators and Disease
Some fishes will eat zoanthids, though most find them distasteful. The box snails (Heliacus spp.) feed specifically on zoanthids, and can be considered a pest if one is trying to grow zoanthids, or a savior if one is trying to rid aquaria of zoanthids.
Heliacus spp. are not always easily seen, as the shell is round and about the size of a closed zoanthid polyp. As with most cnidarians, there are seaslug predators of zoanthids. These present a real threat to captive populations because they can reproduce quickly and rapidly decimate colonies. Careful quarantine is necessary to be sure to avoid introducing such pests.
The common “brown jelly” protozoan infections that affect corals can also kill zoanthids and can spread from zoanthids to corals. If a section of zoanthids remains closed, deflates, and then develops a clear brown gelatinous mass over it, carefully siphon off the mass and cut out the affected polyps. Give the colony a short 1-minute fresh water bath, if possible, and then replace it in the aquarium with a strong stream of water over the colony to prevent reinfection.
Newly imported zoanthids often come associated with encrusting sponges. When the sponges are damaged by shipping and handling, they die and rot, promoting anaerobic conditions within the rock and among the zoanthid polyps. This has a telltale rotten-egg smell and often is associated with white films produced by bacteria of the genus Beggiatoa.
These white films may suffocate zoanthids along with the fouling sponge. It is important to inspect newly imported zoanthids and remove any loose dead or dying sponges. Also, because sponges within the rocks commonly die, the newly imported zoanthid colonies should be placed where they receive extremely strong water flow, to ensure that the rocks are well oxygenated to prevent anaerobic conditions.