Zoanthid

The zoanthid, or "zoa" for short in the hobby, is a colonial animal related to anemones and made up of small, interconnected polyps. It does not build a skeleton, so it is not a true stony coral, but in the reef aquarium it is mentioned together with corals. Each polyp has an oral disc about 0.5–1.5 cm across and a "skirt" of short tentacles around it. The polyps are connected at the base by a thin tissue, and the colony spreads by budding new polyps.

Most colorful zoanthids in the hobby are Zoanthus species from the Indo-Pacific. Because the species are very similar and color is not enough to tell them apart, colonies are sold not under species names but under trade names such as "Rasta" and "Eagle Eye." These names are not based on any official registry; the same form may be found under different names from different sellers. Aquacultured colonies are common.

Palytoxin warning: Palytoxin is one of the most potent natural toxins known. The highest levels are found in Palythoa species; in Zoanthus they are usually much lower, but they can occur in misidentified colonies. Treat every zoanthid as toxic: wear gloves and protective eyewear when handling and cutting, do not work with cuts on your hands, and do not squeeze the colonies. Never put zoanthid-covered rock in boiling water, and never clean it with steam or scrub it out of the water; steam and splashing droplets have caused serious poisonings. If you develop fever, shortness of breath, muscle pain or burning in the eyes, see a doctor immediately (in Türkiye, the National Poison Information Center: 114).

• Rasta: One of the best-known classic forms; recognized by its small, multicolored polyps.
• Eagle Eye: A common form whose oral disc shifts from red to orange and yellow depending on the light.
• Fruit Loops: One of the popular starter forms, with small, multicolored polyps.
• Radioactive Dragon Eye: A form recognized by its fluorescent green polyps and solid, darker green skirt.
• Utter Chaos: A valuable form with large heads and distinctive white spots on the surface, mixing purple and orange tones.

In the Indo-Pacific and the Caribbean it forms wide carpets on shallow reef flats, on rocks occasionally exposed at low tide and on fringing reefs with currents. It is used to strong sunlight and wavy water carrying sand.

• Light: Moderate light (about 100–250 PAR) suits most forms. In strong light the polyps become smaller and denser and the colors brighten; in weak light the stalks stretch.
• Flow: Moderate, random flow; detritus should not build up on the polyps.
• Placement: The middle or lower-middle zone of the rockwork; since it spreads, leave space between different colonies.
• Water parameters: Salinity 1.024–1.026; KH 7–11 dKH, calcium 400–450 mg/L, magnesium 1250–1350 mg/L. Keeping nitrate at a measurable level (2–10 mg/L) preserves color; in very stripped water the polyps shrink.

It gets most of its nutrition from the zooxanthellae in its tissue. Its polyps also catch small particles and fine zooplankton from the water; a fine powdered food once a week can speed up growth but is not required.

It has no stinging tentacles or strong chemical warfare; however, as it spreads it can creep in at the base of slow-growing corals and shade them. Keep different zoanthid forms on separate rocks or with 3–5 cm of space between them; colonies that touch each other gradually mix, and one may suppress the other. It is itself weak against stinging LPS corals and anemones.

Some dwarf angelfish, butterflyfish, zoanthid-eating nudibranchs and sea spiders eat the polyps; check new colonies in quarantine.

Polyps are cut from their base with a scalpel or scissors, singly or in small groups, and glued to frag plugs. Do the cutting with gloves and eye protection, under water or in a separate container; rinse the tools with plenty of water afterward. The colony also spreads on its own by putting out new polyps.

• Polyps not opening: Pests (nudibranchs, sea spiders), a new placement and sudden changes in light are the most common causes; check between the polyps with a magnifying glass.
• Stalks stretching: The light is insufficient; move the colony higher.
• Color loss: Very strong light or overly stripped water can be the cause.