Most zoanthid pests come in on new frags, so quarantine and careful inspection prevent most problems.
Zoanthids have a short list of serious pests, and almost all of them arrive on new corals. Learning what they look like, and checking every frag before it goes in your display, saves a lot of heartache. Removing pests, polyps, or plugs means handling zoanthids, so wear gloves and eye protection and follow the Palytoxin Safety guide.
Zoa-eating nudibranchs
Small nudibranchs that feed on zoanthids are some of the hardest pests to spot. They are often colored to match the polyps they eat and hide at the base of the colony.
- Signs: polyps disappearing a few at a time, colonies that won't open, tiny egg spirals or ribbons on the mat or plug.
- Action: remove adults by hand or with a baster, dip affected frags, and repeat dips to catch hatchlings. Eggs often survive a single dip.
Sea spiders
Sea spiders (pycnogonids) are thin-legged arthropods that pierce polyps and feed on tissue.
- Signs: polyps closing or melting, small spider-like animals clinging to the colony, often visible at night.
- Action: remove visible spiders with tweezers, dip the colony, and keep checking for several weeks, since dips don't reliably dislodge them.
Sundial snails
Sundial snails (Heliacus spp.) have flat, coiled, patterned shells and eat zoanthid polyps, usually at night.
- Signs: missing polyps with a small snail tucked into the colony or on the underside of the rock.
- Action: remove by hand. Check the colony with a flashlight after lights out. They are easy to miss during the day.
Zoa pox
Zoa pox shows up as small white bumps or pustules on the stalks of the polyps. Its cause hasn't been confirmed. It is often blamed on a bacterial infection, and it may be more than one condition.
- Signs: white spots on the column, polyps that stay closed, colonies that slowly shrink.
- Action: isolate affected frags, improve flow and water quality, and dip. Some keepers report success with carefully dosed hydrogen peroxide dips, but results vary. Remove badly affected polyps.
Fish and invertebrates
Not every problem is a classic pest. Some angelfish, butterflyfish, and other fish nip at zoanthid polyps. Some crabs and shrimp pick at colonies too. If polyps close whenever a certain animal is nearby, watch the tank for a few days before blaming water quality.
Algae and cyanobacteria
Nuisance algae and red slime can smother zoa colonies. These are a nutrient and flow issue rather than a true pest. Improve flow over the colony, cut back feeding, and clean the mat gently with a baster.
Prevention routine
- Inspect every new frag closely under bright white light, ideally with a magnifier.
- Check the underside of the plug for snails, eggs, and hitchhikers.
- Dip every new coral before it enters the display. See the Dipping guide.
- Quarantine new zoas in a separate tank for several weeks if you can.
- Remove the frag plug or rubble the coral arrived on, when practical, since eggs often sit on it.
When an outbreak happens
Act quickly. Remove the worst-affected colonies, dip everything that may be affected, and repeat over several weeks. Most pests have eggs or juveniles that survive a single treatment. Keep notes on dates so you don't skip the follow-up dips.