Zoa-eating nudibranchs

Identification

Tiny soft slugs about 5–15 mm long with rows of finger-like cerata along the back. Color matches the zoanthid colony they are feeding on — browns, greens and mottled whites are typical — so they are easiest to spot at night with a flashlight, in a dip bucket, or against closed polyps. Look for zoanthids that stay closed for days, melt from the edges, or show C-shaped white egg ribbons on polyp stalks and at the colony base.

Often confused with

Berghia nudibranchs — Berghia are pale white, eat only aiptasia and never sit on zoanthids chewing polyps. Also confused with sundial snails (hard checkered spiral shell, feeds openly on zoas) and with harmless amphipods sheltering in a colony, which is why experienced keepers confirm the ID by dipping and inspecting what falls off.

What it targets

Zoanthids and palythoa (Zoanthus and Palythoa spp.). Adults feed on polyp tissue and can work through a prized colony polyp by polyp.

How it spreads

Hitchhikes on new zoanthid frags and colonies, usually as eggs that survive shipping and standard dips. Eggs hatch in the display and juveniles spread colony to colony.

Control (safest first)

Manual removal plus egg control: siphon or pick off adults at night and wipe or blot egg ribbons off stalks with a damp paper towel or soft brush. Dips do not kill the eggs, so inspection has to repeat for weeks. PALYTOXIN CAUTION for all zoa handling: wear gloves and eye protection, work in a separate container, and never boil, steam or pressure-wash zoanthid rock — heating zoanthid tissue can aerosolize palytoxin. Coral dips (CoralRx, Revive, Bayer-type dips) on a repeat schedule: dips knock adults off within seconds to minutes per keeper reports, but eggs survive, so repeat every 4–7 days for 3–4 weeks to catch hatchlings. Quarantine prized zoas during the cycle. Freshwater / RO dip matched for temperature and pH: an older forum-standard method that makes nudibranchs drop off; still egg-proof rather than egg-killing, so it needs the same repeat schedule. Predator fish: melanurus and other Halichoeres wrasses, six-line wrasse and yellow coris wrasse pick at nudibranchs and eggs between dips. Reports call them a helpful backstop rather than a complete cure — a wrasse tank can still harbor nudibranchs under a colony base. Flatworm Exit at multiples of label dose is reported by some long-time zoa keepers to kill adults fast, while other guides state flatworm products do not touch these nudibranchs. Treat this as disputed and risky in a display: test in a dip bucket first and never rely on it as the only method.

Natural predators

• Melanurus wrasse→
• Six-line wrasse→
• Yellow coris wrasse

Field notes

A Reef2Reef keeper who found a single adult on a healthy-looking colony noted only one Nirvana polyp had closed, then lost the slug when a powerhead blew it off and ended up pulling half the rockwork out to recover it — a good picture of how one visible animal turns into an evening of hunting. Another keeper on Manhattan Reefs blamed a breakout for eating through more than 100 zoa heads and laying eggs across the colony, crediting six-line and coris wrasses with help but not a full clear-out. Veteran zoa keepers on Reef2Reef disagree about Flatworm Exit: some dose at several times label strength and report adults melting in seconds, while others stick to Revive dips repeated every 4–5 days because eggs survive every dip either way.