Montipora-eating nudibranchs are the most effective pest in the hobby at hiding in plain sight. They are the same colour as the coral they are eating, they feed on the underside and the shaded edges, and by the time you notice bare white skeleton advancing across a plate, you have a breeding population.
The signs, in order of appearance
- A receding edge of clean white skeleton on a montipora, usually starting at the base or the underside of a plate, advancing steadily. Not bleached tissue — genuinely stripped bone.
- Polyps that will not extend in the area next to the damage.
- Tiny white or cream egg spirals laid on bare skeleton near the damaged margin. This is your confirmation. Once you know what the egg masses look like, you will find them before you find an adult.
- The adults themselves — a few millimetres, colour-matched to the coral, moving very slowly. Flip the frag over. That is where they live.
If you keep montipora and you have never once turned a piece over to look at the underside, do that this week. It takes ten seconds and it is the entire early-warning system.
Treating them
The routine is the same shape as flatworms, and the same rule dominates it: dips kill adults, dips do not reliably kill eggs.
- Remove and dip the affected coral in a proper coral dip, agitating so adults are dislodged rather than just annoyed.
- Blast the skeleton with a baster while it is in the dip — underside, base, every crevice, and specifically the bare margin where the eggs are. Physically removing egg masses is the step that determines how long this takes.
- Scrape or cut away bare skeleton carrying visible egg spirals. Dead skeleton is not doing anything for you and it is a nursery.
- Rinse twice in clean, temperature-matched tank water, and dump every drop of dip and rinse water down the drain.
- Repeat every 5–7 days, three times minimum. You are outrunning the hatch, not landing one knockout.
Treat every montipora in the tank, not just the visibly damaged one. Nudibranchs move, and a single missed colony restarts the whole thing. This is the mistake that turns a three-week job into a six-month one.
Where a colony is badly infested and glued down, the honest answer is often to frag the clean growing edge, dip it hard, mount it on a fresh plug, and discard the rest. Saving every square inch of an infested plate has cost a lot of people their entire montipora collection.
Prevention costs nothing
These arrive on frags, without exception. They do not appear from nowhere. Dip and inspect every montipora that enters your system — underside included — and if a plug has egg spirals on it, mount the coral on a new plug and throw the old one away. Quarantine new arrivals somewhere you will look at them daily for a few weeks.
Every reefer with a clean montipora collection is doing exactly this, every time, without exceptions for trusted sources.
Know the difference
Not every small thing on your coral is eating it, and several harmless hitchhikers look alarming under a loupe. Our Coral Pest ID & Treatment Guide shows montipora-eating nudibranchs and their egg masses alongside the look-alikes, with the treatment for each, across 29 pages of photographs. Instant PDF download.
For dip times, dosages and the repeat schedule, see The Complete Coral Dipping Guide.