Montipora-Eating Nudibranchs

What Are They?

Montipora-eating nudibranchs are tiny coral-eating sea slugs that feed directly on living Montipora tissue.

They’re strongly associated with Montipora and are one of the better-known SPS pests in reef aquariums.

They’re small, pale, well camouflaged and very easy to miss, especially because they tend to sit around:

• The underside of plating Montipora.
• Shaded areas.
• Branch junctions.
• Dead skeletal margins.
• The edge of active tissue loss.

Discovery and Identification

The first sign is usually fresh white skeleton or unexplained tissue loss.

If you suspect Montipora-eating nudibranchs, inspect the coral underwater.

Hold it upside down and look closely around the growth margins, underside, base and any freshly exposed skeleton.

Underwater, the nudibranchs keep that pale, frilly appearance, which makes them much easier to recognise.

Out of water, they flatten down and can look like nothing more than a bit of slime stuck to the coral.

And if you find one, don’t assume that’s the only one.

Treat it as confirmation that the problem is established and immediately start looking for more nudibranchs and egg masses.

Understanding the Breeding Cycle

At around 24°C, larvae can become capable of settling at roughly day 6; once they encounter living Montipora cues, they settle and metamorphose.

Treatment must therefore be treated as a cycle: repeatedly remove mobile stages and physically remove egg masses until no animals remain to reproduce.

With Monti nudibranchs, the real battle is breaking that breeding cycle, not just knocking off the adults you see. The eggs develop fast. At around 24 degrees, larvae can be ready to settle within about a week once they’ve got the right host, which means if you stop after one round, you’re basically just waiting for the next generation. I’d frame treatment as a cycle. Remove adults, remove eggs, inspect again, and keep going until you’re certain the cycle is broken, not just until you can’t see adults anymore.

Immediate Containment

Once you’ve spotted one, I’d go straight into containment mode.

First, take that Montipora out of the display if you can and inspect it underwater from every angle. Remove every nudibranch you can see, then start looking for egg masses around the growth margins, underside, base, plug and any dead skeleton.

Then check every other Montipora in the system. Don’t assume it’s just on the one colony.

After that, I’d move the affected Montis into quarantine if possible. You can use a coral dip to help knock off mobile nudibranchs, but don’t trust the dip to deal with the eggs. The eggs need to be physically found and removed.

And then comes the important bit: repeat the inspection and treatment cycle. You’re trying to catch each new generation before it gets established and starts laying more eggs.

Treatment

For an active Montipora-eating nudibranch problem, I’d keep the treatment simple and consistent. Use a proper coral dip such as Coral Rx, or an iodine-based dip, to target the mobile nudibranchs. While the coral is in the dip, use a turkey baster or gentle jet of water to really work around the underside, base, branch junctions and any folds or crevices where they can hide. The important bit is the eggs. Don’t assume the dip has dealt with them. Inspect the coral closely and physically remove any egg masses you find.

During an active outbreak, repeat the process every 3-4 days. The aim is to catch newly hatched juveniles before they have time to mature and start laying more eggs. Keep going until you’ve had several completely clean inspections with no adults, no juveniles, no eggs and no fresh feeding damage.

I’d leave potassium permanganate in the guide as a historical, high-risk option rather than something we recommend as standard, because although it has been reported to affect eggs as well as adults, it can also be very hard on Montipora tissue.

Potassium Permanganate - Historical High-Risk Option

Potassium permanganate is one of the more aggressive treatments that has been used for Montipora-eating nudibranchs.

A well-known protocol associated with Eric Borneman used around 50 mg/L for approximately 90 minutes, with the aim of killing both the nudibranchs and their eggs.

It can work.

But it is extremely harsh on Montipora, and there is a very real risk of badly damaging or even killing the coral you are trying to save.

So although potassium permanganate is a recognised treatment option, it is not the approach we would recommend as the first line of treatment.

Our preference would be to use a normal coral dip to deal with the mobile nudibranchs, physically remove any eggs you can find, and then repeat that process often enough to interrupt the breeding cycle.

In other words, rather than trying to destroy everything in one extremely aggressive treatment, the aim is to:

• Remove the adults.
• Remove the eggs you can see.
• Catch each new generation as it emerges.
• Keep going until the lifecycle is broken.

Follow-Up

Don’t stop treatment just because you can’t see any nudibranchs after the first dip. The mobile adults and juveniles are usually the easy part - the eggs are what keep the infestation going. Commercial dips and iodine-based treatments generally don’t reliably affect the eggs, so repeated treatment and manual egg removal are essential.

During an active outbreak, inspect and treat the affected Montipora every 3-4 days. At each check, look closely around the underside, base, dead skeleton, shaded areas and tissue margins. Remove any egg masses you find and flush or dip the coral to remove mobile nudibranchs.

Keep that rhythm going until you’ve had several completely clean inspections in a row - no adults, no juveniles, no eggs and no fresh feeding damage.

Even once everything looks clean, keep inspecting for a while afterwards rather than immediately assuming the problem has gone. At Signature Frags, we’d rather over-check than stop too early. One missed egg mass can restart the whole cycle.

Quarantine

Anything new or even slightly suspicious should be kept away from established Montipora until you are satisfied it is clean.

Inspect the coral from every angle, not just the visible upper surface. Look closely for feeding damage, adult or juvenile nudibranchs and the characteristic egg masses. Any eggs found should be physically removed rather than relying on a dip to deal with them.

If there is any doubt, keep the coral isolated. Don’t move a questionable Montipora between systems just because it looks clean after one inspection or one dip.

Montipora-eating nudibranchs are one of those pests where being overly cautious at the quarantine stage is considerably easier than trying to remove them from a mature system later.

If every Montipora host is removed from an affected system, host removal may also help break the cycle because these nudibranchs are strongly host-associated. That does not replace repeated inspection of removed coral and wet equipment.

Coral Recovery

Once the feeding has stopped, give the coral a chance to recover.

Pay attention to:

• Alkalinity.
• Calcium.
• Magnesium.
• Salinity.
• Temperature.
• Flow.
• Lighting.

Don’t keep handling or dipping the coral unnecessarily once you’re confident the pest has gone.

Prevention

With Montipora-eating nudibranchs, prevention is far easier than trying to eradicate them once they’re established.

For us, that starts with sourcing. We don’t bring in fresh-import Montipora. The adults can be extremely difficult to spot and the eggs are even easier to miss, so for us the risk simply isn’t worth taking. Our Montipora are sourced with aquaculture in mind rather than repeatedly introducing fresh wild or imported colonies into established systems.

Any new Montipora should still be inspected carefully, paying particular attention to the underside, base, areas of exposed skeleton and any folds or crevices where adults or eggs could be hidden.

Signature Frags Perspective

When we get fresh Montipora in, we’re extremely careful. I use what I call “coring” the Montipora: I cut a perfect circle out of the centre/top of the colony, then cut all the flesh from underneath away, leaving a circular piece of living tissue with the edge and underneath bare skeleton. I discard the base and underside because Montipora-eating nudibranchs are usually found underneath, around the edges and growth margins, and work their way inwards from below. That way, the only thing going into the system is living flesh.