Part of our constant coraling and coral reef maintenance routine, is to remove Drupella snails from the reef. Drupella snails are causing a lot of damage on shallow coral reefs all over Indonesia. Regrettably, there is not a single place we travel to in Indonesia where we don’t find them. Indeed, on most of the dives we come back to shore with few hundred Drupella snails that we’ve pulled off different coral colonies.
A nocturnal Mollusk:
During the day, Drupella snails can be a little bit difficult to spot as they hide at the base of the coral, among algae and other sessile organisms living around the coral. But at night, they climb on top of corals to devour the coral tissue. Drupella snails are colonial mollusk and live in groups. So all you have to do is find an aggregation, and you’ll be able to collect many snails in a quick time.
Luckily, the way they eat corals is very characteristic. But it can be confused with white band disease. Drupella snails don’t eat coral as fast as Crown-of-Thorn starfish, Acanthaster. Drupella snails eat slower and will leave a thin white band of newly eaten coral tissue; below that, a large patch of algae-covered dead coral skeleton and live tissue on the other side. Crown-Of-Thorn starfish will leave a wide band like a highway of newly eaten, bright white coral skeleton.
Cause of outbreak:
Overfishing of their predator is the main cause of Drupella snail outbreaks. Plus, probably like Crown-Of-Thorn starfish, fertilizer runoff causing increased phytoplankton availability provided extra nutrients that increase the survival of the Drupella snail larvae.
But also, it seems a correlation between El Nino event and Drupella snail outbreak exists. One possible explanation could be that increased surface temperature with lower currents probably increases larval survival.
Another interesting publication states that Drupella snails are attracted to stressed corals, with important mucus production. Unfortunately, there are not many places left with unstressed corals. Degraded coral patches are usually a Drupella magnet. One more reason why coral reef restoration sites should be monitored for Drupella outbreaks. From our experience, Drupella snail outbreak is one of the main causes of reef restoration failure of long-term projects in Indonesia. That’s why they should be carefully monitored.
Their natural Predator are gone:
Drupella snail predators are triggerfish, porcupinefish, wrasses, snappers, and emperor breams. For example, wrasses have a pharyngeal bone inside their throat that is designed to crush shells. Napoleon wrasses (Cheilinus undulatus), are one of their main predators. The wrasses can suck the Drupella snail right out of the coral, crush its hard shell, then throw away the shell bits through its gills, and ingest the meat. Unfortunately, Napoleon wrasses are gone from most Indonesian reefs. These curious fish are an easy target for spear fishermen.
Favorite coral they prey upon:
Acropora corals are the favorite food of Drupella snails, with table Acropora eaten first. Acropora millepora and Acropora hyacinthus are a Drupella magnet. Fimbriaphyllia ancora are probably their next best prey.
Technique to remove them:
Unfortunately, manual removal is the only technique found so far. A fastidious but necessary job in any reef restoration project. You don’t want your newly planted corals to be quickly eaten before they have a chance to grow. That’s why Drupella snail control is an important component of reef restoration and maintenance work. Unfortunately, it’s often underestimated and overlooked causing a quick decline in coral survival and overall success rate of a project.
Removing them is best done at night, as the mollusk tends to go out on coral tips and are easier to access.
Few tools are needed, such as long tweezers for large Drupella snails, long thin metal sticks for branching corals, and small tweezers for small plating Acropora coral. A coral cutter can be necessary to cut off any dead coral skeletons and access them easily.
It’s best to immerse the collected Drupella snail in freshwater for a couple of hours to ensure their death before discarding them on land.
It’s also good to keep records of collected Drupella snails, just to monitor effectiveness and possible outbreaks. During our Ocean Gardener courses, we teach how to identify and safely remove drupella snails from corals.