The Drupella Dredger

by nhrcp in 

The Drupella Dredger
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Finding a better way to manage outbreaks of coral eating snails that are threatening reef health, using modified gold dredging equipment.

by nhrcp in

Project OverviewColelcting Drupella Snails

Drupella are small coral eating snails which have reached outbreak proportions in many locations around the globe since the 1980’s. These little snails are now the most abundant gastropod on the reefs of Koh Tao, and are eating their way through much of the island’s coral populations.

We have been collecting Drupella snails by hand since 2010, and so far have removed over 28,800 of them. This has taken a huge effort, as the small snails are difficult to see and grab, with well over 4,000 diver minutes invested by our team alone.

Removing over 28,000 of the snails feels like a great start, but it has done little to dent their populations. In order to address this issue and save our reefs from decline we are trying to ramp up our population control efforts by borrowing some technology from gold dredgers. Our Drupella Dredge would allow divers to remove Drupella snails much more quickly and efficiently with little to no negative impact to the living corals.

Why remove Drupella Snails?

Drupella snails are small coral eating gastropods, measuring no more than about 5 cm in shell length. A single snail can eat about 1.8 cm2 of coral per day, and live for 45-50 years1. Although they have occurred in the Indo-Pacific Ocean for millions of years, they have always been in small abundances. The first outbreak of Drupella was recorded on Japan’s reefs in 19822, and since then outbreaks have occurred in the Red Sea, Hong Kong, Kenya, Thailand, and Australia1.

After reaching sexual maturity, a single female Drupella snail will produce about 160,000 embryos per year1. In normal conditions, very few of these embryos or larvae would survive, with most starving before being able to find a good reef to settle on, and the rest being eaten by fish or filter feeding animals. Development, deforestation, and agriculture are all increasing the amount of nutrients in the sea, meaning that more of the larvae survive. On top of that, over-fishing and coral reef degradation means there are fewer predators to feed on the larvae and juveniles. Once the Drupella reach adult stage, very few animals will feed on them, meaning that populations can quickly get out of control2,3,4.

An outbreak of Drupella is commonly defined as more than about 6 individuals per square meter of reef1, and now, on some of the reefs of Koh Tao, average abundances of over 13.6 individuals per square meter can be found.


How will this project help?

Drupella DredgerThe first time that an outbreak of Drupella snails was observed on Koh Tao was follwoing a major belaching event in 20105. For the last 5 years, our marine conservation team has been removing Drupella snails by hand, using SCUBA diving equipment. So far we have put in well over 100 diver hours into the work, and collected more than 28,870 Drupella snails.

Collecting the snails by hand is difficult and time consuming. The snails are cryptic, well camouflaged, and generally recede down into the corals during the day time. Often a diver may observe more than 20-30 snails on a coral, but may only be able to collect 5-10 due to inaccessibility or risk of damaging the coral.

The Drupella dredge will allow divers to precisely suck-up the snails from the interior of the coral using a 2 inch pipe. The pipe will be able to reach further into the corals than a diver’s hand, and can be used with little to no contact to the reef. Rather than having to pick-up each snail, they will be sucked up to the raft above, where they will be held in a large basket. After collection is complete, the basket contents will be sorted by volunteers and any hermit crabs or non-Drupella snails inadvertently sucked up will be released back to the sea.

We believe that this device could remove thousands of Drupella snails per dive, rather than hundreds. This will increase the effectiveness of our population management efforts, and in the end reduce the costs we ensure by decreasing the number of divers and dive tanks needed.

How will the Drupella Dredger be built and used?

The unit will be constructed using primarily materials sourced locally, to facilitate other reef managers to utilize the design. The unit will consist of pontoons constructed out of HDPE drums, with a small platform installed on top to hold the pump and engine. A power jet designed for gold dredging will facilitate the suction needed to remove the Drupella from the reef and pipe them the basket at the front of the raft.

A team of divers from our Reef Conservation Team will swim along the reef looking for Drupella aggregations and marking them with floating buoys. Two snorkelers tending the raft will swim it to the buoy and allow the diver controlling the dredge nozzle to remove the snails before moving to the next buoy. After the site has been cleared of the snails, the unit can be towed by boat back to shore or to the next removal site.

How else can it help?

Another problem related to nutrient enrichment of our planets coastal areas is the degradation of coral reefs as they are overgrown by Macro-algae. Macro-Algae takes advantage of nutrients in the water, which the corals cannot use, and can grow rapidly to kill living corals and prevent new coral larvae from settling. In areas where macro-algal coverage is high, the dredger can be used to remove algae and open the reef back up for coral growth.

Experimental projects in other areas have shown that the removal of macro-algae greatly encourages the redevelopment of coral in degraded areas, but also notes it is very time consuming. With this dredger out volunteers can remove the macro algae, and be able to sort through the collected matter before disposal to ensure that no crustaceans or other small animals are killed in the process.

Although using the dredger to remove Drupella or macro-algae does not address the root causes of the problem, declining water quality and over-fishing, it does allow us to address some of the symptoms and maintain our reef abundance and diversity until the larger problems can be adequately addressed on a global scale.




Conservation Collage

Our Story

At New Heaven Reef Conservation Program we believe that all divers and dive business should be active in the protection and restoration of the ecological resources they utilize. We believe that through creative business practices, conservation can become an important part of the diving industry while at the same time contributing to professional research and restoration programs. By training divers in reef ecology, research, and restoration it is possible to not only reduce the threats to reef health, but to provide widespread positive impacts to local and global coral reefs.

Since 2007 we have been giving students in our program the theory, training, and skills to help manage the reefs of Koh Tao, and wherever they may dive next. With over 600 students through our program already, we hope that we are helping to change the tide of reef decline and encourage responsible stewardship of our planet's marine resources.

What we do

Participants of our program are actively involved in a wide variety of research, restoration, and protection projects and receive locally and internationally recognized certifications. Short term students contribute back by providing a steady revenue stream for the program and also contributing to ongoing projects. The long-term students contribute in the same way here, but also have the potential to make much larger impacts elsewhere. The training they receive not only makes them better and more aware divers, but provides the access to free-market based conservation jobs. The skill sets they receive will help to increase awareness on reef problems and create more individuals trained in promoting the resilience of coral reefs. By increasing reef protection and reducing threats through this manner, global coral reef resources (valued at over 375 billion USD per year globally) will be put under less stress while still promoting their use through responsible tourism and recreation.

Our Team

Chad Scott, Program Director and Marine Conservation instructor since 2007

Rahul Mehrotra, Marine Conservation Instructor since 2013

Pau Urgell, Marine Conservation Instructor since 2013

Sriploy Macintosh, Marine Conservation Instructor since 2014

Spencer Arnold, Marine Conservation instructor since 2014


[1] Cumming, R.L. (2009). Populations Outbreaks and Large Aggregations of Drupella on the Great Barrier Reef. Research Publication (Great Barrier Reef Marine Park Authority. Online) 96. Science 172: 1155–1157.

[2] Moyer, J.T., Emerson, W.K., and Ross, M. (1982). Massive destruction of scleractinian corals by the muricid gastropod Drupella in Japan and the Philippines. The Nautilus 96(2):69-82.

[3] McClanahan, T.R. (1994). Coral-eating Snail Drupella Cornus Population Increases in Kenyan Coral Reef Lagoons. Marine Ecology Progress Series 115:131-7.

[4] McClanahan, T.R. (1997). Dynamics of Drupella Cornus Populations on Kenyan Coral Reefs. Proc. 8th Int Coral Reef Symposium 1:633-638.

[5] Hoeksema, B.W., Scott, C.M., True, J.D. (2013). Dietary Shift in Coralivorous Drupella snails following a major bleaching event at Koh Tao, Gulf of Thailand. Coral Reefs 32(2):423-428.


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