Drupella Snails & Cleaning Mission

Marine Conservation - regular monitoring dives to assess numbers of Drupella Snails

Eco Koh Tao & Crystal Dive – Snail Mission!

Drupella snails feeding on an acropora
branching coral in Chalok Ban Kao.

Eco Koh Tao and Crystal Dive have teamed up to go on a mission. The marine conservation team of Eco Koh Tao are actively working on a number of projects including the building and maintenance of our own artificial reef, Junkyard Reef.

Other projects include coral restoration, reef check survey’s, debris clean ups, giant clam nurseries, but also predation monitoring. The main predators we are looking at are Crown of Thorns starfish and Drupella Snails.

Coral Reefs have natural predators that feed on it. However, with increasing numbers of these predators the coral reef becomes overrun and can quickly degrade.

For this reason, we conduct regular monitoring dives to assess numbers of Crown of Thorns and Drupella numbers. 

From time to time we get an outbreak of Drupella Snails in the coral reefs here around Koh Tao and while they are interesting to look at they are responsible for extensive coral reef damage, once an outbreak occurs we need to remove them from the reef for the balance to be restored.

Drupella snails feed off of living coral tissue, particular favourites being the stag horn and plate coral colonies here on Koh Tao. They are frequently found hiding in cryptic crevices during the daytime.

Drupella Snails Feeding off Live Coral Tissue

Drupella eat live coral tissue by stripping the tissue from the coral skeleton and leaving white feeding scars that can quickly become covered by algae.

Adults have a robust looking shell, 2-3cm long, are covered in small spikes and are deep purple in colour. Juveniles are usually 0.5-1cm long and are white in colour.

These outbreaks, or overpopulations, have led to dramatic loss in living coral tissue, reduced reef resilience and recovery, population regime shifts, and possibly increased disease occurrence.

Causes of outbreaks of Drupella include human impacts such as terrestrial run-off, overfishing of Drupella predators, and increased reef damage along with natural causes.

The periodic outbreaks suggest some relationship with oceanographic oscillations (e.g., El Niño Southern Oscillation).

The outbreaks occur due to more of the larvae and juveniles surviving, just like the case of Crown of Thorns Starfish, one female can produce over 100,000 eggs, which will turn to free-swimming larvae.

Drupella Snails

Drupella Snails

Most of these larvae should not survive, they should be eaten by fish and plankton feeders (including the corals themselves!) or starve to death in the clean waters of the ocean.

But, when the fish are removed due to over fishing, or the water quality decreases due to nutrient inputs, then more larvae survive to adulthood.

Once the become adults they have very few natural predators, as they are a very unattractive food source.

Regular Drupella Snail Clean Up Dives around Koh Tao

This is why we organize regular drupella snail monitoring dives and we try to get all our Divemaster candidates involved in these eco projects.

As mentioned before, if we do notice overpopulation of drupella snails on the reef then we need to remove them to restore the balance.

It is important to make sure removal is of the snails only and not the common hermit crab which also takes up residence in an empty Drupella snail’s shell, some easy clues for checking are:

  • Gently wave your hand creating small water disturbance, if the shell falls off, then it’s a hermit crab inside
  • If the shell is stuck to the coral and suction becomes stronger when removing, then it is definitely a snail
  • Also the hermit crab will re-emerge from its shell relatively quickly once removed and in your hand

For collecting the snails, we use long tweezers and then place them in a bottle during the dive. Upon return to the dive shop we place them in fresh water for 24 hours. On average we manage to collect between 700 – 1500 snails on a single dive.

The numbers and amount of predators are recorded for future study.

There are a lot of projects researching this in Thailand especially. Apart from the Gulf of Thailand they have been found in large aggregations or outbreak populations in areas such as Kenya, Australia, Hong Kong and the Red Sea.

Special considerations and things to watch out for is further damaging the coral when trying to remove the snails.

This is why we use the special tools and make sure our participants have good buoyancy control.

We only remove the most easily accessible Drupella snails to avoid breaking or damaging any coral during the process.

Crystal Dive Koh Tao

7/1 Moo 2
Tambon Ko Tao
Koh Tao
Surat Thani
84360
Thailand

Phone+66(0)77456106
E-mail: [email protected]

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