"Drupella Snails are responsible for extensive coral reef damage, once an outbreak occurs removal must take place for the balance to be restored. Drupella snails use a special mouth 'radula' and feed off of living coral tissue, particular favourites being stag horn and plate coral colonies here on Koh Tao. The snails leave white tissue scars on affected coral and can usually be found congregated at the base or deep down in-between the coral branches.
Adults have a robust looking shell, 2 to 3cm long, are covered in small cones/ spikes and are deep purple in colour. Juveniles are usually 0.5 to 1cm long and are white in colour.
* wave your hand gently creating small water disturbance, if the shell falls off, then its a hermit crab inside
* if the shel is stuck to the coral and suction becomes stronger while removing then its definitely a snail
* also the hermit crab will re-emerge from its shell relatively quickly once removed and in your hand
Special considerations and things to watch out for is further damaging the coral when trying to remove the snails, there's no point 'helping the coral' by removing snails if you break the coral in order to do so.... we only remove the most accessibly Drupella snails.
Once we've removed what we can from a dive we place the snails in fresh water for 24hrs before then replacing the shells back onto the coral reef so the shell can then bio-degrade, this then enriches the water with calcium carbonate which other organisms will intern use to build their own exoskeleton (anything from coral, octopus, all marine shell's, cuttlefish, clams..etc).
Taking a negative impact and replacing with a positive one....its a healthy balance :)"
written by Janice l. Zutt - PADI Open Water Sucba Instructor