Hermit crabs are common at all of Koh Lanta's dive sites, but you need to know wheat you're looking for in order to spot them during a dive.
The family Diogenidae comprises the 'left-handed hermit crabs' as the left claw is enlarged, rather than the right claw which is the case for the majority of hermit crabs.
There are 429 species of left-handed hermit crabs, however these are not actually 'true crabs' as they do not have an external shell on their soft abdomen.
The lack of shell on the abdomen leaves these crabs easily open to predators, and so to protect themselves, they live in abandoned snail shells.
Each crab has a pair of claws and 2 pair of walking legs. The last 2 pairs of legs are small and modified and along with appendages at the end of the abdomen, are used to clamp onto the internal whorls of the shell.
They can retract their entire body can be into the shell to protect themselves when needed. Eventually they outgrow their shell, and will search for a new one.