Unlike Crabs these Decapod (ten legged) Crustaceans walk in a straight line, and facing forward. The one in the picture seemed to be in no hurry. The moment I toppled the Crab, it retracted itself completely into the shell and came out the very next moment and started walking as though it didn’t care about my presence. At the time of taking the photograph, in Maldives, I had no clue about this fascinating creature, except that it was a sort of Crab. Allow me to share some tid-bits about the ‘Hermit Crab’ in this post.
Most species of Hermit Crabs have long, spirally curved abdomens, which are soft, unlike the hard, calcified abdomens seen in related crustaceans. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract. Most frequently hermit crabs use the shells of sea snails. The tip of the hermit crab’s abdomen is adapted to clasp strongly onto the columella of the snail shell.
As the hermit crab grows in size, it has to find a larger shell and abandon the previous one. This habit of living in a second hand shell gives rise to the popular name “hermit crab”, by analogy to a hermit who lives alone. Several hermit crab species, both terrestrial and marine, use “vacancy chains” to find new shells: when a new, bigger shell becomes available, hermit crabs gather around it and form a kind of queue from largest to smallest. When the largest crab moves into the new shell, the second biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on. More, here.