Critter of the Week: The Hermit Crab

Move over Question of the Week! It's marine month, and there's a new feature in town. A host of creatures are fighting for your attention as Critter of the week. And to get us started, Katie Haylor has been at home with a hermit crab... 

Katie - Name: hermit crab. Scientific name: the common hermit crab is also known as Pagurus bernhardus. Location: habitats range from the Arctic to South America. Special abilities: upcycling, i.e. finding homely snail shells, and being good in a fight.

Mark Briffa from Plymouth University makes the case for this contender for Critter of the Week.

Mark - These peculiar creatures are fascinated observers of the natural wild for literally thousands of years. Aristotle described them as “looking like a cross between a spider and a crayfish.”

Katie - Drawing from the sci-fi realm, Mark also says that hermit crabs aren’t too dissimilar from the face huggers out of the Alien movies…

Mark - The distinguishing feature of hermit crabs is their soft abdomen which lacks a protective exoskeleton. Instead, they use empty snail shells as portable burrows with hermit’s, head, legs, and claws poking out of the opening. A naked hermit crab looks decidedly lopsided. The long, soft abdomen hangs behind its last pair of legs and usually curls round to one side, ending in a little hardened anchor. The front part is protected by a small carapace, and this is equipped with a pair of stumpy little appendages which brace against the snail shell, two pairs of long walking legs, and a pair of claws to the front with one claw being much bigger than the other. This wonky body helps hermits to fit snuggly inside their coiled snail shells. When threatened, they can close off the shell’s opening using their larger claw as a trap door.

Katie - So, apart from good home security measures, what makes the hermit crab such a cool critter?

Mark - Hermit crabs put a huge amount of effort into finding, investigating and even fighting over the empty snail shells that they rely on, and they will just as happily do these things in the lab as in the field. This makes them superb models for scientists interested in how our animals gather information. How they make decisions and, especially, in how animals fight.

In hermit crab fights and attacker vigorously wraps its shell against the shell of a defender. And if successful the attacker will evict the reluctant defender and get to move into a newly vacated and upgraded home. It’s not just about brute force either. Attackers adjust their tactics as the fight goes on and skillful fighting is also important.

Katie - Well they do sound formidable. To be named Critter of the Week, surely hermit crabs must have some amiable qualities…

Mark - They’re true global citizens, they recycle used shells and, in some species, they even form orderly queues for empty shells.

Katie - There you have it: fighting prowess, and eye for a good shell property and, for some, a love of queuing make the humble hermit crab the Critter of the Week...