Hermit crabs are decapod crustaceans found all over the world in a diverse range of marine ecosystems. In coral reef ecosystems, these crabs are often found on sandy sea beds or rocky surfaces, and are occasionally found living on coral branches. They are omnivores with a diet mainly consisting of detritus.

Unlike other crabs, Hermit crabs do not produce a calcified exoskeleton. Instead, these softer bodied organisms locate empty snail shells, which they climb into and occupy. These acquired shells act as a house, providing them with protection from predators. Their asymmetrical 'spiralled' body shape and difference in left and right claw size allow hermit crabs to fit perfectly into these shell homes.

Unoccupied shells are chosen not just by sight, but by Hermit crabs completing a series of highly important checks. Using chemical cues to detect levels of calcium carbonate present in the shell is an important test of shell strength, and turning the shell over allows the crab to assess its size. Like buying new shoes, hermit crabs then slip their legs inside to make sure the fit is right. Once inside, a 'squat lift' exercise provides information in the shells weight, something that is important to know if they are to carry it everywhere they go.

These shells play a crucial role in their survival, therefore picking the best one is extremely important. As hermit crabs grow, they will need to find new larger shells that provide a better fit. Although hermit crabs may discover well-fitting empty shells, often the best shells are already occupied by other individuals, and so hermit crabs living in suboptimal shells will often initiate fights in an attempt to upgrade their home.

Engaging in a fight is a complicated process for both attackers and defenders. It is important for defenders to learn the fighting ability of their attacker, so as not to waste time and energy competing against a superior rival. As the defenders retreat deep into their shell, this is something that is hard to assess and so they must rely on communication from their opponent. Attackers climb onto the defender’s shell and make several attempts at pulling their opponent out its shell. In between efforts, attackers will also bang their existing shell against their opponent’s shell in a series of bouts. Studies have found that this type of behaviour allows hermit crabs to advertise information to their opponent on their strength and endurance, and that the information gained from this banging together of shells is important for defenders when making decisions on whether to continue or withdraw from the contest. If the defending crab loses and gives up its shell, the winner then carefully examines both shells and decides which one it would prefer to live in. All the meanwhile the loser sits and waits for whichever is left or finds a new one for themselves. 

Decisions made to withdraw from a fight are often based on information gathered on the value of the resource an animal is fighting for, or the fighting ability of an individual’s opponent. Studies that have researched contest behaviours in hermit crabs have significantly helped us learn how animals make decisions during contests based on different types of information available to each member of the fight.

 -- Click here to see more of Piers' underwater photos of marine organisms. --