Hermit crabs prefer to occupy darker-coloured seashells, especially if they are living in a dark and gloomy environment.
These tiny crabs often swap shells as they try to find their ideal home. Research has previously shown that hermit crabs on the hunt for a new shell consider its size, condition and shape – features that vary depending on the species of animal to which the shell originally belonged.
To see if colour is also on their list, James Rimmer at the University of St Andrews, UK, and his colleagues painted empty common periwinkle (Littorina littorea) shells either solid black or solid white. Then they collected wild, adult common hermit crabs (Pagurus bernhardus) from the coast of Fife in Scotland.
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In their first experiment, the researchers removed 67 crabs from their current shells and put each one in a well-lit tank with two black and two white shells. Then they repeated the experiment with another 60 naked crabs, this time with the tank in total darkness.
In the light, two thirds of the crabs grabbed the darker shells, says Rimmer. But in the dark, when the crabs couldn’t see the colours, they selected randomly, with a 50/50 split between the black and white shells.
Encouraged by these results, the researchers placed 96 naked crabs – 12 at a time – in a larger, well-lit tank with either a black or a white background, along with 20 black and 20 white shells. The crabs had 24 hours to pick and choose. Then the experiment was run again with another 96 crabs, using red and yellow shells and backgrounds instead.
With ample time to try out different shells and reconsider their choices, at the end of the day, the 192 crabs showed an overall preference for the darker-coloured shells (those that were either black or red), says Rimmer. This was particularly true when the background was black or red, with only around 25 per cent of crabs opting for a light-coloured shell.
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When the background was white or yellow, around 40 per cent of crabs opted for a light-coloured shell over a dark one. This suggests that selecting a camouflaged home might be important for these hermit crabs, according to Rimmer. But he speculates that personality might also have something to do with it, as “bolder” crabs might take more risks with colour compared with other features.
“My feeling is there are a lot of things they’re weighing up – size, weight, volume, shape – in addition to colour,” says Rimmer. “You can imagine this hierarchy of decision-making, and how some criteria might have more impact on their selection behaviour than others.”
Behavioural Processes
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