Sometimes families just outgrow their homes: everyone keeps bumping into each other and there is never enough space. But even if you do upgrade, your new abode may not tick all the boxes, and it turns out that home-hunting hermit crabs are often in the same quandary. ‘Hermit crabs can be forced into “best of a bad job” scenarios in which they must accept whatever relatively suitable shell becomes available’, says Guillermina Alcaraz, from Universidad Nacional Autónoma de México. This compromise might also affect where the nomadic crustaceans choose to dwell. ‘It may be more feasible for crabs to choose a habitat that will be better suited for the shell they have been able to acquire, rather than choosing the shell based on their preferred habitat’, explains Alcaraz. Clibanarius antillensis hermit crabs seem to use loose-fitting, heavy-pointed Stramonita biserialis snail shells when battered by waves, in contrast to more tightly fitting light and rotund Nerita scabricosta snail shells, which they don in sheltered rock pools, so Alcaraz wondered whether the crabs’ choice of shell affects their cost of living and, ultimately, where they choose to settle.
Risking the surf on Mexico's Pacific coast, Alcaraz, Brenda Toledo and Luis Burciaga went in search of Calcinus californiensis hermit crabs in waves crashing at up to 160 cm s–1; ‘we had to avoid being dragged away’, recalls Burciaga. After gathering other crabs from tranquil rock pools nearby, the trio returned to Mexico City, where they encouraged the crustaceans to vacate their homes briefly, and checked how spacious and hefty the shells were. Impressively, the intrepid crabs that were resident in the surf tended to occupy Stramonita shells that were 72% larger than the Nerita shells inhabited by the crabs that opted to shelter in rock pools.
After they had returned the animals to their shells, Alcaraz and her students placed a claw hairclip on the crabs’ shells – to distort the weight and to encourage them to move out – while providing a selection of larger and smaller alternative shells (of the same species that they had previously occupied) in either still water or simulated waves to find out how the environment affected the crustaceans’ choice of shell. Allowing the crabs 24 h to relocate, it was evident that the wave-swept crustaceans preferred to inhabit heavier Stramonita shells, even though the shells are more cumbersome and could impede foraging.
But why were the crabs selecting heftier Stramonita shells that could hamper them when lighter shells would surely ease their burden? Alcaraz measured the amount of oxygen consumed by the animals as they were pummelled by the simulated waves. Remarkably, the crabs clad in heftier Stramonita shells used 84% less energy when hanging on in the surf than the crustaceans that squeezed themselves into smaller Stramonita shells. ‘The heavier the gastropod shell, the lower the net effects of hydrodynamic forces and, therefore, the less energy required to overcome water flow’, Alcaraz explains. In contrast, the crabs that were given lighter, more rotund Nerita shells used more energy clinging onto the ground; the Nerita-clad crabs are more easily dislodged by the seething water and the globular shells are less streamlined, forcing the crustaceans to use more energy to withstand the force of the waves.
Alcaraz suspects that hermit crabs are essentially pragmatic; when it comes to finding a desirable residence, they're prepared to put up with whatever they can squeeze into until a better opportunity comes along. However, their choice of new abode may require that they retreat to a more tranquil environment if their new home isn't up to going with the flow.