Crabs compromise to avoid competition

Rather than compete for shelter, two closely related hermit crab species appear to have adapted to differently shaped shell homes so they can coexist on the same beaches, according to a study published in BMC Ecology.

Land hermit crabs have soft bodies and live in shells that they can carry on their back for shelter and protection against predators, but the availability of empty, well-fitting shells can be limited.

Such finite resources for food and shelter can create competition amongst similar species, and research suggests some may have adapted so they can coexist – a phenomenon known as resource partitioning. 

To explore this, you need to find animals limited by only a single resource. Sebastian Steibl and Christian Laforsch, from Germany’s University of Bayreuth, got lucky. 

Two hermit crab populations, Coenobita rugosus and C. perlatus, live on the same beaches of 11 coral islands in the Maldives, suggesting they don’t need to compete for shell homes – and they are not restricted by habitat, clearly, or food as neither is very fussy.

This made them ideal for studying two potentially competing species, says Steibl, as their survival appeared to only be restricted by the availability of empty shells.

The pair collected 150 of each species from the island of Naifaru and a variety of 150 empty shells. They removed the crabs from their original homes and presented them with two new appropriately sized shells with different shapes to choose from.

They found that each species showed a different shell preference: C. rugosus chose short, spherical shells, while C. perlatus preferred longer shells with a narrow opening.

Given the shells’ similarities in colour, pattern and texture, it’s likely the different preferences were based on their different shape and structure.

“These preferences may reflect different survival strategies developed by each species in response to environmental pressures,” says Steibl.

Heavy, narrow shells provide greater protection against predators but are harder to clutch, he explains, while the lighter round shells increase predation risk but are easier to clutch and move around, and lend themselves better to burrowing and reproduction.

Thus, by minimising predation risk and maximising reproduction, the authors write, “the two hermit crab species might have evolved different strategies to respond to the overall selective pressures in their natural habitat”.

Understanding resource partitioning may explain how the diverse myriad of species on Earth co-exist, and could help predict the impact of dwindling species on ecosystem function.

Steibl and Laforsch suggest hermit crabs offer an ideal model to further understand how closely related species partition resources and differentially evolve.

The crabs and shells were returned to their original habitat.