A marine snail shell newly vacated by its gastropod owner (left) and a shell that has been remodeled by a hermit crab. (Photo : University of California, Berkeley)

Social interaction is commonly found in animals in order to protect their own from predator attack or to grab attention from the females for mating. It also helps animals to capture bigger prey.

But a new study on terrestrial hermit crabs sheds light on their bizarre social behavior, which seems to serve their own agenda - to chase another crab out of its shell and move into larger shells.

Most species of hermit crabs live in the ocean, where there is more availability of snail shells, but some species do live on land as well. But it is very rare to see snail shells on land, and that is possible only when the snails wash ashore from the sea. The crabs choose snail shells because it is very difficult for the predators to break open snail shells.

According to the researchers from University of California, Berkeley, when more than two hermit crabs living on land come together, they form a conga line (like a long, processing line), from the smallest crab to the largest, holding on to the crab which stands before it in line. The crabs are intent on displacing other crabs and occupy larger shells. Once a helpless crab is kicked out, the crabs move into larger shells.

The crab that is thrown out of its home gets the smallest shell, which might not help it to protect itself from predator attack.

The UC, Berkeley research team studied the hermit crabs (Coenobita compressus) living on the Pacific shore of Costa Rica. Lead author of the study Mark Laidre, a UC Berkeley Miller Post-Doctoral Fellow, tied individual crabs to a post and observed the fight for larger shells that typically started within 10-15 minutes.

He also noticed the importance of remodeled shells for the crabs. Rare availability of snail shells on land has led the hermit crabs to remodel the shells and make them lighter and spacious. When Laidre pulled the crabs out of their homes and offered them new homes, none of the crabs survived. Even if some crabs could fit in new homes, they had to spend time and energy to hollow it out, which crabs of all sizes would prefer to avoid, said Laidre.

The study gives insights into how animal behavior affects their own evolution.

"No matter how exactly the hermit tenants modify their shelters, they exemplify an important, if obvious, evolutionary truth: living things have been altering and remodeling their surroundings throughout the history of life," wrote UC Davis evolutionary biologist Geerat J. Vermeij in a commentary in the same journal.

The findings of the study are published in the Current Biology.