The majority of terrestrial hermit crab species worldwide have used trash as shells, according to a study by experts at two Polish universities to be published next month.

The study analyzed publicly available images of hermit crabs online and past scientific literature. It found 386 examples of the crustaceans encased in garbage instead of a seashell, of which about 85% were using plastic caps and the remainder using metal and glass. They included individuals from 10 of the 16 species of hermit crab that live on land, and were found throughout the tropics from Africa to Central America.

“We confirm for the first time that the use of artificial materials by hermit crabs is a behavior occurring on a global scale,” the study authors wrote in the paper, which will appear in the Science of the Total Environment journal.

“Plastic is the most pervasive element of marine waste, with harmful impact on wildlife,” they wrote.

The authors posited that hermit crabs may be choosing plastic homes because they make for better camouflage in a polluted environment, or that there may be more pieces of trash than fitting seashells available on some coasts.

Other factors may include using a unique shell in sexual signaling to attract a mate, the fact that artificial shells may be less heavy, and odor cues, they wrote.

A 2021 study found that hermit crabs seem to be attracted to a chemical emitted by plastic.

Plastic can be dangerous for hermit crabs. A 2019 study of the Cocos (Keeling) Islands, a remote Australian territory in the Indian Ocean, found that over half a million of the islands’ hermit crabs had crawled inside items such as bottles, gotten stuck, and died. The researchers discovered that 414 million pieces of trash had washed onto the shores of the minimally populated islands.

Unlike most crabs, which have naturally calcified abdomens, hermit crabs are born with soft bodies. They evolved to find seashells to live inside, which they can retract into for protection or extend their legs from and scuttle around. They find bigger shells and move into them as they grow.

Massive amounts of human-generated trash in the oceans could set hermit crabs on a new course, the study’s authors wrote, using the proposed term for our present geological epoch as defined by human activity. “Are artificial shells setting the scene for a novel evolutionary trajectory in hermit crabs,” they wrote, “or are they an ecological and evolutionary trap of the Anthropocene?”