Hermit crabs are mistaking plastic for shells and the problem has killed more than half a million of the crustaceans, a new study by the Natural History Museum has found.

The creatures do not make their own shells but instead move from discarded shell to discarded shell as they grow. They are not used to plastic in their environment so do not know to avoid it.

Once they crawl into a piece of plastic debris, the crabs frequently get stuck and starve to death.

Researchers said that if even just one crab mistakes some plastic debris for a shell, this can cause a "gruesome chain reaction", as when one dies it emits a signal alerting others there is a new shell. This causes scores of crabs to come scurrying across the island and fall into the plastic trap.

The team carried out several surveys across a range of sites to ascertain of how many containers there were, including how many were open, how many were in a position likely to trap crabs, and how many contained trapped crabs.