Crabs keep evolving to go from the sea to the land — and back again

Crabs have evolved to migrate from the sea to land then back again multiple times over the last 100 million years, scientists have discovered. 

A new study, published Nov. 6 in the journal Systematic Biology, found that true crabs (Brachyura) — which consist of 7,600 species across 109 families — have evolved to migrate from marine to land habitats between seven and 17 times. (True crabs are distinct from other crustaceans that have developed crab-like bodies).

They also found that on two or three occasions, crabs even went back to the sea from land. "It is 100% harder going from being on land to water," lead author Joanna Wolfe, a researcher in organismic and evolutionary biology at Harvard University, told Live Science. 

Related: Why do Animals keep evolving into crabs?

Most arthropods left the ocean just once during evolutionary shifts more than 300 million years ago, in a process known as terrestrialization. 

In the new study, researchers set out to discover how often and when true crabs left the marine environment for land. They compiled three new datasets for 333 species of true crabs from 88 families, including both marine and non-marine groups. 

Using the entire crab fossil record, the researchers applied two mapping pathways: one where the crab goes from a fully marine environment to land directly through intertidal zones such as beaches, and a second where the species migrates from fully marine to land indirectly, through estuaries, fresh water, riverbanks, coastal forests and jungles.