Crabs are the 20-somethings of the sea, continuously evolving to seek out the new and find where they best fit in. This usually involves moving away from the ocean to other environments, and new research has revealed that they may have done so up to 17 times over the last 100 million years.

The study, conducted by an international team of researchers, looked at how many times true crabs – members of the taxonomic group Brachyura – left the marine environment throughout history and when they did so. This included moving exclusively to land, known as terrestrialization, as well as to semi-terrestrial habitats like intertidal zones and estuaries.

The team used three different datasets to find the answers: one detailing the evolutionary relationships between 344 crab species on a DNA level; the entire documented crab fossil record; and a collection of natural history data such as morphology and behavior.

By connecting this data, they not only discovered that true crabs are 45 million years older than previously thought, first appearing in the Middle Triassic, but that they left the oceans between seven and 17 times over the course of the last 100 million years. This makes crabs unique among arthropods, the vast majority of which have likely only undergone terrestrialization a maximum of a few times, more than 300 million years ago.

The study also found that there were two pathways to terrestrialization: crabs either moved from the seas to land via intertidal zones, beaches, and coastal forests, or more indirectly through estuaries, submerged freshwater, and riverbanks, before reaching coastal forests and jungles.

To do so, they would have had to evolve certain traits that adapted them to their new environments – and the results suggest that different crabs evolved these same traits independently, in a process known as convergent evolution.

“We saw quite a large number of convergent events of terrestrialization with certain sets of traits,” explained Joanna Wolfe, lead researcher, in a statement. According to Wolfe, this could help scientists to determine what happened in other groups of animals that moved from sea to land and predict future evolution of the traits that allow for it. 

“That’s the goal, to be able to apply what we see across the tree of life,” said Wolfe.

There was also an unexpected finding hiding in the data – crabs were found to have returned to a fully marine life on at least two to three occasions in the same period of time.

Perhaps the coastal rent got too high, they started to feel the pinch and had to move back in with their parents. The actual conclusion, albeit more scientific, isn’t actually too far off – it’s difficult for crabs to fully move away from more watery environments.

“Crabs do not have the goal of living on land,” Wolfe explained. “Our results suggest it’s easier to move from fully marine to intertidal or estuarine environments, and it’s up to one hundred times more difficult to transition to living more independently from water, like a fiddler crab, which is mostly on land, because they have more physiological adaptations. Evolution is ongoing, and the niches occupied by organisms change over time; there is no perfect adaptation than can persist forever.”

The study has been accepted for publication in Systematic Biology.