杏吧原创

Crabs have a habit of evolving to live away from the ocean. The crustaceans have clambered out of the sea and made homes in land-based habitats between seven and 17 different times since they arose about 230 million years ago.

The findings highlight crabs鈥� extreme evolutionary flexibility, as they have repeatedly used beaches, estuaries, rivers and mangroves to colonise terrestrial habitats. Determining how they have done so may give insights into understanding how other animals may have broken their bond with the sea.

Various invertebrate groups 鈥� such as arachnids, insects and molluscs 鈥� evolved lifestyles away from the ocean, but this typically happened hundreds of millions of years ago. And unlike with vertebrates, little is known about the series of evolutionary steps it required, says at Harvard University.

There are more crab species than mammal species, says Wolfe, and crabs live in a huge range of habitats, from deep hydrothermal vents to streams cascading from the slopes of the world鈥檚 tallest mountains. Wolfe and her colleagues knew that multiple crab families had left the ocean, but to figure out when and how they adapted to a range of land and freshwater habitats, they reconstructed the crabs鈥� evolutionary history.

The team analysed and compared 10 genes from 333 crab species across 88 families of 鈥渢rue crab鈥�, a group called Brachyura. These are evolutionarily distinct from lots of other crustaceans that aren鈥檛 closely related but are still known, falsely, as crabs 鈥� such animals have undergone carcinisation, the tendency of crustaceans to evolve a similar body shape of a crab. The researchers combined the genetic analysis with data from crab fossils to create an evolutionary tree. Then they added information on living crabs鈥� degree of adaptation to land-based habitats, which let them reconstruct the likely traits of ancestral crabs and map the timeline of crabs鈥� recurrent retreat from saltwater.

The researchers found that crab lineages left the ocean up to 17 different times, a frequency that surprised them, says team member at Florida International University. One subset of crabs in particular, the Thoracotremata, left at least six times, says at West Virginia University, who wasn鈥檛 involved with the research. This suggests that the varied adaptations in body shape and physiology of some crab groups may make them more predisposed to leave ocean habitats, he says.

The findings contrast dramatically with other invertebrate groups, most of which only left the ocean once. Many of the crabs鈥� transitions were in the last few tens of millions of years, but some got their start earlier, when non-avian dinosaurs roamed the landscape.

Some crab lineages colonised the land directly out of the ocean, through beaches and mangroves. Others went indirectly, through estuaries and rivers first. At least two groups returned to the sea after living on land.

鈥淢ost of the arthropods, they never go back [to the ocean],鈥� says Wolfe.

The study鈥檚 treatment of terrestrial living as not a binary, either-or option but as a graded process will be useful for understanding other moves out of the ocean across animals鈥� long evolutionary history, says Lamsdell.

鈥淚t would be very interesting to see these methods applied to other groups that make the transition onto land to test whether there are shared patterns across these evolutionary events,鈥� says Lamsdell.