During evolution, most land plants and animals left the ocean only once.
But the crabs emerged from the sea more than a dozen times, and at least two groups later returned to a marine lifestyle, according to a study published in the Oxford Academic journal Science News.
Crabs are an extremely diverse group that has colonized almost every type of habitat on Earth. Determining when crabs first moved from one habitat to another during evolution has been difficult.
Biologist Joanna Wolfe of Harvard University notes that crabs do not have the large fossil footprint found in early vertebrates. Therefore, past studies have often treated marine, freshwater, and terrestrial crabs as separate subgroups, even though they are more like a continuum.
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Wolfe and her colleagues collected genetic data from 333 species of crabs from the Brachyura group.
They are evolutionarily distinct from the rest of the crustacean group, but share similarities with the king crab and the hermit crab.
Scientists combined genetic data with several dozen fossils to create an evolutionary tree of the crab. In this way, information was collected on the adaptation of each type to life on land.
The researchers found that true crabs diverged from other crustacean lineages about 230 million years ago during the Triassic period.
The study also showed that during their evolution, crabs adapted to a terrestrial lifestyle 17 times. They tried to settle in mangrove forests, freshwater mouths of rivers and seas.
On at least two occasions, crabs have returned to their marine lifestyle long after they left.
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