Crabs are evolutionary masters of the land-sea transition
In short: While most animals made the leap from ocean to land only once, crabs are different. These crustaceans have repeatedly abandoned the waves for terrestrial life, demonstrating an extraordinary evolutionary flexibility that allows them to move between environments multiple times throughout their history.
The process of moving from water to land is known as terrestrialization. For most arthropods, this was a singular event occurring over 300 million years ago. However, true crabs (Brachyura)—a group containing 7,600 species across 109 families—have taken a much more repetitive approach. A study in Systematic Biology indicates that these crabs transitioned from sea to land between seven and 17 times.
This movement isn't always a one-way street. Scientists have discovered that on two or three occasions, crab lineages actually reversed course, returning to the sea from the land. Joanna Wolfe of Harvard University notes that these transitions are incredibly difficult, stating that moving from land to water is "100% harder."
These transitions occur through various pathways. Some species move directly from the ocean to land via intertidal zones like beaches. Others take a more indirect route, migrating through estuaries, fresh water, riverbanks, or even coastal forests and jungles. This ability to exploit diverse habitats has allowed crabs to persist for hundreds of millions of years, with some lineages dating back to the mid-Triassic period, roughly 251.9 to 201.3 million years ago.
This evolutionary flexibility faces modern challenges. In South Korea, researchers observed that land crabs face significant road mortality during their annual spawning migrations. During a study in 2018, 95% of 739 roadkilled carcasses were female, a demographic imbalance that could lead to population declines and threaten the connectivity between land and sea habitats.
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Sources
- Summary by Intellect · source: Why Crabs Keep Leaving the Sea for the Land
- Also: Crabs keep evolving to go from the sea to the land — and back again | Live Science
- Also: Crabs evolved to live away from the ocean up to 17 different times | New Scientist
- Also: Coastal road mortality of land crab during spawning migration | Scientific Reports
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