True crabs may have achieved global evolutionary success by abandoning forward movement in favor of a sideways gait [1].

This shift in locomotion allowed the infraorder Brachyura to colonize a vast array of environments. By changing how they move, these crustaceans could better escape predators and exploit new ecological niches, leading to a massive diversification of the species.

Scientists trace this iconic scuttle back to a common ancestor that lived approximately 200 million years ago [2]. This single evolutionary event provided the biological foundation for the crabs to spread across the planet, from the deepest ocean trenches to terrestrial habitats [1].

There are now approximately 8,000 described species of true crabs [1]. The transition to sideways movement is viewed as a primary driver of this proliferation, as it offered a competitive advantage in survival and adaptation.

While many crustaceans exist, the specific anatomical adaptations of the Brachyura allowed them to thrive in diverse climates. This mobility played a critical role in their ability to dominate various marine and land-based ecosystems over millions of years [1].

“A change in locomotion—abandoning forward movement and adopting a sideways gait—may have driven their evolutionary success.”

The discovery that sideways locomotion evolved once in a common ancestor suggests that a single morphological innovation can trigger a massive radiation of species. For true crabs, this specific movement pattern was not a quirk, but a survival strategy that enabled them to outcompete other organisms and occupy nearly every available aquatic and semi-aquatic habitat on Earth.