Evolution Keeps Making Crabs - But a Key Feature Has Only Evolved Once

Key Points:

• True crabs (Brachyura) are distinguished by their characteristic sideways walk, a behavior believed to have evolved once about 200 million years ago and contributed to their ecological success and rapid diversification.
• Researchers observed 50 crab species and found that 35 predominantly walk sideways, while 15 walk forwards, suggesting that forward-walking crabs evolved from sideways-walking ancestors.
• The sideways locomotion likely provided a defensive advantage, enabling crabs to quickly dodge predators and colonize diverse habitats worldwide, including terrestrial, freshwater, and deep-sea environments.
• The study highlights that while crab-like body shapes have evolved multiple times (carcinization), the behavioral innovation of sideways walking is rare and may have been key to the evolutionary success of true crabs.
• Limitations of the study include using only one individual per species and the arbitrary cutoff angle for categorizing walking direction; further research is needed to explore anatomical influences and adaptive advantages of crab locomotion.