Why Crabs Move Sideways: A 200-Million-Year-Old Mystery Unveiled
The sideways movement of crabs has long intrigued scientists, raising questions about its causes and evolutionary history. A new study published in the journal eLife presents the largest dataset ever collected on crab locomotion, allowing researchers to trace this unusual movement pattern back to a common ancestor that lived around 200 million years ago.
The Environmental Significance of Sideways Movement
Sideways movement is a hallmark of “true” crabs (Brachyura), the largest group within the decapod crustaceans. This unique mode of movement may offer significant advantages to crabs. For instance, it can help crabs evade predators by making their movement direction less predictable.
Professor Yuko Kawabata, a co-author of the study and an associate professor at Nagasaki University, explains that sideways movement may have greatly contributed to the ecological success of true crabs. There are approximately 7,904 species of true crabs, far exceeding the number of species in their sister groups.
Exploring the Evolution of Crab Movement
The research team, led by Kawabata, studied the movement of 50 species of true crabs. Each species was recorded for ten minutes using a standard video camera inside a circular plastic arena resembling their natural habitat. Due to practical constraints, only one individual per species was observed.
The team combined these observations with data from a previously published phylogenetic tree of crabs, which mapped the evolutionary relationships of Brachyura using 10 genes from 344 species across most major branches.
The Singular Evolution of Sideways Movement
Among the 50 species studied, 35 moved sideways, while 15 moved forward. When these behaviors were plotted on the evolutionary tree, a clear pattern emerged. It appears that sideways movement evolved only once, from an ancestor that moved forward at the base of the Eubrachyura group.
Kawabata notes that this singular event sharply contrasts with the phenomenon of carcinization, which has occurred repeatedly across decapod crustaceans, highlighting that behavioral changes like sideways movement may be rare.
A Key Innovation for Survival
The researchers suggest that this single shift to sideways movement played a crucial role in the success of true crabs. Sideways movement allows crabs to quickly navigate in any direction, facilitating their escape from predators.
On the other hand, sideways movement is uncommon across the animal kingdom, perhaps because it might interfere with other important activities such as digging, mating, and feeding.
Evolution and Environmental Opportunity
The study also suggests that evolutionary success is not driven by biological innovations alone; environmental factors play a significant role. Researchers estimate that sideways movement in true crabs originated around 200 million years ago.
This period coincided with major environmental changes such as the breakup of Pangaea and the expansion of shallow marine environments, creating new opportunities for species diversification.
Conclusion
The current findings highlight that sideways movement in true crabs is a rare yet innovative trait that may have contributed to their ecological success. Such innovations can open up new adaptive opportunities while remaining constrained by genetic history and environmental contexts. This study provides a deeper understanding of how locomotion patterns in animals have diversified and persisted over evolutionary time.