Sideways Walking in True Crabs: A Single Evolutionary Origin 200 Million Years Ago

A new study suggests sideways walking in true crabs evolved just once, approximately 200 million years ago, and may have fueled their remarkable ecological success.

Overview

A study published as a peer-reviewed preprint in eLife reports that sideways walking—a common characteristic of true crabs (Brachyura)—likely originated in a single common ancestor approximately 200 million years ago, shortly after the Triassic-Jurassic extinction event. The research, led by behavioral ecologist Yuuki Kawabata from Nagasaki University, examines the evolutionary history of this distinctive locomotion mode.

Methodology

Researchers analyzed the walking behavior of 50 true crab species in a laboratory setting. Each species was filmed for 10 minutes in a circular arena designed to mimic its native environment. Due to practical constraints, only one individual per species was observed. The study classified 35 species as predominantly sideways-walkers and 15 as forward-walkers.

Behavioral data were combined with a published phylogenetic framework based on 10 genes from 344 species. The evolutionary tree was simplified to include 44 genera, plus five families and one superfamily.

Key Findings

Genetic analysis indicates that sideways walking evolved once, at the base of the group Eubrachyura, from a forward-walking ancestor.

According to the study, this trait has been largely retained by most true crabs, while forward-moving species appear to have secondarily evolved from sideways-walking ancestors.

The researchers note that this single evolutionary origin contrasts with "carcinization"—the convergent evolution of crab-like body shapes—which occurred multiple times.

Timeline and Environmental Context

The estimated origin of sideways walking coincides with several environmental changes:

• The breakup of the supercontinent Pangaea
• The expansion of shallow marine habitats
• The early Mesozoic Marine Revolution, following the Triassic-Jurassic extinction

Suggested Advantages

The authors propose that sideways locomotion may aid predator evasion by allowing rapid escape in either lateral direction. However, the study notes that this form of movement is rare in the animal kingdom, potentially because it can interfere with other behaviors such as burrowing, mating, and feeding.

Significance and Diversity

True crabs comprise approximately 7,904 species, exceeding their sister group Anomura (hermit crabs) and closest relatives Astacidea (clawed lobsters and crayfish). The researchers propose that the sideways walk may have contributed to this ecological success.

Researcher Statements

"Sideways locomotion may have contributed significantly to the ecological success of true crabs." — Yuuki Kawabata, senior corresponding author

Kawabata added: "To disentangle the relative roles of innovation and environmental change, we need further analyses of trait-dependent diversification, fossil-informed timelines and performance tests."

Limitations

The study acknowledges several limitations:

• Behavioral data were limited to one individual per species, which may not represent typical behavior for that species
• Peer reviewers noted that the 60-degree threshold used to classify walking direction lacks biological justification and may not be consistent across taxa
• The phylogeny did not perfectly align with observed species, requiring simplification

Implications

The research provides evidence that a single behavioral innovation, combined with environmental opportunity, may have driven the diversification of true crabs. Further research is needed to assess the relative roles of body plan versus locomotion style and to test the adaptive advantages of sideways movement.