Crabs keep evolving to go from the sea to the land — and back again

Crabs' Remarkable Evolutionary Journey Revealed in New Study

A recent study, published in the journal Systematic Biology on Nov. 6, unveils the fascinating evolutionary history of true crabs (Brachyura). These crabs, comprising 7,600 species across 109 families, have astoundingly transitioned between marine and land habitats multiple times in the last 100 million years.

Iterative Migrations: A Surprising Pattern

The research, led by Joanna Wolfe from Harvard University, indicates that true crabs have undergone migrations from sea to land and back between seven and 17 occasions. Notably, a few instances saw crabs reversing the journey, returning from land to the sea, a feat deemed significantly more challenging.

Unveiling the Unique Crustaceans

True crabs stand apart from other crustaceans due to their distinctive bodies. In contrast to most arthropods, which left the ocean only once over 300 million years ago during a process known as terrestrialization, true crabs exhibited a more dynamic evolutionary pattern.

Unraveling the Mystery: Methodology and Discoveries

To discern the frequency and timing of these migrations, researchers compiled datasets for 333 true crab species from 88 families. These datasets covered both marine and non-marine groups. Utilizing the entire crab fossil record, the team employed two mapping pathways: direct migration from fully marine to land and an indirect route through estuaries, fresh water, riverbanks, coastal forests, and jungles.

Grading Terrestriality: Classifying Crab Adaptations

The study classified each crab species based on their terrestrial adaptability, revealing a gradient of terrestriality. Applying methods originally designed for studying virus evolution, the researchers estimated that true crabs emerged approximately 45 million years earlier than previously believed, possibly dating back to the mid-Triassic period, making them contemporaries of some of the earliest dinosaurs.

Convergent Evolution: A Common Thread

The crabs' iterative migrations resulted from convergent evolution, where different organisms independently develop similar traits. Interestingly, the majority of crabs predominantly thrive in semi-terrestrial habitats. Land-based crabs are concentrated within a specific species-rich group of the family tree, dispelling the misconception that crabs aim to permanently dwell on land.

Conclusion: A Dynamic Evolutionary Saga

In summary, the study unveils the dynamic and intricate evolutionary journey of true crabs, showcasing their ability to navigate between marine and terrestrial environments multiple times. This newfound understanding challenges previous notions and emphasizes the complexity of life's adaptations over millions of years.