Crab Convergence: Evolution's Crab-shaped Blueprint Across Species

Curious Cases investigates how crab-like body plans emerge across unrelated lineages through convergent evolution. The episode brings together evolutionary paleobiologist Matthew Wills, evolutionary biologist Jo Wolf, and Ned Sissat Williams, director of the Crab Museum in Margate, to unpack why crabs look alike across oceans and eras. They discuss true crabs versus false crabs, the anatomy behind crustacean shells, and how environment and function drive morphology. The conversation blends fossil insight, modern genetics, and engaging museum demonstrations to show that crabs are more than a quirky curiosity—they are a window into the creativity and limits of evolution.

Introduction and Crab Curiosity

The episode opens with a playful exploration of the idea that evolution sometimes carves a crab-shaped path across very different creatures. The hosts set up a discussion with three guests: Matthew Wills, professor of evolutionary paleobiology at Bath University; Jo Wolf, an evolutionary biologist affiliated with Harvard and UC Santa Barbara; and Ned Sissat Williams, director of the Crab Museum in Margate. The conversation centers on why crab-like forms appear again and again and how memes about a world of crabs reflect real evolutionary principles rather than inevitability.

“Crabs are weird and funny looking, and yet they are part of the world in which we inhabit.” - Ned Sissat Williams

Convergent Evolution and Crabs

The panel delves into convergent evolution, describing how unrelated species can arrive at similar morphologies due to shared environmental pressures. Matthew Wills articulates that convergence is not about shared ancestry but about similar selective forces producing parallel solutions. The group uses crabs as a vivid example to illustrate how different lineages arrive at crab-like forms without a single common crab ancestor.

“There are various forms of convergent evolution, which is basically where Evolutionary forces on different species will, come with the same results.” - Matthew Wills

True vs False Crabs and the Morphology

The discussion moves to taxonomy, explaining two major groups: true crabs (brachiura) and false crabs (anomala). Hermit crabs and other relatives are sometimes mistaken for true crabs, highlighting how morphology can be misleading. The guests explain that crabs share a common functional package, yet their bodies can diverge significantly, leading to debates about what truly constitutes a crab. The exchange clarifies how morphology and phylogeny can diverge, especially when modern genomics reshapes classic anatomical assumptions.

“Hermit crabs are not really crabs.” - Joe Wolf

Ecology and the Pace of Change

Environment emerges as the key driver that shapes crab-like forms. The crew discusses crustacean diversity—from mangrove engineers that aerate roots to deep-sea crabs in hydrothermal vents—and notes that evolution can be rapid in certain lineages. They touch on the fossil record, noting that some crab-like shapes persist for tens of millions of years, while other lineages diverge dramatically in relatively short geological timescales. The episode emphasizes that carcinization is one pattern among many convergent outcomes shaped by ecological context.

“Evolution can happen really, really quickly.” - Matthew Wills

Takeaways and Reflections

Concluding, the guests reflect on the meme of everything becoming crab-like, arguing that, while convergence is powerful, there are limits set by available anatomical building blocks and ecological opportunities. The episode closes with an appreciation for crabs as ecosystem engineers and a reminder that taxonomy, paleontology, and genomics together reveal a nuanced, non-linear story of evolution that defies simple memes.

“Crabs are ecosystem engineers.” - Ned Sissat Williams