Crabs and Convergent Evolution: How Crab-Like Forms Recur Across Life

Curious Cases invites listeners to explore why crab-like forms repeatedly appear across diverse life forms. The episode centers on carcinization, the idea that evolution can converge on a crab-shaped body in unrelated lineages, and it features expert guests Matthew Wills, Joanna Wolf, and Ned Sess Williams from the Crab Museum in Margate. The discussion unpacks true crabs versus false crabs, the taxonomy of brachiura and anomura, and how environmental pressures shape morphology. Through lively examples and fossil records, the hosts illustrate how crabs are more than just a beach sight — they are a window into the dynamics of evolution, convergence, and ecological engineering. The conversation leaves listeners with a fresh appreciation for crabs and the power of convergent evolution.

Introduction and premise

This episode of Curious Cases delves into one of evolution’s most entertaining patterns: convergent evolution leading to crab-like body plans in unrelated groups. The hosts examine why the crab form is so appealing to natural selection, and how memes about crabs have influenced public imagination. The panel of guests—Matthew Wills, Joanna Wolf, and Ned Sess Williams—provide expert lenses on evolutionary biology, taxonomy, and the public display of crabs in museums. The discussion emphasizes that crabs are not a single lineage but a recurring solution to ecological challenges, prompting a broader look at how shape, function, and environment interact across life on Earth.

"Crabs are weird and funny looking" - Dara O'Brien

Crab biology and classification

The episode clarifies the distinction between true crabs (brachiura) and false crabs (anurea), explaining that a crab-like appearance has evolved many times across different groups. The conversation covers the taxonomic rules that separate true crabs from their look-alike relatives, such as hermit crabs and porcelain crabs, and discusses how morphology can be decoupled from direct ancestry. The experts also touch on the dynamic nature of crustacean taxonomy, where appearance does not always reveal evolutionary relationships.

"Hermit crabs are like my favourite crab, and they're not a crab" - Ned Sess Williams

Convergent evolution and the meme

Convergent evolution is framed as a widespread phenomenon, with crabs serving as a memorable example. The discussion acknowledges that while crabs are a striking instance, similar evolutionary outcomes have occurred in many lineages, underscoring the importance of environmental pressures and functional demands in shaping form.

"It's not necessarily more of a thing than other convergent evolutions. It's just sort of won the PR game" - Ned Sess Williams

Case studies and fossil record

The guests discuss fossil evidence, including a 90-million-year-old crab fossil, showing that crab-shaped morphology has persisted across deep time. They contrast this with other rapid evolutionary shifts, such as whale evolution, illustrating that some lineages remain crab-like for long periods when the form remains advantageous.

"Crabs live in every ocean, pretty much" - Ned Sess Williams

Ecology, environment and ecosystem roles

The conversation expands to the ecological roles of crabs, from their function as ecosystem engineers in mangroves to their diverse habitats in deep oceans and on land. The experts highlight how the morphology of crabs supports a range of lifestyles and how this shape contributes to ecological processes that sustain other organisms and ecosystems.

"Crabs are ecosystem engineers" - Ned Sess Williams

Public engagement and future directions

The episode closes with reflections on how the crab meme has helped people engage with evolutionary science and how museums and researchers communicate complex ideas through relatable examples. The panel suggests that studying crab-like forms can illuminate broader patterns of evolution, convergence, and adaptation across the tree of life.

"Crabs are everywhere. They are a hugely important part of the world around us" - Ned Sess Williams