What land crabs reveal about evolution's repeats

How can bodies adapt to survive environments completely different from the ones they evolved for?

To help answer this big question, Hanna Gray fellow Victoria Watson-Zink—a postdoctoral researcher at the Harvard University lab of HHMI investigator Cassandra Extavour—turns to a small animal with a lot of experience: the land crab.

"All crabs are, originally, aquatic species. That means that every genus of terrestrial crab has changed its biology significantly to live on land: their diet, how they cope with stressors, even their anatomy," she explains.

Of the changes she's studied, their dietary adaptations are among the most extreme. Many land crabs are herbivorous and eat primarily brown or green leaves—tough, lignified plant matter that's difficult for any animal to break down. Land crabs, descended from ancestors that ate an entirely marine diet, have tiny digestive systems that seem particularly ill-suited to the task.

To make sense of this apparent contradiction, Watson-Zink turned to the crabs' gut microbiome—the community of bacteria living inside their digestive systems. Her thinking: Perhaps land crabs recruited beneficial microbes to help digest this new food source and adapt to their new environment.

Converging on digestion, differently

Of course, not all terrestrial crabs are alike—some lineages have been on land far longer than others, and their degrees of terrestrial adaptation vary. To capture that range, Watson-Zink developed a six-point scale of "terrestriality," from crabs that still spend most of their time submerged to a handful of species that can survive in full desert conditions.

The scale also sorts crabs by which of four evolutionary routes they took onto land, since crabs arriving via freshwater faced different pressures than those that colonized directly from the sea. It also accounts for the fact that different crab lineages likely faced different evolutionary constraints as they adapted to land.

She later joined colleagues in applying that framework to a broader family tree of crabs, estimating that the transition from water to land has happened independently as many as 17 times.

This framework for classifying land crabs by lineage and degree of adaptation proved key to her study of their gut microbiomes. Examining samples collected from 14 crab species across six genera, she and her colleagues used genetic sequencing to identify which bacteria live in each crab's gut—and what those bacteria can do.

They found that the more land-acclimated crabs were likelier to carry the full set of bacterial genes needed to break down lignocellulose, the tough plant material that makes up much of their diet. But which specific bacteria were doing that work turned out to be a surprise: Each host crab genus had its own distinct microbe profile, and some microbes appeared only in a single host genus.

"It's a great example of functional convergence," explains Watson-Zink. "Every crab uses its own unique microbial tools to solve the same problem."

Diet is just one piece of the puzzle for Watson-Zink. Over the course of her career, she hopes to investigate the full suite of adaptations that land crabs have made for life on land—from how they breathe and reproduce to how they manage extreme water loss during the dry season.

In the past, she also studied how land crabs cope with their most pervasive stressor on land: the threat of drying out. To find out, she tracked which genes crabs turned on and off as they responded to dehydration.

As with the microbiome work, she found a pattern of convergence—species relied on a mix of shared genetic tools and their own unique ones. But the study ran into a familiar obstacle: Land crabs remain so understudied that many of their genes have never been characterized.

"Sometimes we'd find similar adaptations at the genetic level, but because we weren't familiar with those genes, we couldn't be sure what their functions actually were," Watson-Zink explains. "Studying an animal like the land crab is a bit like trying to fly an airplane while you build it."

Looking ahead, she hopes to eventually map the full genomes of multiple land crab species, creating a resource the broader scientific community can draw on. She's also excited about a future project on respiration: When these crabs colonized land, they independently evolved lungs to replace their gills. Those lungs all serve the same purpose, but their structures differ—and Watson-Zink wants to identify which genes each species used to build them.

Although she's relatively early in her career, she's already seen her science translate into real-world impact: Her work studying coconut crabs helped inform the International Union for Conservation of Nature's decision to add the species to its Red List of Threatened Species.

She hopes that her research will prove useful to scientists studying other land colonizers, like snails. As climates shift, more species will be forced to adapt to unfamiliar conditions—and Watson-Zink sees her work as a piece of that larger puzzle.

"Fundamentally, I want to understand what genetic toolkit animals reach for when they're facing a particular challenge," she says. "If we understand how—and how often—those adaptations occur, that could help us keep more species alive."