This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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.
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