Revealing how crabs make their escape

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On the coast of Argentina, in the estuaries and salt marshes of the Southwestern Atlantic, countless mud crabs burrow into the sand. Daniel Tomsic, associate professor of neuroscience at the University of Buenos Aires, grew up watching the ubiquitous creatures, called Neohelice granulata, scuttle in and out of their nests whenever he visited the beach.

Tomsic now studies how the crab’s ganglia control visually guided escape behavior in the same crabs. Gulls prey on the creatures, which are just barely bigger than a guitar pick. When a crab sees a gull flying overhead, it must decide whether to scurry and in what direction. It’s the same computation a tennis player makes in reading a 200-kilometer-per-hour serve and deciding when to swing, Tomsic says. “Nobody knows how these time-to-collision calculations are made,” he says, adding that the crab is a tractable model organism for studying this question.

Tomsic inherited the model from his mentor, Hector Maldonado, who introduced Neohelice as a neuroethological model in Argentina after returning from 18 years in exile. Tomsic advanced the work by making intracellular recordings of live, behaving animals, something that had previously been done only in insects and only after removing the animal’s legs. The crab’s rigid carapace and accessible brain region, reachable through a small hole drilled in the cuticle, made it possible, Tomsic says.

Intracellular recordings revealed the circuits that connect visual input to motor output. The decision to flee is triggered once the apparent size of an approaching object has grown to a certain size, according to work by Tomsic and his colleagues. The crab continuously adjusts its running speed as the threat expands, slowing immediately if the stimulus stops growing. A single giant neuron, the MLG2 neuron, encodes how fast the threat is expanding and drives that speed adjustment in real time, and another neuron, MLG1, controls a crab’s freezing response.

Field studies in collaboration with Jan Hemmi, professor of biological sciences at the University of Western Australia, revealed that crabs behave differently in the lab than in the wild. In the lab, crabs consistently flee in the opposite direction from an approaching stimulus, and they wait until the looming object is fairly large before moving. In the field, however, crabs bolt in response to smaller stimuli and run toward their burrows to hide, even if it’s in the same direction as a predator. Without a place to hide, lab animals freeze to avoid detection. “That was a big lesson for me,” Tomsic says. The mismatch showed that laboratory conclusions about natural behavior carry real risk without field checks, he says.

More recently, Tomsic’s focus has shifted to the decision-making behind the escape behavior, and how being surrounded by other crabs changes when and how an individual crab chooses to flee. And, because Neohelice prey upon each other, the model enables researchers to study predatory behavior as well.