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Quick claw

This tiny amphipod can snap its claws at dizzying speeds.

Scientists have just discovered one of the fastest and most energetic motions on the planet – and it’s not what you think.

Turns out that the claws of the male amphipod (Dulichiella appendiculata) can repeatedly snap shut in less than 0.01% of a second, shooting out high-energy water jets and popping sounds.

That’s some serious speed for such a little critter. Amphipods are tiny, shrimplike crustaceans that only grow to a few millimetres long. They tend to live in cool, scummy water, spending their days scrounging for dead algae and seaweed – and, apparently, snapping their claws.

“What’s really amazing about these amphipods is that they’re sitting right on the boundary of what we think is possible in terms of how small something can be and how fast it can move without self-destructing,” says lead researcher Sheila Patek from Duke University in the US. “If they accelerated any faster, their bodies would break.”

Their results are published in the journal Current Biology.

You might assume that the fastest motions in nature might come from large animals or robots, but in fact they come from much smaller organisms and structures – including cnidarian stinging cells, fungal shooting spores, and the mandible strikes of ants, termites and spiders.

“These diverse systems share common features: they rapidly convert potential energy — stored in deformed material or fluid — into kinetic energy when a latch is released,” the researchers explain in their paper.

“However, the fastest and smallest known movements often cannot be used multiple times, because mechanical components are broken or ejected.”

It’s especially difficult to produce fast motions in water, which has a higher density and viscosity than air.

But the snaps of these amphipods in water were not only ultra-fast, but also repeatable.

In order to study the breakneck motion, the researchers caught the species by hand off a dock in North Carolina, then put them in a lab setting and teased them into snapping by dangling human hairs in front of them.

This had to be done in a lab setting due to the expensive and specialised nature of the ultra-fast camera equipment – which captured the action at 300,000 frames per second.

The result was among the smallest and fastest of any repeated movement ever seen.

“Some mandible strikes by snap-jaw and trap-jaw ants and termites (asterisks) are faster, but their motions are performed in air,” the authors write.

The acceleration of the amphipods’ claws is even similar to the speed of the mantis shrimp’s punch.

Intriguingly, the observations also revealed that sometimes the resulting water jets caused “cavitation”, where rapid changes in water pressure cause bubbles to form – and when they pop, they release an immense amount of energy, enough to degrade the steel of boat propellers.

But why do amphipods snap their claws in the first place?

“The claws make up a third of a male’s body weight,” says Patek. “We want to know why they invest so much into this action, whether it plays into male-female interaction or territorial disputes. That’s something we’re excited to pursue.”

This high speed video filmed at 30,000 fps and played back at 30 fps shows a cavitation bubble forming. Credit: Patek Lab.

Lauren Fuge

Lauren Fuge

Lauren Fuge is a science journalist at Cosmos. She holds a BSc in physics from the University of Adelaide and a BA in English and creative writing from Flinders University.

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