Unless Quarter-inch long, the amphipod – a crustacean that looks a bit like a shrimp – lives a quiet life, sifting through algae along the East Coast. Well it is superficially quietly, as scientists have just discovered. A male amphipod wields a massive claw that can exceed a third of its mass, and when it breaks it in less than 10,000th of a second, it mobilizes a supercharged water jet to voice its displeasure. Thanks to a $ 150,000 camera that films at 300,000 frames per second, researchers for the first time captured a male amphipod in the act, a snap so violent it’s almost enough to blow up the animal.
You’re probably wondering how you experimentally piss off a male amphipod – specifically, the species Dulichiella cf. appendiculata. So I’ll tell you. In the lab, researchers stuck toothpicks on the animals’ backs, then attached the toothpicks to “micromanipulators,” devices that allowed them to precisely position amphipods. All they had to do was hang single hairs from a brush near the amphipods, violating their personal space. Then, BREAK. “So they’re clearly using it in an aggressive context,” says Sheila Patek, a Duke University biologist who co-authored the paper.
With the super-fast camera roll, Patek and his colleagues made the invisible suddenly visible. “In a way, it’s almost magical,” Patek says. Previously, you could hear or feel an amphipod snapping only if you had one in a tray, not if you observed one in the wild. “But then, to get everything crisp and beautifully lit, you can suddenly see this little appendix fill the screen, load, and then break,” she says.
The critical piece of this appendix, known more formally as the gnathopod, is called the cocksfoot. In the image above, it’s the long blade-like structure at the top of the claw. It is not thicker than a human hair. To break, the amphipod contracts a muscle, straightening that dactyl and storing an incredible amount of energy. Patek and his colleagues need to do more work to fully understand the morphology of how the snap works, but it’s likely that a latch will hold the typist in place. When the animal is ready to snap, it releases the latch, suddenly releasing the stored energy from the claw.
“And then when we looked even further, we thought, ‘Wait, there’s a water jet leave the! Patek says. Specifically, the force of the claw click seems to push the water at an oblique angle, rather than perfectly straight ahead. “And then, oh my God, every once in a while the water spray seems to cause cavitation, which is the formation of these vapor bubbles, which occurs when you have a flow rate at these extraordinary speeds. When these tiny cavitation bubbles collapse, they explode, releasing an explosion of energy. This type of force is so powerful, in fact, that when boat propellers create their own cavitation bubbles, over time the force eats away at the metal of the blade.