At less than a quarter-inch long, the amphipod—a crustacean that looks a bit like a shrimp—lives a leisurely life, sifting through algae up and down the East Coast. Well, it’s superficially leisurely, as scientists just discovered. A male amphipod wields a massive claw that can exceed a third of his mass, and when he snaps it in less than a 10,000th of a second, it marshals a supercharged jet of water to make his displeasure known. Thanks to a $150,000 camera that shoots at 300,000 frames per second, researchers have for the first time captured a male amphipod in the act, a snap so violent that it’s nearly enough to explode the animal.
You’re probably wondering how you experimentally piss off a male amphipod—more specifically, the species Dulichiella cf. appendiculata. So I’ll tell you. Working in the lab, the researchers glued toothpicks to the animals’ backs, then attached the toothpicks to “micromanipulators,” devices that allowed them to precisely position the amphipods. All they had to do was dangle single hairs from a paintbrush near the amphipods, violating their personal space. And then, SNAP. “So they’re clearly using it in an aggressive context,” says Duke University biologist Sheila Patek, coauthor on the paper.
With the ultra-high-speed camera rolling, Patek and her colleagues made the invisible suddenly visible. “In a way, it’s almost magical,” Patek says. Previously, you might only hear or feel an amphipod snap if you had one in a tray, not if you were eyeballing one in the wild. “But then to get the whole thing in focus, and beautifully lit, suddenly you can see this little appendage filling the screen, loading, and then snapping,” she says.
The critical bit of that appendage, known more formally as a gnathopod, is called the dactyl. In the above image, that’s the long, blade-like structure at the top of the claw. It’s no thicker than a human hair. To snap, the amphipod contracts a muscle, cocking back that dactyl and storing an incredible amount of energy. Patek and her colleagues need to do more work to fully understand the morphology of how the snap works, but it’s likely that a latch keeps the dactyl in place. When the animal is ready to snap, it releases the latch, suddenly freeing the claw’s stored-up energy.
“And then when we looked even further, we’re like, ‘Wait, there’s a water jet coming out of there!’” says Patek. More specifically, the force of the claw snap seems to be pushing water at an oblique angle, rather than perfectly straight ahead. “And then, oh my gosh, every once in a while, the water jet seems to cause cavitation, which is the formation of these vapor bubbles, which happens when you have flow at these extraordinary speeds.” When these tiny cavitation bubbles collapse, they explode, unleashing a blast of energy. This kind of force is so powerful, in fact, that when boat propellers create their own cavitation bubbles, over time the force chews away at the metal of the blade.