Let's work through those points. When it comes to fluid dynamics, aquatic creatures as tiny as the amphipod are up against some serious forces. To them, says Patek, the water is “like honey. If you're going to try to capture prey when you're this size, if you're going to swim up to it or move slowly to it, you literally push the prey away from you. You actually can't reach it.”
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To overcome this, small critters use high acceleration. Take how jellyfish use their stingers, which are loaded with nematocysts, specialized cells that accelerate tiny barbs into those unfortunate enough to come in contact with them. This remains the highest-known acceleration in the animal kingdom. “They're basically going so fast that they can transition out of the honey realm and into the water realm,” Patek says, enabling the stinging cells to actually touch the prey, instead of pushing it away. The amphipod has to manage the same task: to somehow get the dactyl to accelerate such that it’s moving like it’s cutting through water, not honey.
The second bit is storing enough energy to make this happen. That energy initially comes from the animal contracting the muscle in the gnathopod, which in turn loads that energy into some kind of spring mechanism. (Patek’s team isn’t quite sure yet what this mechanism is.) But how much energy can the amphipod store in the material that makes up its claw, and how far can it push that material before it fails?
“Well, the answer is, the way to get the most energy out of the material is to break it, like to take it all the way to failure,” says Patek. That’s what a jellyfish does with its ultra-fast-firing nematocyst cells: When a stinger is triggered, pressure in the cell rapidly spikes, propelling the dart coiled up inside to rupture through the cell. But that’s not an option for our little amphipod, who’d really rather keep his gnathopod long-term. So instead, he pushes the claw juuust up to the limit of breaking. “The closer you can get to that, the more energy you're going to be able to get out of the material,” Patek adds.
But what is it about the material—both what it’s made of and how it may be structured—that makes the claw so strong? “That's something we've been working on with engineers,” Patek says. “Because they're curious about how much energy you can really store in something without breaking it.”
Amphipods aren’t the only crustaceans snapping all over the seafloor. The aptly named snapping shrimp , aka pistol shrimp, wields a strikingly similar claw. It too has a dactyl, which it cocks back and fires, moving a jet of water, plus cavitation bubbles, which together knock its prey out cold. And the infamous mantis shrimp wields double hammers that it cocks and releases with enough force to blow clams to bits . That impact also creates cavitation bubbles, which helps to break through shells. But what’s fascinating is that these three distantly related crustaceans didn’t retain the weapon from an ancient common ancestor—the trait evolved three separate times.