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Wednesday, May 22, 2024

How This Teeny-Tiny Sea Critter Punches Like Mike Tyson

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.

In other words, the amphipod is managing a one-two punch: The jet of water is itself extremely powerful, and that’s followed by the explosive bubbles releasing their own energy. It’s all so violent, it’s a wonder that the amphipod doesn’t explode—and Patek says there are good reasons why it doesn’t. “There's a really cool intersection here,” she says, “of not blowing up, being able to store enough energy, materials-science-wise, and also the fluid dynamics.”

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.”

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.

So now, the teeny-tiny amphipod joins an exclusive crew of crustacean super-punchers. “This fascinating work elicits childhood memories of Where the Wild Things Are, where the wild things ‘roar their terrible roars and gnash their terrible teeth,’” says University of Colorado Boulder biologist Lindsey Dougherty, who researches mantis shrimp but wasn’t involved in this new work. “In this case, the gnashing is in the form of a gnathopod claw that exceeds 24,000 g's.”

For context, you’ll experience 5 g’s on humanity’s wildest roller coaster, and you'd pass out at around 7—and the amphipod is managing this through the viscous medium of water, not air. “Never mind the size of the claw, which at a third of the animal’s body mass, would be like giving your friend a high-five with a 50-pound thumb,” says Dougherty. “I tell my students that anything we can do, nature does better—in this case, it's literally too fast for us to see.”

Strangely, though, in their investigation of 241 amphipod strikes, Patek’s team detected cavitation bubbles only six times. By contrast, for the mantis and snapping shrimp, such bubbles are a regular feature of their strikes. (Snapping shrimp do it so often—so incessantly, really—that the collapsing bubbles create a constant chorus of cackling in the sea.) It’s a mystery why these male amphipods are perfectly capable of producing powerful exploding bubbles, yet don’t do it regularly, at least in the lab.

Another unknown is why the amphipod would evolve such a formidable weapon in the first place. A male tends to mate with a group of females, so the claw may well help him fend off usurpers. Or, before it comes to blows, snapping may convince rival males to retreat.

“It could be for territoriality, it could be for mate attraction,” says Patek. “I think this is going to be a Duke undergrad project for the next decade.”

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