Seahorses Are Stealthy, Brutally Effective Assassins

Slate’s animal blog.
Dec. 13 2013 12:38 PM

Seahorses Are Stealthy, Brutally Effective Assassins

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The head of a seahorse (Hippocampus erectus) as it stalks its prey.

Courtesy of Brad Gemmell

Copepods are tiny crustaceans that live in virtually every aquatic environment on earth. Like translucent, antennae-ed grains of rice but just a fraction of the size, these creatures form the foundation for entire ecosystems and get gobbled up by everything from birds and corals to jellyfish and whales. But for all the billions of mouths trying to eat copepods, nowhere is more dangerous in all the ocean’s depths than the millimeter of water next to a seahorse’s snout.

Now, that’s kind of a weird statement to make for two reasons. First, copepods have evolved to be masters of evasion, capable of jetting away from predators at a rate of 500 body lengths per second. (For comparison, the world’s fastest land animal, the cheetah, can reach speeds of only 20 body lengths per second.) Second, the seahorse is not what you’d call an agile predator. In fact, watching them bumble around might lead you to the conclusion that seahorses suffer from severe vertigo, or perhaps alcoholism.

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So how does such an ungainly fish—and weird as they look, seahorses really are fish—take down the ocean’s most elusive prey? New research suggests the seahorse’s oddly shaped head is designed specifically for copepod assassination.

Marine scientist Brad Gemmell explains that copepods are blind, so they can’t see danger approach. “The only way they can detect the presence of a predator is by sensing the disturbance that it makes in the water,” Gemmell said. And when they sense a disturbance in the aqua-force, the crustaceans shift into hyperdrive within 2 to 3 milliseconds of perceiving the threat—one of the fastest response times on record.

But you can’t evade what you don’t sense. Using a high-speed-macro-laser-hologram-awesome-sauce camera, Gemmell and his team were able to measure the water displaced by a seahorse’s strike. Or more specifically, the water not displaced. (Their research was published last month in Nature Communications.)

As you can see from the video, the seahorse is able to get extremely close to the copepods without triggering their escape plan. Gemmell’s measurements show that this is because the seahorse’s head somehow cuts through the water with almost no disturbance. And to a copepod, a predator that makes no waves might as well be invisible.

Once the copepod is within the seahorse’s halo of death, the latter uses its large epaxial muscles along its spine to snap the head upwards, where it can suck prey into its mouth like an eyedropper. Best of all, research shows other copepods in the area of the strike are less inclined to notice the seahorse attack—meaning the seahorses are basically equipped with silencers, which allows them to reposition and attack again and again without scaring off potential meals. All of this adds up to a 94 percent success rate of seahorse-on-copepod carnage.

Personally, I think it’s a good thing I don’t have access to a seahorse aquarium, because I’d be plopped in front of that thing 24/7 like it was the Mirror of Erised. Gemmell seems more interested in the copepods though, and his other paper out right now details the way another fish catches them.

“Zebrafish actually create a little suction to mask their signal,” he said. The suction negates the bow wave made by their approach and cloaks the predator until it’s close enough to strike. Gemmell says zebrafish and other predators hunt with only about 30 to 40 percent accuracy, but it at least shows there’s more than one way to skin a copepod.

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