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Seahorses Have a Built-In Stealth Mode to Capture Speeding Prey

Seahorses are slow swimmers, so how do they eat?

by Derek Mead
Nov 26 2013, 6:05pm
A dwarf seahorse slowly glides into frame before bam, a copepod is eaten. Video by Brad Gemmell, Ed Buskey and Jian Sheng

Conjure up an image of a seahorse in your head, and you'll probably envision a gentle, beautiful creature with a certain majesty that's unexpected for its diminutive size. (You might also imagine a zebra with snorkel gear, which I recommend thinking about, because it's awesome.) You probably didn't picture seahorses as being speedy killing machines.

It's a fair assessment, as seahorses aren't as speedy as their terrestrial namesakes. In fact, they're pretty slow swimmers. This is a problem, for seahorses have to eat, and their prey of choice, small crustaceans known as copepods, are notably quick. So how does the average seahorse catch a meal?

As it turns out, stealth, not speed, is the key: According to new research published in Nature Communications, seahorses' heads have evolved to glide through water without creating any turbulence. So instead of chasing down skittish, speedy prey, seahorses can sneak into feeding range without prey noticing, before slicing their heads through the water to suck in a tasty copepod.

The snout area (circled) of a seahorse's head was modeled to have extremely
low turbulence, which helps them sneak up on prey. Via Nature

"Seahorses have the capability to overcome the sensory abilities of one of the most talented escape artists in the aquatic world—copepods," lead author Brad Gemmell, of UT Austin, said in a release. "People often don't think of seahorses as amazing predators, but they really are."

Copepods are the most abundant group of crustaceans around, but you might not ever see one, as they average just a millimeter or two in length. Despite that, they can really haul ass: One species, Euchaeta rimana, has been clocked in bursts at up to 8.9 cm per second, a stunning speed for a tiny creature.

The dwarf seahorse (Hippocampus zosterae), which was the focus of the study, measure just a few centimeters in length; much bigger than a copepod, but still far from able to keep up. Despite this, the seahorses are successful at catching prey the vast majority of the time, a success rate that's quite rare for predators.

To figure out how seahorses could catch their fast prey, the research team used high-speed digital holography methods developed by co-author Jian Sheng. The techniques involve using a microscope combined with a laser and high-speed camera to capture the seahorses in motion.

Another angle of the pivot feeding maneuver, via the authors

The holography setup is capable of capturing fluid flow along with motion, as you can see in the video above. Or perhaps as you can't see; the most notable aspect of the video is that there's almost no turbulence around the front of the seahorse's head as it moves in on its copepod prey. 

This area of low turbulence above the seahorse's snout allows it to sneak up behind a copepod before jerking its head up and sucking in a meal, a technique called pivot feeding. The relationship between snout morphology and pivot feeding is a consistent area of interest for seahorse research, as it's a fascinating example of evolutionary pressure between predator and prey. Just as missile subs are designed to be as quiet as possible, seahorses have evolved stealthy hunting abilities.

"It's like an arms race between predator and prey, and the seahorse has developed a good method for getting close enough so that their striking distance is very short," Gemmell said.

And while seahorses may not swim very quickly, they can strike with extreme speed—completing the pivot move in as a little as a millisecond, according to the research team. But to pull it off, they have to get as close as a millimeter away, hence the need for stealth. Pivot feeding isn't limited to seahorses, but the ability to strike without creating a telltale wake appears to be.

It's fascinating research, and naturally it comes with larger ramifications. Copepods anchor the world's food chains, and as we've seen, predator-prey relationships can have important cascade effects throughout marine systems. Understanding exactly how copepods evolved in tandem with predators like seahorses—and how other predators, like small fish, have not—can help ecologists understand how the reefs they live in work, which is key to modeling those habitats' health. For embattled coral reefs, that's crucial knowledge.

@derektmead

Front page image via Wikipedia
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