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The stealthy seahorse: Weird-shaped head makes it an awesome predator

November 26, 2013|By Deborah Netburn

Don't be deceived by the seahorse's gentle demeanor or slow, regal movements. These bizarrely shaped fish are actually stealthy predators, ideally adapted for slowly sneaking up on one of the ocean's greatest escape artists, and then devouring it in a flash.

Seahorses are opportunistic eaters, but one of their preferred foods is the elusive copepod--a small crustacean that varies in size from 1/10 of a millimeter to a few millimeters across.

Copepods cannot see predators coming at them, but they sense them by detecting subtle changes in water flow when one approaches. And when a copepod is tipped off that a predator may be nearby it snaps into action fast. Its response time is 2 to 3 milliseconds, and it can swim away at speeds of 500 body lengths per second.

To put that in perspective--if a 6-foot person could swim as quickly as a copepod, then she would be cutting through the water at 2,000 mph.  

In theory, the copepod should be at a major advantage when it comes to escaping the seahorse. Seahorses live in the relatively calm waters of grassy seabeds, where it is easier for copepods to detect a disturbance in the water flow. Also, the seahorse's feeding mechanism, which involves whipping its neck back and suctioning in its prey, means it has to get very close to whatever it wants to eat. 

And yet, researchers at the University of Texas at Austin observed that when a seahorse attempts to prey on a copepod, it is successful 90% of the time. 

So, how does the seahorse beat the copepod nearly everytime? The answer is in the weird, elongated shape of its head, according to a new study in the journal Nature Communications.

"A seahorse's mouth is pretty small, and it is extended away from the thicker, blunter part of the head that generates most of the water disturbance," said Brad Gemmel, a researcher at the University of Texas at Austin and the lead author of the paper. "Having this elongated snout minimizes the water disturbance around the prey."

Gemmel is specifically interested in how animals interact with fluids to capture and avoid prey. In this study, he put a seahorse in a small tank with a few copepods, and then seeded the water with small, buoyant particles. The particles acted as markers that helped the scientists see what was happening with the fluids. Then they used a technique called high speed holography that let them observe the fluid motion.

That experiment showed that there was very little water disturbance right at the strike zone, and a normal amount of water disturbance around the head of the seahorse. To make sure that the living seahorse wasn't moving the water in some way--perhaps by slightly sucking it back, the scientists re-created the experiment with preserved animals, to see how their head shape affected the water.

The result was the same. The lack of water disturbance was related to the seahorse's long snout. 

While other fish with blunter heads also feed on copepods they are much less successful at it. Their success rate is more like 40%, Gemmel said. But then again, their feeding process is less complex.

"It takes the seahorse a little time to get in that strike zone," Gemmel said. "So even though they are very successful, the rate at which they strike prey may be less than other fish."

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