Seahorse Heads Have a ‘No Wake Zone’ That’s Made for Catching Prey

Nov. 26, 2013

AUSTIN, Texas — Seahorses are slow, docile creatures, but their heads are perfectly shaped to sneak up and quickly snatch prey, according to marine scientists from The University of Texas at Austin.

Video still of dwarf seahorse

Watch a video of a dwarf seahorse catching a copepod in slow motion: http://youtu.be/_tHJdoqXnqI

“A seahorse is one the slowest swimming fish that we know of, but it’s able to capture prey that swim at incredible speeds for their size,” said Brad Gemmell, research associate at the University of Texas Marine Science Institute, which is part of the College of Natural Sciences.

The prey, in this case, are copepods. Copepods are extremely small crustaceans that are a critical component of the marine food web. They are a favored meal of seahorses, pipefish and sea dragons, all of which are uniquely shaped fish in the syngnathid family.

Copepods escape predators when they detect waves produced in advance of an attack, and they can jolt away at speeds of more than 500 body lengths per second. That equates to a 6-foot person swimming under water at 2,000 mph.

“Seahorses have the capability to overcome the sensory abilities of one of the most talented escape artists in the aquatic world — copepods,” said Gemmell. “People often don’t think of seahorses as amazing predators, but they really are.”

In calm conditions, seahorses are the best at capturing prey of any fish tested. They catch their intended prey 90 percent of the time. “That’s extremely high,” said Gemmell, “and we wanted to know why.”

For their study, Gemmell and his colleague Ed Buskey, professor of marine science, turned to the dwarf seahorse, Hippocampus zosterae, which is native to the Bahamas and the U.S. To observe the seahorses and the copepods in action, they used high-speed digital 3-D holography techniques developed by mechanical engineer Jian Sheng at Texas Tech University. The technique uses a microscope outfitted with a laser and a high-speed digital camera to catch the rapid movements of microscopic animals moving in and out of focus in a 3-D volume of liquid.

dwarf-seahorse-square

Seahorses like this one have a head shape that allows them to sneak up on sensitive prey before they can escape. Photo provided by Brad Gemmell.

The holography technique revealed that the seahorse’s head is shaped to minimize the disturbance of water in front of its mouth before it strikes. Just above and in front of the seahorse’s nostrils is a kind of “no wake zone,” and the seahorse angles its head precisely in relation to its prey so that no fluid disturbance reaches it.

Other small fish with blunter heads, such as the three-spined stickleback, have no such advantage.

Gemmell said that the unique head shape of seahorses and their kin likely evolved partly in response to pressures to catch their prey. Individuals that could get very close to prey without generating an escape response would be more successful in the long term.

“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,” he said.

Seahorses feed by a method known as pivot feeding. They rapidly rotate their heads upward and draw the prey in with suction. The suction only works at short distances; the effective strike range for seahorses is about 1 millimeter. And a strike happens in less than 1 millisecond. Copepods can respond to predator movements in 2 to 3 milliseconds — faster than almost anything known, but not fast enough to escape the strike of the seahorse.

Once a copepod is within range of a seahorse, which is effectively cloaked by its head shape, the copepod has no chance.

Gemmell said that being able to unravel these interactions between small fish and tiny copepods is important because of the role that copepods play in larger ecosystem food webs. They are a major source of energy and anchor of the marine food web, and what affects copepods eventually affects humans, which are sitting near the top of the web, eating the larger fish that also depend on copepods.

Gemmell, Buskey and Sheng published their research this week in Nature Communications.

For more information, contact: Lee Clippard, College of Natural Sciences, 512-232-0675; Brad Gemmell, University of Texas Marine Science Institute, 512-983-0244, brad.gemmell@utexas.edu; Ed Buskey, University of Texas Marine Science Institute, 361-749-3102, ed.buskey@utexas.edu

8 Comments to "Seahorse Heads Have a ‘No Wake Zone’ That’s Made for Catching Prey"

1.  Robert said on Nov. 26, 2013

Very cool study! Who ever thought such a docile looking creature would turn out to be so ferocious.

2.  Mike Gunter said on Nov. 27, 2013

Cool!

3.  Roy Niles said on Nov. 27, 2013

Did the adaptive behavior require the evolution of the head, or did accidental changes in the head require the evolution of the behavior? I vote for behavior as responsible, in this case anyway.

4.  Roy Niles said on Nov. 27, 2013

I wonder whether the trial and error learning of behavior then evolved the head or whether the accidental changes of the head evolved the behavior.

5.  Roy Niles said on Nov. 27, 2013

Thought provoking article. Was the no wake zone actually made, as the title indicates, or is this an example of how the accidental nature of evolution works wonders?

6.  Gordo said on Dec. 5, 2013

Evolution is a continuous series of mutations some beneficial some not. This was a good one, so the next generation of that individual all get it which for a seahorse is probably about a million. Next one will be cooperative behavior and they will start attacking swimmers. Don't think so, wait a million years and you'll see.

7.  Spencer said on Dec. 5, 2013

To Roy Niles: I believe this evolved morphology would always be a random change that happened to be beneficial and was selected for because it made their behavior more advantageous. It starts to sound a bit too Lamarckian if you say behavior is driving evolution.

8.  judith walker said on Dec. 11, 2013

Sea Horses and the many relations they have, all seem to have this basic horse shaped head. Even with amazingly diverse body shapes and styles. And most eat copepods.