One of the most effective predators in the animal kingdom is smaller than your thumb, and it won’t win any races.
The dwarf sea horse is less than an inch long, and with its S-shaped body and small dorsal fin, it’s going nowhere fast. But oddly enough, this unusual body shape and lack of speed make the sea horse a menacing hunter.
Unlike fish with protruding jaws, the sea horse has a long, thin snout that it rotates toward prey in a swift snatching motion called pivot feeding. This millisecond maneuver creates suction that pulls in the sea horse’s prey, but it works only at extremely close range.
“We knew that these sea horses were feeding successfully by doing this short-range swinging motion,” says Brad Gemmell of the University of Texas at Austin’s Marine Science Institute. “But they must first overwhelm the ability of the prey to escape. Our question was: How do they get so close without alerting their prey?”
This is no mean feat. Copepods, the tiny crustaceans that dwarf sea horses eat, are highly sensitive to changes in the water around them. They rely on small sensitive hairs to detect motion, and once they sense danger they have one of the fastest escape responses of any organism on the planet: They are able to flee at 500 body lengths per second. By comparison, a cheetah can manage only 30 body lengths per second.
Gemmell and a team of researchers used high-speed cameras to measure the movements of sea horses and the velocity of the water around them as they approached their prey. They found that the water just above the snout of the sea horse was significantly less turbulent than above or to the side of its body. As a sea horse orients itself toward its prey, the calmer water directly above its mouth allows it to sneak up and pounce.
Once the creature is in range, the sea horse’s mouth covers the distance to the copepod in less than a millisecond, giving it little chance of escape. In Gemmell’s observations, sea horses that were able to get within a millimeter of the copepods caught them 79 percent of the time. They were far more successful when they approached slowly.
Adam Jones at Texas A&M University in College Station studies the behavior of sea horses and pipefish. He says stealthy behavior is a big part of a sea horse’s way of life. “Prey seem entirely oblivious to the sea horse’s presence until it’s too late,” he said, “probably because the sea horses move so little while they forage.”