Stealthy seahorses sneak up on prey

Calm water above the predator's snout lulls prey into false sense of security

PUBLISHED : Sunday, 08 December, 2013, 6:18am
UPDATED : Monday, 13 June, 2016, 12:45pm

Those cute seahorses are actually stealthily effective ocean predators. Make that one of the most effective predators in the animal kingdom.

The dwarf sea horse is less than 2.5cm 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 seahorse a menacing hunter.

Unlike fish with protruding jaws, the seahorse has a long, thin snout that it rotates towards prey in a swift snatching motion called pivot feeding. This millisecond manoeuvre creates suction that pulls in the seahorse'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 seahorses 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 seahorses and the velocity of the water around them as they approached their prey. They found that the water just above the snout of the seahorse was significantly less turbulent than above or to the side of its body.

As a seahorse orients itself towards 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.

In Gemmell's observations, seahorses that were able to get within a millimetre of the copepods caught them 79 per cent of the time.

Adam Jones at Texas A&M University says stealthy behaviour is a big part of a seahorse's way of life. "Prey seem entirely oblivious to the seahorse's presence until it's too late," he said, "probably because the seahorses move so little while they forage."