What does it take to catch your prey 90 percent of the time?
For the seahorse, the shape of its head, along with its stealthy approach and lightning speed, literally makes a meal of some of the ocean’s most sought after prey: the copepod, according to a new study.
A long-snouted, or hairy seahorse, seen in Mallorca, Spain, in 2006.Photography by Visuals Unlimited/Corbis
“When you’re a copepod everything in the ocean essentially wants to eat you,” said study author Brad Gemmell, a marine biologist at the University of Texas at Austin. “They’ve evolved some pretty impressive ways to avoid being captured by predators.”
This includes sensitivity and speed. Copepods, a type of small crustacean, can’t see images and rely on a remarkable sensitivity to fluid disturbances to sense predators approaching.
Once they detect them, they’re outta there, roadrunner style.
“They have one of the shortest response length times seen in the animal kingdom,” Gemmell said: About two or three milliseconds from the time they sense a signal to the time they scram. They can reach speeds of over 500 body lengths per second.
To catch something that fast you have to be just a bit faster, and a previous study had shown that seahorses were better at getting copepods under calm conditions than other fish. Gemmell says they had a 90 percent success rate compared to the 30-40 percent success rate of other predators. (Related: “First Video of Rare Brightly Colored Seahorse.”)
This remarkable lead prompted the question of why seahorses are such wicked predators, and the new study shows that the head shape of the seahorse, “creates this zone with very little disturbance, which allows them to get really close to these very sensitive, highly evasive copepods,” said Gemmell.
Speedy Seahorse
It might seem counterintuitive: the head of a sea horse seems bulkier than other, sleeker fish, but they have what’s called pivot or pipette method of feeding, a trait of the Syngnathid fish family which includes seahorses, pipefish, and sea dragons.
The seahorse snout is narrow, elongated, and rounded on top, and since it’s quite small, it creates less disturbance in the water, allowing it to get within one millimeter of a copepod. It then sucks in prey with its mouth, which isn’t much bigger than the prey itself. It can also extend its mouth away from the thicker part of its head and swing it “forward and upward and cover the distance to the prey in about one millisecond.” (Related: “How Seahorses Evolved to Swim ‘Standing Up.’“)
So even the zippy copepod’s getaway speed of two or three milliseconds is no match for the one millisecond strike of the seahorse.
“Once the seahorse gets within range the story is pretty much written, as far as the fate of the copepod goes,” said Gemmell, whose study is published today in the journal Nature Communications.
According to the study, large tendons of the epaxial muscles—muscles along the vertebrae—are “hypothesized to store and release energy to accelerate the head.”
The researchers used high-speed digital techniques to capture the movements of the live dwarf seahorse, Hippocampus zosterae, and fluid disturbance in 3-D imagery at the strike zone. They also used preserved seahorse specimens and positioned them in flume studies, a method that allows researchers to control the flow of the water so that any disturbance would be clearly generated by the head of the animal.
“The result was this zone of low disturbance is still there, even on a dead fish, so it can be done merely with the shape of the head,” added Gemmell.
Calmer Seas Crucial
Seahorses, Gemmell points out, are less successful in more turbulent environments where other fish have an easier time capturing copepods. The slow, deliberate stealth of the seahorse works in the calmer environments of deep water and coral reefs.
The study refers to the calm space in the water that the seahorse’s head creates as a “quiet zone,” which, combined with other factors, is bad for the copepod.
Guess they’re not familiar with the old stand by, “Things are quiet… a little too quiet.”