Why do seahorses resemble horses? – January 25, 2011
The wacky, curvaceous shape of a sea horse makes it easier to catch its prey without having to go to the trouble of swimming very far, a new study in Nature Communications has found.
Seahorses employ a sit-and-wait strategy, hiding behind lush sea grasses till their meal – small shrimp or fish larvae – happen along. At that point, they snap their heads upwards toward the prey and use suction to draw the meal into their snouts.
The tendons of the rotational muscles of the fish are like elastics that snap the head upward remarkably quickly as the prey passes by. The whole process is called pivot feeding and takes about 5 milliseconds. Pipefish and sea dragons, which also fall in the Syngnathid family of fishes, use the same mechanism.
But why the seahorse evolved a head that is bent in relation to the rest of the body in a horse-like manner has been a mystery. Their evolutionary ancestors resembled the pipefish, with their trunk and head in a straight line.
Sam Van Wassenbergh at the University of Antwerp and his colleagues now suggest the bent head gives the seahorse an advantage by allowing it to move its head further and strike at a greater distance.
Consider the linear pipefish, Syngnathus leptorhynchus, which also uses the pivot feeding mechanism. When the creature rotates its head toward the prey to capture it, the sudden momentum is transferred down the body to create movement in the trunk. The head movement itself is restricted.
This is not a problem for fish that swim around while hunting, such as the pipefish, because they can compensate for a decreased head strike range by moving themselves forward, say the scientists. But for a creature that refuses to budge to hunt for food, even small improvements in its strike range can confer a fitness advantage.
In seahorses, the trunk is at a sharp angle to the head and so has significant inertia. When the head rotates, the trunk reacts but less than in a pipefish. In addition, there is a compressed region in the bent neck that snaps the head forward to increase the speed of the strike.
“When you shoot a rifle, you get a recoil movement. Same thing happens with seahorses,” says Wassenberg.
The researchers created a biomechanical model to study feeding, and confirmed their model by studying real world data collected from videos of prey capture by various seahorse and pipefish species.
“Results were quite spectacular, I think,” says Wassenberg.