Beautiful, often elusive, and a favorite fish with people everywhere, the seahorse gets its name because of its resemblance to members of the equine family – that is, it looks a lot like a tiny horse! These unique little creatures are found in shallow temperate and tropical waters throughout the world, preferring to inhabit sheltered areas such as mangroves, coral reefs, estuaries, and beds of seagrass. For some time now, scientists have been puzzled about the fish’s horse-like head – and now, researchers from the University of Texas at Austin have arrived at an answer.
Blame it On Copepods
Copepods are tiny crustaceans with no sense of sight. They rely on sensitivity to disturbances in the water around them to detect predators, and when movement is sensed, they flee rapidly. Brad Gemmell, a marine biologist working on the seahorse study, reports that these little critters have one of the shortest response length times seen anywhere in the animal kingdom, scrambling to elude predators just two to three milliseconds from the time they sense a disturbance nearby.
What does the copepod have to do with the way a seahorse’s head is shaped? As it turns out, everything. To catch prey that moves as quickly as a copepod, a predator has to be even faster; seahorses are better at catching copepods than other fish are, particularly when conditions are calm.
Their success is directly linked to their head shape, which according to Gemmell, “creates this zone with very little disturbance, which allows them to get really close to these very sensitive, highly evasive copepods.” The seahorse’s narrow, elongated snout creates barely any disturbance in the water, allowing the fish to get to within a millimeter of its prey undetected. From that close distance, the seahorse then vacuums the prey into its mouth. Seahorses can also extend their mouths away from the thicker portion of their heads, after which they swing the mouth forward and upward, covering the distance to the prey in just about a millisecond – half the time it takes a copepod to bail out.
During the study, researchers used high-speed digital cameras to capture movements made by dwarf seahorses, as well a fluid disturbance in 3-D within the fish’s strike zone. The results were amazing; even when the seahorse was at rest or when preserved seahorse specimens were positioned in a flume, very little disturbance occurred around the fishes’ heads and mouths.
This groundbreaking study also provides clues to another question: Why is it that seahorses prefer to inhabit the areas they do? The answer it seems is a surprisingly simple one: when water is turbulent, other fish have an easier time catching copepods because the crustaceans can’t tell whether disturbance is happening because of current or because of the presence of a predator. As a result, there are more copepods for seahorses to eat in areas where water tends to be calm, and where the seahorse’s quiet, deliberate stealth provides it with an advantage.