Although they have a majestic, equine-like head and neck that look like those of horses, seahorses don’t gallop — they swim very slowly. Forty-six species of these small marine fish, and they are all part of the genus Hippocampus, a word that comes from the ancient Greek hippókampos – meaning “horse” and “sea monster.”
Ironically, the hippocampus is also the name of an important and complex region in the brain embedded deep into temporal lobe that has a major role in learning and memory. The human hippocampus and fornix (arch) in the hippocampus has been compared with a seahorse because it looks like one.
Aside from the horse-like structure of its head and, the seahorse also has segmented bony armor, an upright posture, a bent neck, a distinctive trunk and tail and a curled prehensile tail. Its long snout is used to suck up food; its eyes can move independently of each other like those of a chameleon. It is also highly unusual as it is the only species in which the male gets pregnant and gives birth.
Seahorses are found mostly in shallow tropical and temperate salt water throughout the world, living in sheltered areas such as estuaries, seagrass beds, coral reefs, and mangroves. Three species live in the Mediterranean Sea – H. guttulatus (the long-snouted seahorse), H. hippocampus (the short-snouted seahorse), and H. fuscus (the sea pony).
Seahorses are very small, ranging in size from 1.5 to 35.5 centimeters (5⁄8 to 14 inches). They are named for their equine appearance. They also swim very poorly, rapidly fluttering a dorsal fin and using pectoral fins to steer. The slowest-moving fish in the world is H. zosterae (the dwarf seahorse), with a top speed of about 1.5 meters (five feet) per hour. Since they are poor swimmers, they are most likely to be found resting with their prehensile tail wound around a stationary object – just like poor human swimmers hold onto the edge of a swimming pool instead of going into deep waters.
However, despite being relatively slow swimmers, they are skillful at preying on small, quick-moving animals. In a new study conducted at Tel Aviv University (TAU), researchers have succeeded in characterizing the incredible preying capability of seahorses, discovering that they can move their head up at the incredible speed of 0.002 seconds. The rapid head movement is accompanied by a powerful flow of water that snags their prey right into the seahorse’s mouth.
The study was led by Prof. Roi Holzman and the doctoral student Corrine Jacobs of the School of Zoology at TAU’s George S. Wise Faculty of Life Sciences and the Steinhardt Museum of Natural History and was conducted at the Interuniversity Institute for Marine Sciences in Eilat. The study was published in the Journal of Experimental Biology under the title “Rearing seahorses trigger powerful gulps.”
The researchers explained that, in addition to the male pregnancy, seahorses are fish that possess unique properties such as square tail vertebrae and the unique eating system. For most of the day, seahorses are anchored with their tail to seaweeds or corals with their head tilted downward, close to their body. However, when they detect prey passing over them, they lift their head at incredible speed and catch it. According to Holzman, while preying, seahorses turn their body into a kind of spring. Using their back muscles, they stretch an elastic tendon and utilize their neck bones as a “trigger” just like a crossbow. The result is faster than even the fastest muscle contraction found anywhere in the animal world.
But until now it was not clear how the spring-loaded mechanism enabled seahorses to eat. Just as anyone who tries to remove a fly from a cup of tea knows, water is a viscous medium, and the fish needs to open its mouth to create a flow that draws the prey in. But how do seahorses coordinate snagging in prey with their head movement?
In their new study, the TAU scientists succeeded in characterizing and quantifying seahorse movement by photographing their attack at a speed of 4,000 images per second and using a laser system for imaging water flows. This measurement showed that the “crossbow” system serves two purposes – to promote head movement and generate high-speed suction currents that are 10 times faster than those of similar-sized fish. These advantages enable seahorses to catch particularly elusive prey.
The new measurements also help shed light on the ecology of various species of seahorses, distinguished from each other by the length of their noses. “Our study shows that the speed of head movement and suction currents are determined by the length of a seahorse’s nose,” Holzman added. “From the evolutionary aspect, seahorses must choose between a short nose for strong suction and moderate head raising or a long nose for rapid head raising and weaker suction currents. This choice, of course, corresponds to the available diet. Long-nosed species catch smaller, quicker animals, while short-nosed species catch heavier ones.”
Seahorses are not the only instance of the impressive spring mechanism, he continued. Actually, seahorses are counted among the family of fish bearing the appropriate scientific name “Misfit Fish,” including species such as alligator pipefish, shrimpfish, and cornetfish or flutemouths.
“These fish are called that because of their odd shape which enables stretching their body into a spring. The big question applies to the evolution of the spring mechanism, how it was formed and when it developed. I hope our recent study will lead to further studies designed to help solve the riddle of spring fish.”
