The sea horse's signature "S" shape holds a secret weapon: It is an adaptation for the ambush hunting style favored by these tiny, carnivorous fish, according to new research that looked at the biomechanical properties of the sea horse's curvaceous neck region.
Sea horses evolved from long, narrow swimmers similar to pipefishes. Anchored by their prehensile tails, they hover in sculptural stillness near coral reefs and sea grass beds until a tiny shrimp or larval fish swims by. Then, with a subtle tilt of the head and a slurp, the diminutive predators suck in unassuming prey. Both sea horses and pipefishes employ the tilt-and-slurp technique, known as pivot feeding. Sea horses, however, do not pursue their prey as pipefishes do, so the reach of their necks is an important determinant of their hunting success.
"From the first videos I saw of feeding pipefish, I realized that there are some very important things, mechanically, going on in the neck region," says Sam Van Wassenbergh of the University of Antwerp in Belgium, co-author of the research, published in the January 25 issue of Nature Communications. "From there, my idea started to grow that the sea horse shape could play an important role in capturing prey." (Scientific American is part of Nature Publishing Group.)
The researchers used high-speed video and mathematical modeling to show the extent to which the sea horse's equine mien improves its performance during pivot feeding. Studying videos of sea horses and pipefish, the team discovered that although the up-and-down motion of the head was similar in both, the sea horse could also extend their heads forward. "Compared with straight-bodied pipefish, all sea horse species studied consistently show an additional forward-reaching component in the path traveled by the mouth during their strikes at prey," the researchers wrote.
Two mathematical models were then developed, one based on a pipefish, the other on a sea horse. Researchers calibrated the models using a computer program, fine-tuning the flexibility, inertial properties and hydrodynamic resistance of each body segment until the models could simulate the pivot feeding motion seen in the videos. "Mathematical models have always been an important tool for functional morphologists and biomechanicists," Van Wassenbergh says.
By manipulating these models into hypothetical intermediates between sea horses and pipefishes—straight-necked seahorses and bent-necked pipefishes—researchers could test the functional effects of body shape. Although the seahorse's curvy neck provides a longer reach (strike distance), it comes at the cost of speed (strike velocity), they found. Bending the neck on the pipefish model, for example, resulted in a 28 percent increase in strike distance. Conversely, straightening the neck on the sea horse model increased strike velocity 36 percent.
Sea horses need to slurp up dozens of tiny shrimp each day in order to survive. Although their curves make them clumsier swimmers than their streamlined cousins, the gain in striking distance seems well adapted to their stealthy sit-and-wait feeding strategy.
"Sea horses would have colonized habitats where pipefishes were already present, and the increased strike distances would have allowed the newcomers to compete successfully for the same food resources," says Peter Teske, who researches sea horse evolution and ecology at the Rhodes University in South Africa and was not involved in the study. "Overall, I think this is an interesting piece of the puzzle that explains why sea horses look as unusual as they do…. What remains to be fully answered is what the implications of this study are in terms of understanding the evolution of sea horses," he adds.
A clue may be found in the pygmy pipehorse, a fish that resembles both sea horses and pipefishes. Like sea horses, pygmy pipehorses have a prehensile tail, but swim horizontally like pipefish. As pygmy pipehorses share a more recent ancestor with sea horses than they do with pipefish, "their phylogenetic position suggests that evolution of a tail-attached lifestyle preceded the evolution of the bent head in sea horses," the researchers wrote.