The pectoral fins improve manta ray swimming efficiency by combining up and down oscillations with an undulating wave that travels out towards their tips.
Introduction
The distance between the tips of a giant manta ray’s two pectoral fins can reach to nearly the height of a three-story building, yet the marine animal’s behemoth size does not hinder its grace under water. As manta rays glide through the ocean, they flap their massive triangular fins to pick up speed, not unlike a bird fluttering its wings while riding the sky.
The Strategy
On average, giant manta rays swim about 9 miles per hour (14.5 kilometer per hour), but to evade danger, they can sprint as fast as 22 mph (35.4 kph). To reach these speeds, they combine fin oscillations with undulations.
The oscillations flap the fins up and down, while the undulations send a traveling wave outward from its body to the flexible fins’ tips. As giant rays flap in this manner, their fins disturb the water and create vortices, which push against the surrounding water and propel the animal forward.
Scientists use a mathematical relationship to estimate the difficulty that an animal experiences when moving through a fluid, such as water or air. The “Strouhal number” relates the dimensions of the wing or fin, the animal’s velocity, and the frequency with which vortices are generated by the animal’s flapping.
Researchers who modeled the shape of giant manta ray fins found that as the fins flap, they can create one of two unique wake patterns. The two wakes, which occur under different conditions, correspond to separate peaks in swimming efficiency: one that occurs at a Strouhal number of 0.2, and the other at 0.3. The ability to generate two wake patterns may allow the animals to better control fluid flow around their bodies and appendages, increasing their movement efficiency and allowing them to conserve energy while swimming.
The Potential
Studying flapping manta ray fins may improve our understanding of aerodynamics and yield more efficient underwater vehicles that consume less fuel and emit fewer pollutants. Swimming robots could be used to inspect underwater pipeline integrity and improve our ability to explore the ocean.
