Fish Flight

The archer fish is known for its stealthy ability to spit jets of water at unsuspecting bugs, knocking them from overhanging leaves into the water below. Once an insect is shot down, however, it becomes fair game for any passing predator. But it just so happens that the fish can also launch itself out of the water from a near standstill to catch the tumbling insect in midair, ensuring that no other competitor steals its prey.

MIT engineers have detailed the hydrodynamics of the archer fish’s rocket-like jumps in the Journal of Experimental Biology. Coauthors are Anna Shih, SM ’10, Leah Mendelson, SM ’13, PhD ’17, and Alexandra Techet, SM ’98, PhD ’01, associate professor of mechanical engineering.

After training five small archer fish to reliably jump for food, the researchers placed them in a tank of water seeded with polyamide particles—tiny beads invisible to the naked eye. A laser placed below the tank illuminated the beads with infrared light; a high-speed camera captured the motion of the beads and therefore the direction and velocity of any eddies produced in the water.

The team then suspended pieces of freeze-dried shrimp above the tank at heights ranging from one-fourth to more than two times a fish’s body length. The camera recorded 98 jumping sequences, averaging about 16 to 24 jumps per fish.

Frame by frame, the team analyzed each fish’s motions while jumping, as well as the direction and velocity of the particles displaced by the fish’s movements, and identified three general jumping phases: hovering, thrust production, and gliding.

By alternately flapping its pectoral fins and caudal fin, a fish can hover in place just under the water’s surface as it looks for prey. It then raises its pectoral and pelvic fins while beating its tail until it produces enough thrust to launch itself out of the water. Once the fish has broken through the water and into the air, it glides up to the bait without producing further thrust.

Remarkably, the fish were able to launch themselves to a height as great as 2.5 times their body length. On average, the maximum velocities during each run ranged from 0.6 to 1.7 meters per second squared.

“The [Olympic] record for the 100-meter freestyle is a little under 50 seconds, so two meters per second,” ­Mendelson says. “So these fish are almost as fast as an Olympic swimmer, but actually going up rather than horizontal.”