Plankton bring the flea circus to the ocean. Like flying trapeze artists or tumbling acrobats, these tiniest of marine animals can really soar above the water's surface. Researchers have caught two species of miniscule crustaceans, called copepods, performing never-before-seen feats of athleticism: To avoid predators, they leap out of the water, completing arcs in the air about 40 times their own length.
Study co-author Brad Gemmell, a marine biologist at the University of Texas, Austin, owes the discovery to his daily routine. On most days after lunch, he takes a short stroll along the university's marina, which sits on the Gulf of Mexico in Port Aransas. On one such walk, he noticed an unusual pattern on the water: "It looked like rain drops hitting the surface," he says. "I thought, 'That's kind of strange.' "
Gemmell ran back to his lab, grabbed a beaker, and scooped up some of the marina water. What he found were lots of copepods—crustaceans smaller than most ants that look like tiny lobsters. He placed the creatures into a tank shared by plankton-eating fish, then watched as some of the copepods literally jumped to safety, landing back down with tiny splashes. Gemmell was amazed. That's not a behavior scientists had ever described before.
So he and his colleagues returned to the marina, this time armed with video cameras. There, they identified two species of jumping crustaceans: Anomalocera ornata—the copepods he had collected on his lunch break—and another related species, Labidocera aestiva. Both are considered big for copepods, reaching a few millimeters in length. The team recorded 89 A. ornata individuals springing for cover as small and predatory fish neared. The animals practically flew, hitting speeds of about 0.66 meters per second in midair and landing up to 17 centimeters away, the group reports online today in the Proceedings of the Royal Society B.
Next, the team collected dozens of L. aestiva copepods and recorded their underwater takeoffs using high-speed video in the lab. Amazingly, the crustaceans launched themselves using just a single swimming stroke in which they kicked each of their five pairs of spindly legs in rapid succession. Just escaping from the water cost the animals up to 88% of their initial energy. But thanks to the low density of air, once the copepods breached, they soared. "They actually travel many times farther through the air than they could with the same jump through the water," Gemmell says. They spun furiously, somersaulting at rates up to 7500 revolutions per minute.
In comparison, other leaping aquatic animals, such as flying fish, lose much less energy in breaching but, due to their greater mass, can't fly nearly as far on their own momentum. Gemmell suspects that A. ornata and L. aestiva both evolved their acrobatic displays largely because they're relatively big, making them easy for fish to spot. Their tumbles likely take them well out of a close predator's line of sight; in the marina, only one of the 89 jumping crustaceans wound up getting eaten on camera.
The study shows just how active some plankton can be, says Jeannette Yen, a biological oceanographer who investigates copepod behavior at the Georgia Institute of Technology in Atlanta. For years, many scientists had assumed that these creatures were only "passive drifters," she says. But that's not the case. Although copepods are pushed and pulled by the currents, she says, "within that realm, they perform amazing behaviors."
Petra Lenz, a marine biologist at the University of Hawaii, Manoa, adds that many biologists likely suspected that some plankton could fly. She used to wonder why the A. ornata individuals she collected always became stuck to the sides of her buckets, well out of the water. It's about time someone finally caught them in the act, she says.