Tiny underwater robots mimic ocean life

Scientists have developed as warm of tiny underwater robots to study ocean currents and understand the movement of plankton - the most abundant life forms in the sea.

The miniature autonomous underwater explorers (M-AUEs) will study small-scale environmental processes taking place in the ocean. The ocean-probing instruments are equipped with temperature and other sensors to measure the surrounding ocean conditions while the robots 'swim' up and down to maintain a constant depth by adjusting their buoyancy.

The M-AUEs could potentially be deployed in swarms of hundreds to thousands to capture a 3D view of the interactions between ocean currents and marine life. Researchers at Scripps Institution of Oceanography at the University of California San Diego in the US deployed a swarm of 16 grapefruit-sized underwater robots programmed to mimic the underwater swimming behaviour of plankton, the microscopic organisms that drift with the ocean currents.

The study was designed to test theories about how plankton form dense patches under the ocean surface, which often later reveal themselves at the surface as red tides.

"These patches might work like planktonic singles bars," said Peter Franks, a Scripps biological oceanographer who has long suspected that the dense aggregations could aid feeding, reproduction, and protection from predators.

Two decades ago Franks had proposed a theory predicting that swimming plankton would form dense patches when pushed around by internal waves - giant, slow-moving waves below the ocean surface. Testing his theory would require tracking the movements of individual plankton - each smaller than a grain of rice – as they swam in the ocean, which is not possible using available technology.

Scripps research oceanographer Jules Jaffe instead invented "robotic plankton" that drift with the ocean currents, but are programmed to move up and down by adjusting their buoyancy, imitating the movements of plankton. A swarm of these robotic plankton was the ideal tool to finally put Franks' mathematical theory to the test.

"The big engineering breakthroughs were to make the M-AUEs small, inexpensive, and able to be tracked continuously underwater," said Jaffe.

The low cost allowed researchers to build a small army of the robots that could be deployed in a swarm. During a five-hour experiment, researchers deployed a 300-metre diameter swarm of 16 M-AUEs programmed to stay 10-metres deep in the ocean off the coast of California.

The M-AUEs constantly adjusted their buoyancy to move vertically against the currents created by the internal waves. The three-dimensional location information collected every 12 seconds showed where this robotic swarm moved below the ocean surface. The results of the study were nearly identical to what Franks predicted. The study was published in the journal Nature Communications.