Could the humble brine shrimp, a few millimeters in length, be partly responsible for the large-scale motion of the ocean? That’s the finding from a pair of Caltech researchers, who used lasers to herd brine shrimp around a tank and track their effects on the water’s movement. The results, published in the journal Physics of Fluids, show that ocean currents may be a function not only of large-scale physical phenomena like wind and tides, but also of living things.
Study coauthor John Dabiri, a Caltech fluid dynamicist, latched on to the idea a few years ago while studying how jellyfish move. He noticed something strange: As the jellyfish swam, they appeared to be carrying a lot of water with them.
Dabiri wondered if the same was true of smaller animals that fill the world’s oceans, including krill and copapods. He decided to study brine shrimp, perhaps best known in popular culture from the grow-your-own-sea-monkey kits. (Sea monkeys are a brine shrimp hybrid not found in nature; the scientists studied a species known as Artemia salina.)
The tiny creatures, only a few millimeters long, are sensitive to light; they rise up during the night to feed on phytoplankton and sink back into the darker depths during the day.
Dabiri and graduate student Monica Wilhelmus wondered if these dramatic vertical migrations — in some cases, hundreds of meters — could be affecting the mixing of the water. But it’s hard to track brine shrimp movements in their natural environment because, unlike the population of jellyfish studied earlier, it’s hard to figure out where the tiny animals will be at any given time.
“We spent a lot of time scratching our heads trying to figure out a good way to study these animals and the role that they play,” Dabiri said.
But the researchers realized they could actually test the animals in the lab — by controlling their movements with light.
Brine shrimp are sensitive to certain wavelengths of light, which the researchers used to guide the animals’ movements. They filled a tank with water and moved a blue laser up through the liquid, which the brine shrimp followed. They shone a green laser in the middle of the tank to keep the brine shrimp away from the edges.
Then they used a red laser sheet to track the water movements, because the brine shrimp don’t seem to notice that particular color. The red light reflected off small silver-coated beads that were added to the water and revealed how the water was flowing. (continued…)
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