A time lapse of migrating brine shrimp (white) and particles suspended in the water (yellow) reveals large, swirling currents created by the swimming animals that mix the surrounding water.
Plankton—tiny organisms that move with water currents—constitute a large fraction of the biomass of the world's oceans. Many plankton migrate upward and downward in response to daylight cues, but the local and global impact of their daily motions on ocean currents has yet to be quantified.
Two researchers at the California Institute of Technology (USA) recently stimulated brine shrimp with lasers and measured the flow of salt water surrounding their vertical movements (Phys. of Fluids, doi:10.1063/1.4895655). The currents generated by their movements indicate that these small-sized crustaceans could have a large effect on global circulation patterns in the oceans.
Brine shrimp, marketed as “Sea-Monkeys” to generations of schoolchildren, exhibit diel vertical migration, meaning that they migrate upward and downward in response to cues from the sunlight that filters through the water. The Caltech scientists used two-week-old, 5-mm-long specimens, which beat their tiny legs in a metachronal pattern (think of “the wave” going through the crowd at a sports stadium).
The researchers corralled the miniature crustaceans with green and red lasers shining through the walls of their acrylic tank and induced their migration by slowly raising and lowering a blue laser on a vertical track. Micron-scale silver-coated beads mixed into the salty water allowed the scientists to visualize the flow of the water in response to the shrimp movements. These movements trigger a Kelvin-Helmholtz instability, which leads to the formation of eddies much larger than the shrimp themselves.
“We use lasers because that allows us to shape the light into sheets and cylinders of various dimensions, which in turn allows us to manipulate the spacing of the organisms,” says John O. Dabiri, a professor of aeronautics and bioengineering. “We also conducted particle image velocimetry, which is most conveniently accomplished with a laser source.”
According to Dabiri, the researchers plan to add a density gradient—variations in salinity and temperature—to the water in the tank to make it a bit more like the ocean. To get true verisimilitude, though, they will eventually need to take their lasers to the sea itself.