Sid Perkins, “Can Sea Monkeys

 This passage is adapted from Sid Perkins, “Can Sea Monkeys

Stir the Sea?” ©2014 by American Association for the

Advancement of Science.

Winds, waves, and tides are crucial for mixing

the surface waters of lakes and seas, transporting

heat downward and simultaneously bringing

nutrient-rich waters up to the surface where light-

harvesting phytoplankton need them to thrive.

But small marine creatures help such processes as

they migrate to the ocean surface each night to forage

and then return to the relative safety of unlit depths

during daylight hours, some researchers think. One

of the most familiar of these travelers, known to kids

worldwide as the sea monkey, is the brine shrimp

Artemia salina, says John Dabiri, a fluid dynamicist

at the California Institute of Technology (Caltech).

Although the small swirls created by the fast-

churning legs of a single sea monkey are not strong

enough to significantly stir the seas, the eddies kicked

up by billions of them might do the trick, Dabiri and

others have proposed. To test the notion, he and

Monica Wilhelmus, also of Caltech, measured the

tiny currents triggered by artificially induced

migrations of brine shrimp in the lab.

Dabiri and Wilhelmus used blue and green lasers

to induce thousands of 5-millimeter-long brine

shrimp to “migrate” to and from the bottom of a

1.2-meter-deep tank. The creatures are strongly

attracted to those colors, Dabiri says. The researchers

shone the blue laser into the tank and moved it

slowly up and down to control the crustaceans’

vertical movements. The tank’s solid walls could

strongly affect the flow patterns generated by the

shrimp as they swam, so the researchers kept the

shrimp away from the edges of the tank by shining

the green laser beam directly down into the center.

To help visualize the swirls and eddies generated by

the shrimp, the researchers added copious amounts

of silver-coated microspheres to the water and

illuminated them with a red laser, a color that doesn’t

seem to affect the shrimps’ behavior.

The team’s high-speed videos of the teeming,

laser-lit migrations captured images of swirls much

larger than the creatures themselves, which resulted

from the interactions of smaller flows created by

individuals. The larger the swirls, the more effective

the mixing might be, Dabiri says. “So even for slow

migrations, there could be strong effects,” he notes.

Previous studies suggest that light-harvesting

phytoplankton, the base of the ocean’s food chain,

collect about 60 terawatts of solar energy,

Dabiri says. Even if marine organisms that consume

phytoplankton convert only 1% of that power into

mixing the oceans, that’s collectively comparable to

the mixing power of winds and tides, Dabiri and

Wilhelmus report.

“This is a really innovative experimental setup

that provides a nice illustration of flow velocities,”

says Christian Noss, a fluid dynamicist at the

University of Koblenz-Landau. Jeannette Yen, a

biological oceanographer at the Georgia Institute of

Technology, agrees. “I like the idea of using [the

shrimps’] behavior to lure them to the camera,” she

says.

But scientists disagree on how effective billions

of churning sea monkey legs might be in blending

ocean layers that are hundreds of meters deep.

“I wouldn’t want to say just yet that [biomixing] is

important at a global scale” solely based on a lab

experiment, says Stephen Monismith, a fluid

mechanicist at Stanford University. André Visser, a

physical oceanographer at the Technical University

of Denmark, agrees. “Most of the energy [from the

shrimp] probably goes into heating the water” rather

than mixing it, he says.

In fact, the upper and lower layers of the seas have

measurable differences in density, a stratification

that, according to theory, would reduce the efficiency

of any biomixing. And subsequently, experiments

similar to Dabiri’s suggested that stratification stifles

mixing. In that research, Noss and colleague Andreas

Lorke, also of Koblenz-Landau, studied the effects of

large crowds of aquatic creatures called Daphnia

(commonly known as water fleas) as they migrated

up and down in a tank of mildly stratified water. As

expected, the stratification squelched the biomixing

generated by the swimming Daphnia, Noss says.

Those results aren’t surprising, Visser says. “It’s

difficult to lift heavy water up and to push light water

down.”

Dabiri and his colleagues’ next set of lab

experiments will look at the effects of sea monkey

migrations in stratified waters, he says. Those

experiments should reveal whether sea monkeys are

better mixers than water fleas.