Shine a Little Light
Every day, like clockwork, what may be the world's largest animal migration unfolds around the globe. The migrants aren't wildebeests, or whales, or monarch butterflies. Rather, they're zooplankton -- tiny copepods, miniature jellyfish, and seafaring larvae, trillions upon trillions of them -- and their journey is vertical. By night they swim up, by day they swim down -- a few feet in the smallest ponds, or as far as 500 yards in the open sea, a distance hundreds of thousands of times longer than their tiny bodies. A human would have to row a boat 500 miles to breakfast each day (then row home again) to appreciate this epic journey.
The diel, or daily, migration is one of the greatest movements of biomass on the planet and is critical to aquatic food webs. Yet biologists barely understand it. Larger animals can be electronically tagged, then tracked with satellites; through the motions of one, the motivations of the crowd can be deciphered. Not so with zooplankton: how do you mark one without sinking it, slowing it, altering its behavior? Studies have revealed that zooplankton likely migrate upward for food and downward to escape predators and ultraviolet radiation. But little is known about the relative importance of these factors or what other factors might be involved, much less how this migration could be altered by environmental change.
"We have been stopped for so many years by not being able to follow small things," laments Lars-Anders Hansson, a freshwater ecologist at Lund University in Sweden. Compared with larger animals, "we know so little about them, but they are equally important."
Now Hansson and his team think they've hit upon the right technology for the job. In recent years, biomedical researchers have embraced the use of engineered nanoparticles called quantum dots, or q-dots. Made of cadmium selenide and other semiconductor materials, q-dots fluoresce brightly when hit with certain wavelengths of light, like molecule-size flashlights. Hansson's colleagues in the chemistry department found a way to get q-dots to adhere to amino acids on the carapace of zooplankton. Bathe some animals briefly in a drop of solution containing q-dots and voilà: the q-dots stick by the thousands, "like small bulbs," Hansson says. Being minuscule, the q-dots don't hinder an individual's motion, nor do they appear to interfere with reproduction or survival rates.
Scientific movies are made of this. In the lab, Hansson's team dressed specimens of Daphnia magna, a common water flea, in q-dots, then let them migrate up and down in glass containers. From time to time a light-emitting diode (of a wavelength that the animals couldn't see) was shone onto the jars; the Daphnia lit up in the darkness. Caught on video, their travels recall those of will-o'-the-wisps -- ghostly, with vague intent.
Although barely out of the box, the technology promises to answer fine-scale questions about the diel migrants and what global environmental change may portend. To start, do zooplankton move differently when predatory fish are nearby? Do they actively search for food? One might then ask how these movements may change as seawater becomes more acidic and parts of the ocean grow warmer.
Q-dots also offer a way to better understand how zooplankton respond to ultraviolet radiation. Some zooplankton turn red when exposed to UV; this is the organism's natural defense against DNA damage, but it also provides scientists with a handy way to gauge an animal's exposure level. Q-dots enable researchers to tag groups of exposed and unexposed zooplankton to compare their behavior and begin to tease out the extent to which UV light drives their migration. The goal, Hansson says, is to "disentangle the mechanisms behind why these massive movements occur." That knowledge might soon be put to use, he adds: as the planet's ozone layer thins and UV exposure becomes more intense, changes in the diel migration could alter food webs in heretofore unexpected ways.
"We've opened a kind of door for really looking at the big issues with small animals," he says. "It's a huge leap forward."
What nanotracking offers is a fundamental change in perspective. We're accustomed to thinking of the world's zooplankton (if we think of them at all) as a mindless plural, like microscopic lemmings. Now, Hansson notes, "we can look at them as individuals." Of course, scientists already know a great deal about their biology from studying them under the microscope. But it's another matter to consider them, one by one, as creatures of volition. As fellow voyagers. This knowledge is potentially overwhelming. Are we prepared to know trillions of copepods on a first-name basis? But it is helpfully sobering, even inspiring, to know that so many work so hard each day to get so far, and how, and why.