For their research, Katija and Dabiri trained their sites not on krill but on small jellyfish, which can also swarm in large schools. They tracked how individual jellyfish carried water as they swam upward in the water column by observing the track of glowing dye injected into the water [see video below] as well as by measuring the kinetic energy the jellies generated in their wakes.
But why settle for such small sea dwellers? Although one might expect massive animals, such as whales, to have more impact on mixing individually, Dabiri, an assistant professor of aeronautics and bioengineering, explains that smaller organisms that travel in large schools—crustaceans and zooplankton for example—would have more of a global impact because they're so widespread and numerous.
Per Darwin's theory, however, it is not just critical mass that matters, but body shape. Dabiri explains that the quickest and most efficient swimmers—those that are smooth and bullet-shaped—are the least effective mixers, whereas slower and more saucer-shaped creatures will drag along proportionately more water.
How much water is moving? For it to have much importance for mixing purposes, water needs to be carried about a meter. From the observations and numerical simulations, Dabiri notes, "We expect that fluid is being carried at least on the magnitude of meters—if not tens of meters."
Extrapolating from their work, Katija and Dabiri suggest that in large schools these organisms likely have an even greater mixing power. In a massive krill migration for example, "it will be much more difficult for water to slip through the cracks" and not be carried along, Dabiri says.
But no one is quite sure how—and whether—the dynamic is actually playing out across the world's oceans. "It's not clear how you will go from that to a global model," Dewar says. Other considerations include how organisms' swimming style would affect water transport and how the combined force of these animals' drift might add up to a worldwide impact on ocean circulation. If it turns out to be as large a component as some are beginning to think, it will need to be incorporated into computer climate models. And that would be no small task because today's models are not nuanced enough to include data at the level of a school, much less an individual animal—to say nothing of complexities involving possible feedback loops down the road.
"Our paper raises more questions than it answers," Dabiri acknowledges. But, he says, it is casting light on what might be an important dynamic of oceans that has been right under our noses—or at least our hulls.