In Disposable Mucus Houses, These Zooplankton Filter the Oceans


When it comes to the flow of carbon in the ocean, “we don’t know nearly as much as we should,” said Kakani Katija, a principal engineer at the Monterey Bay Aquarium Research Institute and the study’s lead author. “If we really want to understand how the system works, we have to look at all the players involved. Giant larvaceans are one important group we need to learn more about.”

In the past, other scientists have tried studying giant larvaceans in the laboratory. But these efforts always failed because the animals’ houses were too fragile to be harvested and collected specimens were never able to build houses outside the ocean.

To study the zooplankton in their natural habitat, Dr. Katija and her collaborators developed a new deep-sea imaging instrument, called DeepPIV, which they paired with a remotely operated vehicle.

DeepPIV projects a sheet of laser light that cuts straight through a larvacean’s mucus house. A high-definition camera on the remotely operated vehicle can then capture the inner pumping mechanisms illuminated by the laser.

Analyzing footage of 10 giant larvaceans, Dr. Katija’s team estimated filtration rates based on how quickly particles were flowing into the animals’ chambers. On average, larvaceans longer than half an inch filtered 11 gallons (about the fuel tank capacity of a small car) of water an hour. The scientists also estimated that the creatures spent approximately two-thirds of their time filtering seawater.

Combining their filtration measurements with long-term data on the abundance of giant larvaceans in Monterey Bay, the researchers calculated that, at peak population density and maximum filtration rate, the bay’s giant larvaceans can sift through all the water in their primary habitat in as little as 13 days.

The filtration rates Dr. Katija’s team calculated are higher than those previously inferred from studying smaller larvacean species in the lab, said Kendra Daly, a zooplankton expert at the University of South Florida who was not involved in this study.

This difference, she added, highlights the importance of “using innovative approaches to actually make measurements in the water.”

The study also demonstrates the usefulness of the new DeepPIV tool, Dr. Katija said. Among other things, she and colleagues want to use the instrument to study how fluid mechanics influence the functioning of other marine organisms and the transport of nutrients through the ocean.

“I recently saw a graphic that said we’ve mapped 100 percent of the moon’s surface and 100 percent of Mars’s surface, but only 5 percent of the Earth’s ocean,” she said. “With more time and investment in developing technologies, I hope we’ll see that start to change.”

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