An Ode to Zooplankton, the Hidden Heroes of the Sea

These charismatic animals power the marine world as we know it.

Composite image of ten different species of zooplankton on black background.
The group of animals known as zooplankton is incredibly diverse. It’s populated by creatures that differ wildly in shape, size, feeding habits and more. Karen Osborn, Smithsonian Institution

Scoop some seawater into your hands during a trip to the beach, and it might not look like much: murky with some flecks of debris. But if you took a sample of that water into the lab and put it under a microscope, you’d see a profusion of life.

Many of the creatures you’d spot on closer inspection are members of a vast and vital group called the zooplankton. They’re denizens of the open ocean, spending their days in the massive water column between the surface and the seafloor.

As they feed, grow and die in this underwater expanse, zooplankton act as a cornerstone for ecosystems in the ocean and beyond. They’re crucial nodes in the marine food web, key links in the global carbon cycle and the leaders of the world’s largest migration.

With that kind of power, these animals stand to cause a lot of chaos if they’re disturbed. As climate change wreaks havoc on the marine environment, putting many species of zooplankton at risk, the consequences could ripple far beyond their microscopic water world.

A cast of characters

Copepods like this one are one of the most populous groups of zooplankton in the ocean. While they graze on plant-like organisms called phytoplankton, many filter-feeders, including large animals like manta rays and some whales, dine on them. Karen Osborn, Smithsonian Institution

Marine species are called plankton when they can’t swim powerfully against a current. The animal members of that group are known as zooplankton.

“It’s such a diverse assemblage of organisms,” said Allen Collins, a zoologist with NOAA Fisheries and curator of gelatinous zooplankton at the Smithsonian National Museum of Natural History.

Some zooplankton, including many copepods, spend their entire lives drifting in the water column. Others, like sea urchins, emerge from their eggs as planktonic larvae but eventually secure themselves to the seafloor to mature into adults. Zooplankton range in size from single-celled organisms like the hard-shelled foraminifera to larger drifters like gelatinous comb jellies, tunicates and siphonophores — which include some of the longest animals in the world.

Although they’re found throughout the world ocean, where exactly they congregate depends on a range of factors, from water temperature and salinity to food availability and predators.

“To us, it looks like one big pot of soup out there,” said Karen Osborn, a zoologist and curator of marine worms and crustaceans at the museum. To the zooplankton, which respond to extremely subtle variations in the water column, the ocean is far from homogeneous. “You can put a sensor down in the water and pick up millimeter-thick layers in the ocean, and there are animals that are keyed in on that layer,” Osborn said.

Single-celled organisms known as foraminifera can live on the seafloor or in the water column as plankton. They build protective shells, called tests, out of calcium carbonate. Brian Huber, Smithsonian Institution

Surface tension

Though they might not be Olympic swimmers, many zooplankton participate in the largest migration on the planet, traveling the hundreds of meters between the sun-filled surface waters and the dark depths on a daily basis.

For many species, this epic trek is the key to surviving in the water column. “It’s like playing hide-and-seek on a football field,” Osborn said. “You’re out in this huge three-dimensional space with no structure and nothing to hide behind, and getting found usually means getting eaten.” To steer clear of predators, zooplankton will retreat to deeper, darker waters during the day and only venture to the surface for food under cover of night.

Not every species moves on the same schedule, and researchers are still untangling all the reasons for this journey. What is clear is that when zooplankton migrate, many other species — including other zooplankton — take advantage of the daily pattern.

Take the foraminifera, for example. One species, covered in spikes that act like fishing poles, lies in wait for migrating copepods. “They effectively form a blanket right above where the copepods go to hide, so the copepods have to swim through this layer of little land mines,” Osborn said. “If the copepods touch the sticky fishing lines, the foraminifera reel them in and eat them.”

Many other animals that feed on zooplankton, like fish, follow the migrators in search of a meal. And the animals that eat those zooplankton predators, like seals, toothed whales and sea turtles, get in on the action, too — meaning that the zooplankton’s game of hide-and-seek is responsible for the daily dynamics of the entire system.

Fueling the food web

Those bigger animals have little choice but to trail the zooplankton up and down the water column. The entire marine food web relies on them. Where zooplankton go, everyone else follows.

“Zooplankton are essential for a healthy ecosystem,” said Paula Pappalardo, an ecologist who studies plankton biodiversity at the museum. “The tiny zooplankton will eat the phytoplankton, which get their energy from the sun through photosynthesis.”

Those zooplankton are eaten by larger zooplankton, which are eaten by fish, which are eaten by marine mammals, seabirds and humans — making zooplankton crucial for survival far beyond the shoreline. Some colossal ocean creatures, like baleen whales, even feed on zooplankton directly, producing a vast web of predators and prey with zooplankton at its center.

All that eating, waste-producing, growing and dying also means that zooplankton play a critical role in the global carbon cycle. “They are very important for what’s known as the biological pump,” Pappalardo said. “Because of their vertical migration, zooplankton help move organic matter to deeper waters, and their fecal pellets and decomposing bodies sink through the ocean as part of the marine snow. Those processes transport carbon down the water column and eventually it gets buried in ocean sediments,” a key step in the movement of carbon around the planet.

Gelatinous zooplankton, like this comb jelly, shape marine ecosystems. They eat enormous numbers of other zooplankton and fish, sometimes dramatically affecting the populations of those species. They also act as a major exporter of carbon to the deep sea and can even change surrounding water chemistry with the waste they produce. Robert Aguilar, Smithsonian Institution

Turning tides

So many intricately connected processes means that jostling the zooplankton population, particularly through human disturbances like climate change, could have cascading consequences.

“Everything seems to be in a delicate balance out there,” Osborn said. “If you really imbalance something, it’s probably going to have a pretty huge effect, and we don’t know what the downstream changes are going to be.”

Researchers are working to unravel how zooplankton respond to a stressed environment. Laboratory studies show that ocean acidification from soaring carbon dioxide can make it harder for zooplankton to build their shells, though the effects differ among species. And, Pappalardo pointed out, there is already evidence that zooplankton populations are shifting geographically to escape warming waters.

That’s just the beginning. Scientists are continually discovering new ways that human-caused disruptions could affect zooplankton.

“With less ice in the Arctic, more sunlight will make it into the water,” Collins said. “The organisms there are likely to respond by changing their vertical migrations” as the threat of detection by predators shifts. Plus, none of these disturbances are playing out on their own. Recent research has investigated how zooplankton respond to multiple stressors, like warming and acidification, that happen at the same time.

All of these threats make research on zooplankton particularly urgent. “Many of these organisms are understudied,” Pappalardo said. “You need a baseline of what a community looks like to see if it’s changing and how it’s changing.”

Researchers use a variety of tools, including nets like this one, to study plankton up close. Bob Embley, NOAA Pacific Marine Environmental Laboratory

One of the efforts to capture that baseline was the StreamCode project, a series of expeditions in 2017 headed by the museum and the Smithsonian Marine Station at Fort Pierce, Florida. Pappalardo, Osborn and Collins all participated in the project, which analyzed organisms based on their genetics and anatomy to document the biodiversity of zooplankton in the Gulf Stream, a major current off the coast of Florida.

Projects like these, plus innovations in acoustic sensors and video identification techniques, offer researchers a clearer picture of this endlessly complex community. With that data in hand, scientists are better equipped to monitor changes in the zooplankton and track the ripple effects on other marine life, coastal environments and humans.

There’s also a simpler reason to protect these animals, Collins pointed out. “I like to emphasize that they are on this planet just like humans are, and they have these evolutionary lineages that go back millions and millions of years,” he said. “Who are we to mess it up for them?”

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