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For Plankton, One Size Doesn’t Fit All

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Krill floats through the ocean depths.
Zooplankton like krill are a major food source for many marine species, including whales, penguins, and many fishes.
David Wrobel/Visuals Unlimited

If you’re looking to understand what makes life in the Earth’s vast oceans possible, you have to start small—very small. You have to start with plankton. 

Plankton isn’t a term for a specific species, nor a genus or family. It’s a catch-all for a staggering variety of marine organisms that share one important trait: they’re drifters. In other words, if it lives in the world’s oceans and can’t swim against a current, then it’s plankton.

There are phytoplankton, single-celled organisms that can be found near the ocean’s surface. And then there are zooplankton, animals that come in a range of sizes from remarkably tiny to easily observed with the naked eye.

Innumerable microscopic species, including bacteria and viruses as well as algae and copepods, will spend their entire existence riding the currents, where they are a vital source of sustenance for other ocean life.


Two comb jellies floating deep beneath the sea.
Megaplankton like comb jellies use cilia on their bodies to propel through water.
S. Zankl/Blickwinkel/AGE Fotostock

“Although small and inconspicuous, phytoplankton are the foundation of the oceanic food chain,” says John Sparks, curator in the Department of Ichthyology and curator of the upcoming exhibition Unseen Oceanswhich opens March 9 for Member Preview Days and to the public on March 12. “Phytoplankton serve as a food source for zooplankton, which in turn feed the largest animals in the ocean, such as whale sharks and blue whales.”

Phytoplankton have another key role on Earth: they produce more than half of the world’s oxygen. “They are primary producers, converting sunlight via photosynthesis into their own food energy, just as land plants do,” says Sparks. Just as they soak up sunlight, phytoplankton also absorb carbon dioxide from the atmosphere, transferring it to the deep ocean in a crucial carbon cycle. 

But just because many planktonic species are small, don’t think that they are simple. Consider the diatom, represented by tens of thousands of living species. Despite being single-celled, many species of diatoms craft cell walls called frustules. While they’re invisible to the naked eye, these cellular armors are often intricate and beautiful pieces of engineering when viewed through a microscope.


Diatom exhibits cell walls with a honey-combed appearance, and floats deep beneath the sea.
Many diatom species, such as Amphitetras antediluviana, feature intricate cell walls called frustules.
Perennou Nuridsany/Science Source

And on the other end of the spectrum, there’s megaplankton: any species that measures over 2 mm. Here you’ll find comb jellies, which use rows of cilia along their bodies to propel themselves through the water, and the Portuguese man o’ war, which uses its venom-loaded tentacles to paralyze and kill prey.


Find out more about plankton in Unseen Oceanswhich opens to the public on March 12. Member Preview Days begin March 9.


A version of this story originally appeared in the Winter 2018 issue of Rotunda, the Member magazine.