Zooplankton explained: what they eat, what eats them, and the examples worth knowing

The name is a straight description. It comes from Ancient Greek zōîon, animal, and planktós, drifter or wanderer. Animal drifters. That is the whole definition, and the important half of it is the second word: plankton are aquatic organisms that cannot swim effectively against a current. They go where the water goes.

This is a category defined by behaviour rather than by ancestry, which is why it contains such wildly unrelated creatures. Zooplankton spans single-celled protozoans at one end and large metazoans at the other. In between sit cnidarians such as jellyfish and the Portuguese Man o’ War; crustaceans including copepods, cladocerans, ostracods, amphipods, mysids and krill; chaetognaths, which are also called arrow worms; molluscs such as pteropods; and chordates, from salps to juvenile fish.

Zooplankton, explained in one paragraph

Plankton splits into two halves by how each half gets its energy. Phytoplankton, meaning cyanobacteria and microalgae, are the plant-like component: they build their own tissue from light and dissolved nutrients. Zooplankton are heterotrophic, which is a formal way of saying they have to eat something else. That single difference cascades into everything below, and it is the reason the two groups occupy different depths, respond to different seasons and matter to oceanographers for different reasons. Almost everything else about the group can be explained from that one starting point.

Phytoplankton vs zooplankton: the difference that decides everything

Set phytoplankton vs zooplankton side by side and you get the base of the marine food web and the layer immediately above it. Because phytoplankton need light, they are concentrated near the surface. Zooplankton are not tied to light in the same way, only to food and to the risk of being seen, and that freedom is what makes their behaviour interesting.

It is worth being precise about the comparison, because phytoplankton vs zooplankton is often treated as a size distinction. It is not. Some zooplankton are smaller than some phytoplankton. The line between them is metabolic, and nothing else.

There is a second distinction inside the animal half, and it matters more than it sounds. Holoplanktonic organisms spend their entire life cycle in the plankton. Meroplanktonic ones spend only part of it there before graduating to the nekton, meaning animals that do swim, or to a sessile life on the seabed. Barnacle and crab larvae are ordinary examples. So a plankton sample is partly a community of permanent residents and partly a nursery, and the balance shifts through the year.

What do zooplankton eat?

Ask what do zooplankton eat and the honest answer is that the group is too broad for one answer. The feeding behaviours recorded across it include filter feeding, active predation and symbiosis with photosynthetic partners. A copepod straining microalgae out of surface water and a jellyfish capturing larval fish are both doing zooplankton feeding, and they have almost nothing else in common.

The most consequential case is grazing on phytoplankton. When you ask what do zooplankton eat and the answer is microalgae, you are describing the transfer step that moves energy from photosynthesis into the animal kingdom. Microzooplankton, the smallest animal grazers, matter out of proportion to their size here, because they also regenerate the nutrients that fuel further primary production. They remain understudied. Routine oceanographic surveys seldom measure microzooplankton biomass or grazing rate at all, even though the dilution technique for estimating herbivory has been available since Landry and Hassett published it in 1982.

What eats zooplankton?

Almost everything, which is the point. What eats zooplankton includes larval and adult fish, baleen whales, seabirds and other zooplankton. The group sits at the narrowest part of the marine energy pipeline: a very large number of consumers depend on it, and it in turn depends on a very thin surface layer of primary production.

This is also why the question of what eats zooplankton has a spatial answer rather than only a taxonomic one. Predation risk is not evenly distributed through the water column, and that is what drives the behaviour in the next section.

Diel vertical migration, and the largest daily movement of life

Zooplankton are carried by currents, but many can move vertically under their own power, and they use it. In diel vertical migration, animals sit deep during daylight where a visual predator cannot pick them out against the surface, then rise towards the food-rich surface layer after dark. Every day, across every ocean, an enormous mass of small animals climbs and descends. It is routinely described as the largest synchronised movement of biomass on the planet, and it is driven by a trade-off that is easy to state: food is up, danger is up, so go up only in the dark.

Push that logic to a place where there is no daylight for months and it should break down. It does not. Work by Kim Last at the Scottish Association for Marine Science and colleagues found that during the Arctic winter, when the sun does not rise, the migration keeps running on the lunar cycle instead of the solar one. Moonlight is faint, but it is enough to be worth hiding from. If you want the reason the sun can be absent for that long, that is the subject of our polar night explainer.

Zooplankton examples you can actually see

Most of this group is invisible without a microscope, but not all of it. The useful examples for getting a feel for the range:

• Copepods. Small crustaceans, a few millimetres long, and among the most abundant multicellular animals in the ocean. If a plankton net comes up with a cloud of tiny hopping specks, this is usually what they are.
• Krill. Shrimp-like crustaceans, large enough to see clearly, and the link between microalgae and baleen whales in polar seas.
• Jellyfish. Proof that being planktonic is about the inability to hold position against a current, not about being small.
• Salps. Barrel-shaped chordates that filter water and can form long chains.
• Larval fish, crabs and barnacles. The meroplankton: temporary members, on their way to another life.

Those zooplankton examples cover single cells to metre-scale animals, which is a reminder that the label is an ecological role rather than a family tree. The zooplankton diet varies just as widely, and so does what preys on them. Nothing about the group can be explained by looking at one member of it.

Why the zooplankton diet matters beyond the food web

There is a carbon argument here too. Animals that feed near the surface at night and sink to depth by day are moving carbon downwards, in bodies and in faecal pellets, on a daily schedule. That flux is part of how the ocean takes carbon out of contact with the atmosphere, which means the zooplankton diet is not only a biology question. It has a climate accounting entry attached to it, and the size of that entry depends on grazing rates that, as noted above, are still rarely measured directly.