Zooplankton are the part of the plankton group that must eat other living things to survive. The word "zooplankton" comes from Ancient Greek: "zōîon" means "animal," and "planktós" means "drifter" or "wanderer," so together it means "animal drifter." Plankton are living things in water that cannot swim strongly against currents. Instead, they move with the water in oceans, seas, lakes, or rivers.

Zooplankton differ from phytoplankton, which are the plant-like part of the plankton group. The prefix "phyto-" comes from the Greek word for "plant," though phytoplankton are not true plants. Zooplankton are heterotrophic, meaning they must eat other organisms for food. Phytoplankton are autotrophic, meaning they make their own food using sunlight and a process called photosynthesis. Because zooplankton cannot make their own food, they eat other organisms, often phytoplankton, which are usually smaller. Most zooplankton are too small to see without a microscope, but some, like jellyfish, are large enough to see with the naked eye.

Many single-celled organisms called protozoans are zooplankton. Examples include zooflagellates, foraminiferans, radiolarians, some dinoflagellates, and tiny marine animals. Larger zooplankton include jellyfish-like cnidarians, ctenophores, mollusks, arthropods, tunicates, and planktonic arrow worms and bristle worms.

The difference between autotrophs and heterotrophs is not always clear in very small organisms. Studies of tiny marine plankton show that more than half of microscopic plankton are mixotrophs, meaning they can get energy and carbon from both sunlight (using internal structures called plastids) and other food sources. Many small zooplankton are mixotrophic, which means they could also be considered phytoplankton.

Overview

Zooplankton are heterotrophic (sometimes detritivorous) plankton. The word "zooplankton" comes from Ancient Greek: "zôion," meaning "animal," and "planktós," meaning "wanderer" or "drifter." Zooplankton include organisms of many sizes, from tiny protozoans to larger metazoans. Some zooplankton, called holoplanktonic, live their entire lives in the plankton. Others, called meroplanktonic, spend part of their lives in the plankton before becoming nekton (free-swimming organisms) or settling on the ocean floor. While water currents move most zooplankton, many can move on their own to avoid predators or find food, such as during diel vertical migration, when they move up and down in the water column.

Zooplankton are not spread evenly throughout the ocean. Like phytoplankton, they form "patches" in certain areas. Some species are limited by factors like salinity and temperature, while others can survive in a wide range of conditions. Zooplankton distribution is also influenced by biological factors, such as reproduction, predation, and the movement of phytoplankton, as well as physical factors like water mixing (upwelling and downwelling), which affects nutrient availability and phytoplankton growth.

Zooplankton play an important role in aquatic food webs. They consume phytoplankton and other food sources, providing energy for larger animals like fish. They also help move organic material through the ocean via the biological pump. Zooplankton respond quickly to changes in phytoplankton abundance, such as during the spring bloom. They are also involved in the spread of pollutants like mercury through the food chain.

Models that include zooplankton help scientists study ecosystems. Examples are biogeochemical models, ecosystem models, and size-spectra models, which also consider how these factors change over time and space.

Ecologically important zooplankton groups include protozoans like foraminiferans, radiolarians, and dinoflagellates (some of which can perform both photosynthesis and consume food). Important metazoan zooplankton include jellyfish, crustaceans like copepods and krill, arrow worms, pteropods (a type of mollusk), and salps. Zooplankton have diverse feeding habits, such as filter feeding, hunting, or living with phytoplankton (as in corals). They eat bacteria, phytoplankton, other zooplankton, detritus, and even nektonic organisms. Because of this, zooplankton are often found in surface waters where food is plentiful.

Zooplankton can also carry disease-causing bacteria. For example, crustacean zooplankton can host Vibrio cholerae, the bacterium that causes cholera. The bacteria attach to the chitinous exoskeletons of these zooplankton, which provides them with nutrients to survive in the ocean.

Size classification

Body size is considered a key trait for plankton because it is a physical feature shared by many organisms across different groups. This trait influences how organisms function in ecosystems, affecting their growth, reproduction, feeding habits, and survival. Over 170 years ago, scientists observed that larger species are often found in colder, higher latitude regions. This idea is known as Bergmann's rule.

In ocean ecosystems, body size plays a major role in how plankton interact with other organisms, which affects how efficiently carbon is moved through the food chain. Body size is influenced by temperature because the processes that control growth and metabolism depend on external heat. Most plankton are ectotherms, meaning their body temperature is controlled by the environment. As a result, ectotherms grow more slowly and reach larger sizes in colder areas. This pattern, called the temperature-size rule (TSR), has been seen in many types of ectotherms, including single-celled and multicellular organisms, invertebrates, and vertebrates.

The reasons behind the relationship between body size and temperature are not yet fully understood. While temperature is a major factor in how body size changes with latitude, other factors like oxygen levels and interactions between physical, chemical, and biological processes may also play a role. For example, oxygen availability affects how ectotherms respond to temperature changes, but it is difficult to separate the effects of oxygen and temperature in natural environments because they are closely linked.

Zooplankton can be divided into size groups that vary in shape, diet, and feeding habits. Microzooplankton are organisms that consume other plankton and include single-celled protists like ciliates and dinoflagellates, as well as young stages of larger zooplankton. These organisms are major grazers, eating about 59–75% of daily marine plant production, more than mesozooplankton. In some nutrient-rich ecosystems, larger zooplankton may consume more because larger plants are more common there. Microzooplankton also help recycle nutrients that support plant growth and provide food for other animals.

Despite their importance, microzooplankton are not often studied. Routine ocean observations rarely track their numbers or how much they eat, even though a method called the dilution technique has been used for over 40 years to measure their feeding rates. Globally, there are about 1,600 records of microzooplankton feeding rates, compared to over 50,000 for plant production. This lack of data makes it harder to model their role in ocean ecosystems accurately.

Mesozooplankton are a larger group of zooplankton, often dominated by copepods like Calanus finmarchicus and Calanus helgolandicus. These organisms are an important food source for fish.

Because plankton are rarely fished, scientists suggest that changes in their numbers and types can help study how marine ecosystems respond to climate change. Many plankton species have life cycles that last less than a year, so they react quickly to yearly climate changes. Even monthly sampling can show changes in their populations.

Taxonomic groups

Protozooplankton are tiny, single-celled animals that float in water. All protozooplankton are protozoans, but not all protozoans are protozooplankton. Some live in soil or as parasites. Marine protozooplankton include zooflagellates, foraminiferans, radiolarians, and some dinoflagellates.

Protozoans are protists that eat organic matter, such as other tiny organisms or organic debris. In the past, protozoans were called "one-celled animals" because they move and hunt like animals and lack cell walls, which plants and algae have. While scientists no longer group protozoa with animals, the term is still used to describe single-celled organisms that move and feed on other organisms.

Radiolarians are single-celled predators with round, silica shells covered in holes. Their name comes from the Latin word for "radius." They catch food by extending parts of their bodies through the holes. When radiolarians die, their silica shells sink to the ocean floor and become part of ocean sediment. These shells, called microfossils, help scientists learn about past ocean conditions.

Radiolarians, like diatoms, come in many shapes. Their shells are usually made of silicate, but some have shells made of strontium sulfate crystals.

Foraminiferans (forams) are single-celled predators with shells that have holes. Their name means "hole bearers." Their shells, called tests, are chambered and grow as they get bigger. Forams usually have shells made of calcite, but some have shells made of sediment or chiton. Most forams live on the ocean floor, but about 40 species float in the water. Scientists study forams because their fossils help them understand past environments and climates.

Amoebas are single-celled organisms that move and eat. Some have shells made of silica or other materials. Others, like testate amoebas, have shells covered in diatoms.

Ciliates are single-celled organisms with tiny hair-like structures. They move and eat by using these structures. Some ciliates, like Stylonychia putrina, eat cyanobacteria.

Dinoflagellates are a group of single-celled organisms with about 2,000 marine species. Some are predators and part of the zooplankton community. Their name comes from Greek and Latin words meaning "whirling" and "whip," referring to their two whip-like flagella used for movement. Most dinoflagellates have red-brown, cellulose armor. Some, like Gyrodinium, lack armor.

Dinoflagellates often live in symbiosis with other organisms. For example, some nassellarian radiolarians host dinoflagellates inside their shells. The dinoflagellates provide the radiolarians with a protective membrane, while the radiolarians provide nutrients. Scientists believe this relationship evolved separately from other symbioses, like those with foraminifera.

Mixoplankton are plankton that can both photosynthesize and hunt for food. Mixotrophs use different sources of energy and carbon instead of relying on one method. Scientists estimate that more than half of all microscopic plankton are mixotrophic. Mixotrophs can have their own chloroplasts, endosymbionts, or acquire chloroplasts through kleptoplasty.

Mixotrophs can use combinations of energy sources, such as photosynthesis and chemotrophy, or autotrophy and heterotrophy. They can be eukaryotic or prokaryotic. Many marine microzooplankton are mixotrophic, meaning they could also be classified as phytoplankton. Studies show that 30–45% of ciliates and up to 65% of amoebas, forams, and radiolarians are mixotrophic.

Phaeocystis is a type of algae that lives inside acantharian radiolarians. Phaeocystis forms large colonies during blooms and plays a key role in marine carbon and sulfur cycles.

Some forams are mixotrophic, hosting algae like green algae, red algae, and dinoflagellates. These forams are common in nutrient-poor waters. Some forams retain chloroplasts from algae to perform photosynthesis.

Dinoflagellates have varied feeding habits. Some are photosynthetic, but many are mixotrophic, combining photosynthesis with hunting prey. Some live inside marine animals and protists, helping coral reefs. Others eat protozoa or are parasitic. The toxic dinoflagellate Dinophysis acuta gets chloroplasts from ciliates, which in turn get them from cryptophytes.

Copepods are tiny crustaceans, usually 1 to 2 mm long, with teardrop-shaped bodies. They have three body sections: head, thorax, and abdomen. They have two pairs of antennae and a tough exoskeleton made of calcium carbonate. Most copepods have a single red eye. About 13,000 copepod species are known, with 10,200 living in the ocean. They are among the most common and important marine organisms.

Role in food webs

Zooplankton, which are tiny single-celled animals, eat a large amount of organic carbon from phytoplankton in the ocean. This process is important for the ocean’s food chain and how carbon moves through the environment. However, scientists know little about how much zooplankton eat because there are few measurements of their grazing. This lack of data makes it hard to create accurate models of how carbon moves, how the ocean’s food web works, and how ecosystems function. To solve this problem, researchers suggest developing better tools to measure how changes in phytoplankton numbers or light properties relate to grazing.

Grazing by zooplankton and other small heterotrophic protists is a major way that organic matter is lost from the ocean’s primary producers, like phytoplankton. This process changes the sizes of particles in the ocean and affects how carbon is transported from the surface to the deep ocean. To understand how ocean ecosystems work and how they might change with environmental shifts, scientists need accurate information about grazing in models that study ocean chemistry, ecosystems, and comparisons between different environments. Studies have shown that the loss of phytoplankton, mostly caused by grazing, explains yearly changes in phytoplankton numbers, how quickly they grow, and how much carbon they send to the deep ocean.

• Pelagic food web
• Pelagic food web and the biological pump. Connections between the ocean’s biological pump, which moves carbon from the surface to the deep ocean, and the pelagic food web. How scientists can study these parts using ships, satellites, and underwater vehicles. Light blue areas in the ocean are the euphotic zone, where sunlight reaches the seafloor, and darker blue areas are the twilight zone, where very little light is present.

Role in biogeochemistry

Zooplankton connect primary producers to higher levels in marine food webs and act as important recyclers of carbon and other nutrients. These nutrients greatly influence marine biogeochemical cycles, such as the biological pump, especially in the open ocean’s oligotrophic waters. Zooplankton release dissolved organic matter (DOM) through sloppy feeding, excretion, egestion, and the leaching of fecal pellets. This DOM controls its cycling and supports the microbial loop. Factors like absorption efficiency, respiration, and prey size affect how zooplankton transform and deliver carbon to the deep ocean.

Excretion accounts for 80% of DOM release by crustacean zooplankton, while sloppy feeding contributes 20%. Fecal pellet leaching was found to have little impact in the same study. Protozoan grazers release DOM mainly through excretion and egestion, and gelatinous zooplankton can also release DOM by producing mucus. Fecal pellet leaching can occur hours to days after egestion, with effects depending on food concentration and quality. Factors such as absorption efficiency (AE), which measures how much food is absorbed by plankton, influence how much organic material is recycled. Low feeding rates often lead to high AE and small, dense fecal pellets, while high feeding rates result in lower AE and larger, more organic-rich pellets. Respiration rates also affect carbon loss through respired CO₂, influenced by oxygen levels, pH, and light. Smaller prey are usually ingested whole, while larger prey are consumed more sloppily, releasing more organic matter. Carnivorous diets also release more dissolved organic carbon and ammonium than omnivorous diets.

Zooplankton support the ocean’s biological pump by exporting carbon through fecal pellets, mucous feeding webs, molts, and carcasses. Fecal pellets are a major contributor to this export, with copepod size likely determining how much carbon reaches the ocean floor. The importance of fecal pellets varies by time and location, such as during zooplankton blooms, which increase fecal pellet production and carbon export. As fecal pellets sink, microbes in the water column alter their carbon composition, affecting how much is recycled in the euphotic zone and how much reaches deeper waters. Fecal pellet contributions to carbon export may be underestimated, but new methods, like analyzing amino acid isotopic signatures, are being developed to better quantify this. Carcasses, such as those from gelatinous zooplankton during blooms, also contribute to carbon export. These large, sinking carcasses may provide significant food sources for benthic organisms due to their high carbon content.