Zooplankton are the heterotrophic component of the planktonic community (the "zoo-" prefix comes from Ancient Greek: ζῷον, romanized: zôion, lit.

Zooplankton can be contrasted with phytoplankton (cyanobacteria and microalgae), which are the plant-like component of the plankton community (the "phyto-" prefix comes from Ancient Greek: φῠτόν, romanized: phutón, lit.

[1] Many protozoans (single-celled protists that prey on other microscopic life) are zooplankton, including zooflagellates, foraminiferans, radiolarians, some dinoflagellates and marine microanimals.

Recent studies of marine microplankton have indicated over half of microscopic plankton are mixotrophs, which can obtain energy and carbon from a mix of internal plastids and external sources.

Although zooplankton are primarily transported by ambient water currents, many have locomotion, used to avoid predators (as in diel vertical migration) or to increase prey encounter rate.

[5] The physical factor that influences zooplankton distribution the most is mixing of the water column (upwelling and downwelling along the coast and in the open ocean) that affects nutrient availability and, in turn, phytoplankton production.

Since they are typically small, zooplankton can respond rapidly to increases in phytoplankton abundance,[clarification needed] for instance, during the spring bloom.

Important metazoan zooplankton include cnidarians such as jellyfish and the Portuguese Man o' War; crustaceans such as cladocerans, copepods, ostracods, isopods, amphipods, mysids and krill; chaetognaths (arrow worms); molluscs such as pteropods; and chordates such as salps and juvenile fish.

[11] One of the oldest manifestations of the biogeography of traits was proposed over 170 years ago, namely Bergmann's rule, in which field observations showed that larger species tend to be found at higher, colder latitudes.

[17] This pattern of body size variation, known as the temperature-size rule (TSR),[18] has been observed for a wide range of ectotherms, including single-celled and multicellular species, invertebrates and vertebrates.

[17] Despite temperature playing a major role in shaping latitudinal variations in organism size, these patterns may also rely on complex interactions between physical, chemical and biological factors.

As plankton are rarely fished, it has been argued that mesoplankton abundance and species composition can be used to study marine ecosystems' response to climate change.

[31][32] Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.

[33][34] Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.

Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes.

[36] They are widely researched with well-established fossil records which allow scientists to infer a lot about past environments and climates.

[42] A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other.

[54] It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms.

Jellyfish, and more gelatinous zooplankton in general, which include salps and ctenophores, are very diverse, fragile with no hard parts, difficult to see and monitor, subject to rapid population swings and often live inconveniently far from shore or deep in the ocean.

[68] But jellyfish bloom in vast numbers, and it has been shown they form major components in the diets of tuna, spearfish and swordfish as well as various birds and invertebrates such as octopus, sea cucumbers, crabs and amphipods.

Traditionally gelatinous predators were thought ineffectual providers of marine trophic pathways, but they appear to have substantial and integral roles in deep pelagic food webs.

[73][74] To overcome this critical knowledge gap, it has been suggested that a focused effort be placed on the development of instrumentation that can link changes in phytoplankton biomass or optical properties with grazing.

[75] In all ocean ecosystems, grazing by heterotrophic protists constitutes the single largest loss factor of marine primary production and alters particle size distributions.

Absorption efficiency, respiration, and prey size all further complicate how zooplankton are able to transform and deliver carbon to the deep ocean.

Leaching of fecal pellets can extend from hours to days after initial egestion and its effects can vary depending on food concentration and quality.

Absorption efficiency (AE) is the proportion of food absorbed by plankton that determines how available the consumed organic materials are in meeting the required physiological demands.

[84] Zooplankton play a critical role in supporting the ocean's biological pump through various forms of carbon export, including the production of fecal pellets, mucous feeding webs, molts, and carcasses.

Fecal pellets are estimated to be a large contributor to this export, with copepod size rather than abundance expected to determine how much carbon actually reaches the ocean floor.

For example, zooplankton bloom events can produce larger quantities of fecal pellets, resulting in greater measures of carbon export.

Because of their large size, these gelatinous zooplankton are expected to hold a larger carbon content, making their sinking carcasses a potentially important source of food for benthic organisms.