Zooplankton are theheterotrophic component of theplanktonic community, having to consume other organisms to thrive. The name comes fromAncient Greekζῷον (zōîon), meaning "animal", andπλαγκτός (planktós), meaning "drifter, wanderer, roamer", and thus, "animal drifter". Plankton are aquatic organisms that are unable to swim effectively against currents. Consequently, they drift or are carried along by currents in theocean, or by currents inseas,lakes orrivers.
Zooplankton can be contrasted withphytoplankton (cyanobacteria andmicroalgae), which are the plant-like component of the plankton community (the "phyto-" prefix comes from Ancient Greek:φῠτόν,romanized: phutón,lit. 'plant', although taxonomicallynotplants). Zooplankton areheterotrophic (other-feeding), whereas phytoplankton areautotrophic (self-feeding), often generatingbiological energy andmacromolecules throughchlorophylliccarbon fixation usingsunlight – in other words, zooplankton cannot manufacture their own food, while phytoplankton can. As a result, zooplankton must acquirenutrients byfeeding on otherorganisms such as phytoplankton, which are generally smaller than zooplankton. Most zooplankton aremicroscopic but some (such asjellyfish) aremacroscopic, meaning they can be seen with thenaked eye.[1]
Manyprotozoans (single-celledprotists that prey on other microscopic life) are zooplankton, includingzooflagellates,foraminiferans,radiolarians, somedinoflagellates andmarine microanimals. Macroscopic zooplankton include pelagiccnidarians,ctenophores,molluscs,arthropods andtunicates, as well as planktonicarrow worms andbristle worms.
The distinction between autotrophy and heterotrophy often breaks down in very small organisms. Recent studies of marine microplankton have indicated over half of microscopic plankton aremixotrophs, which can obtain energy and carbon from a mix of internal plastids and external sources. Many marine microzooplankton are mixotrophic, which means they could also be classified as phytoplankton.
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Zooplankton (/ˈzoʊ.əplæŋktən/;[2]/ˌzoʊ.əˈplæŋktən/)[3] areheterotrophic (sometimesdetritivorous)plankton. The wordzooplankton is derived fromAncient Greek:ζῷον,romanized: zôion,lit. 'animal'; andπλᾰγκτός,planktós,'wanderer; drifter'.[4]
Zooplankton is a categorization spanning a range oforganism sizes including smallprotozoans and largemetazoans. It includesholoplanktonic organisms whose completelife cycle lies within the plankton, as well asmeroplanktonic organisms that spend part of their lives in the plankton before graduating to either thenekton or asessile,benthic existence. Although zooplankton are primarily transported by ambient water currents, many havelocomotion, used to avoid predators (as indiel vertical migration) or to increase prey encounter rate.
Just as any species can be limited within a geographical region, so are zooplankton. However, species of zooplankton are not dispersed uniformly or randomly within a region of the ocean. As with phytoplankton, 'patches' of zooplankton species exist throughout the ocean. Though few physical barriers exist above themesopelagic, specific species of zooplankton are strictly restricted by salinity and temperature gradients, while other species can withstand wide temperature and salinity gradients.[5] Zooplankton patchiness can also be influenced by biological factors, as well as other physical factors. Biological factors include breeding, predation, concentration of phytoplankton, and vertical migration.[5] The physical factor that influences zooplankton distribution the most is mixing of the water column (upwelling anddownwelling along the coast and in the open ocean) that affects nutrient availability and, in turn, phytoplankton production.[5]
Through their consumption and processing of phytoplankton and other food sources, zooplankton play a role in aquaticfood webs, as a resource for consumers on highertrophic levels (including fish), and as a conduit for packaging the organic material in thebiological pump. Since they are typically small, zooplankton can respond rapidly to increases in phytoplankton abundance,[clarification needed] for instance, during thespring bloom. Zooplankton are also a key link in thebiomagnification ofpollutants such asmercury.[6]
Ecologically important protozoan zooplankton groups include theforaminiferans,radiolarians anddinoflagellates (the last of these are oftenmixotrophic). Important metazoan zooplankton includecnidarians such asjellyfish and thePortuguese Man o' War;crustaceans such ascladocerans,copepods,ostracods,isopods,amphipods,mysids andkrill;chaetognaths (arrow worms);molluscs such aspteropods; andchordates such assalps and juvenile fish. This widephylogenetic range includes a similarly wide range in feeding behavior:filter feeding,predation andsymbiosis withautotrophicphytoplankton as seen in corals. Zooplankton feed onbacterioplankton, phytoplankton, other zooplankton (sometimescannibalistically),detritus (ormarine snow) and evennektonic organisms. As a result, zooplankton are primarily found in surface waters where food resources (phytoplankton or other zooplankton) are abundant.
Zooplankton can also act as adiseasereservoir. Crustacean zooplankton have been found to house the bacteriumVibrio cholerae, which causescholera, by allowing the cholera vibrios to attach to their chitinousexoskeletons. Thissymbiotic relationship enhances the bacterium's ability to survive in an aquatic environment, as the exoskeleton provides the bacterium with carbon and nitrogen.[8]
Body size has been defined as a "master trait" for plankton as it is amorphological characteristic shared by organisms across taxonomy that characterises the functions performed by organisms in ecosystems.[9][10] It has a paramount effect on growth, reproduction, feeding strategies and mortality.[11] One of the oldest manifestations of the biogeography of traits was proposed over 170 years ago, namelyBergmann's rule, in which field observations showed that larger species tend to be found at higher, colder latitudes.[12][13]
In the oceans, size is critical in determiningtrophic links in planktonic ecosystems and is thus a critical factor in regulating the efficiency of thebiological carbon pump.[14] Body size is sensitive to changes in temperature due to the thermal dependence of physiological processes.[15] The plankton is mainly composed ofectotherms which are organisms that do not generate sufficient metabolic heat to elevate their body temperature, so their metabolic processes depends on external temperature.[16] Consequently, ectotherms grow more slowly and reach maturity at a larger body size in colder environments, which has long puzzled biologists because classic theories of life-history evolution predict smaller adult sizes in environments delaying growth.[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][19][13]
The processes underlying the inverse relationship between body size and temperature remain to be identified.[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. For instance, oxygen supply plays a central role in determining the magnitude of ectothermic temperature-size responses, but it is hard to disentangle the relative effects of oxygen and temperature from field data because these two variables are often strongly inter-related in the surface ocean.[20][21][13]
Zooplankton can be broken down into size classes[22] which are diverse in their morphology, diet, feeding strategies, etc. both within classes and between classes:
| type of zooplankton | size range |
|---|---|
| picozooplankton | 2μm |
| nanozooplankton | 2–20μm |
| microzooplankton | 20–200μm |
| mesozooplankton | 0.2–20 millimeters |
Microzooplankton are defined as heterotrophic andmixotrophic plankton. They primarily consist ofphagotrophicprotists, including ciliates, dinoflagellates, andmesozooplanktonnauplii.[23] Microzooplankton are majorgrazers of the plankton community. As the primary consumers of marine phytoplankton, microzooplankton consume ~ 59–75% daily of themarine primary production, much larger than mesozooplankton. That said, macrozooplankton can sometimes have greater consumption rates in eutrophic ecosystems because the larger phytoplankton can be dominant there.[24][25] Microzooplankton are also pivotal regenerators of nutrients which fuel primary production and food sources for metazoans.[25][26]
Despite their ecological importance, microzooplankton remain understudied. Routine oceanographic observations seldom monitor microzooplankton biomass or herbivory rate, although the dilution technique, an elegant method of measuring microzooplankton herbivory rate, has been developed for over four decades (Landry and Hassett 1982). The number of observations of microzooplankton herbivory rate is around 1600 globally,[27][28] far less than that of primary productivity (> 50,000).[29] This makes validating and optimizing the grazing function of microzooplankton difficult in ocean ecosystem models.[26]
Mesozooplankton are one of the larger size classes of zooplankton. In most regions, mesozooplankton are dominated bycopepods, such asCalanus finmarchicus andCalanus helgolandicus. Mesozooplankton are an important prey for fish.
As plankton are rarely fished, it has been argued that mesoplankton abundance andspecies composition can be used to study marine ecosystems' response to climate change. This is because they have life cycles that generally last less than a year, meaning they respond to climate changes between years. Sparse, monthly sampling will still indicate vacillations.[30]
Protozooplankton refers toprotist zooplankton (planktonic protozoans).[31] All protozooplankton are protozoans, but not all protozoans are protozooplankton, since some live in environments like soil or as parasites. Marine planktonic protozoans includezooflagellates,foraminiferans,radiolarians and somedinoflagellates.
Protozoans are protists that feed on organic matter such as othermicroorganisms or organic tissues and debris.[32][33] Historically, the protozoa were regarded as "one-celled animals", because they often possessanimal-like behaviours, such asmotility andpredation, and lack acell wall, as found in plants and manyalgae.[34][35] 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 byheterotrophy.
Radiolarians are unicellular predatoryprotists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of theocean sediment. These remains, asmicrofossils, provide valuable information about past oceanic conditions.[36]
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 Radiolarian geometry | |
| Ernst Haeckel's radiolarian engravings |
Like radiolarians,foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often calledtests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made ofagglutinated sediment particles orchiton, and (rarely) silica. Most forams are benthic, but about 40 species are planktic.[37] They are widely researched with well-established fossil records which allow scientists to infer a lot about past environments and climates.[36]
![The Egyptian pyramids were constructed from limestone that contained nummulites.[38]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAll_Gizah_Pyramids.jpg)
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Dinoflagellates are a phylum of unicellularflagellates with about 2,000 marine species.[39] Some dinoflagellates arepredatory, and thus belong to the zooplankton community. Their name comes from the Greek "dinos" meaningwhirling and the Latin "flagellum" meaning awhip orlash. This refers to the two whip-like attachments (flagella) used for forward movement. Most dinoflagellates are protected with red-brown, cellulose armour.Excavates may be the most basal flagellate lineage.[40]
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Dinoflagellates often live insymbiosis with other organisms. Manynassellarian radiolarians housedinoflagellatesymbionts within their tests.[41] The nassellarian providesammonium andcarbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders.[42] There is evidence fromDNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as withforaminifera.[43]
![Oodinium, a genus of parasitic dinoflagellates, causes velvet disease in fish[44]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aArchives_de_zoologie_exp%25C3%25A9rimentale_et_g%25C3%25A9n%25C3%25A9rale_(1920)_(20299351186).jpg)
![Karenia brevis produces red tides highly toxic to humans[45]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aKarenia_brevis.jpg)
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Mixoplankton are mixotrophic plankton, capable of both photosynthesis and predation. Amixotroph is an organism that can use a mix of differentsources of energy and carbon, instead of having a single trophic mode on the continuum from completeautotrophy at one end toheterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.[46] There are two types of eukaryotic mixotrophs: those with their ownchloroplasts, and those withendosymbionts—and others that acquire them throughkleptoplasty or by enslaving the entire phototrophic cell.[47]
The distinction between plants and animals often breaks down in very small organisms. Possible combinations arephoto- andchemotrophy,litho- andorganotrophy,auto- andheterotrophy or other combinations of these. Mixotrophs can be eithereukaryotic orprokaryotic.[48] They can take advantage of different environmental conditions.[49]
Many marine microzooplankton are mixotrophic, which means they could also be classified as phytoplankton. Recent studies of marine microzooplankton found 30–45% of the ciliate abundance was mixotrophic, and up to 65% of the amoeboid, foram and radiolarianbiomass was mixotrophic.[50]
Mixotrophic zooplankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017[51] | ||||||
|---|---|---|---|---|---|---|
| Description | Example | Further examples | ||||
| Callednonconstitutive mixotrophs by Mitra et al., 2016.[52] Zooplankton that are photosynthetic: microzooplankton or metazoan zooplankton that acquire phototrophy through chloroplast retentiona or maintenance of algal endosymbionts. | ||||||
| Generalists | Protists that retain chloroplasts and rarely other organelles from many algal taxa |  | Mostoligotrich ciliates that retain plastidsa | |||
| Specialists | 1. Protists that retain chloroplasts and sometimes other organelles from one algal species or very closely related algal species |  | Dinophysis acuminata | Dinophysis spp. Myrionecta rubra | ||
| 2. Protists or zooplankton with algal endosymbionts of only one algal species or very closely related algal species |  | Noctiluca scintillans | Metazooplankton with algalendosymbionts Most mixotrophicRhizaria (Acantharea,Polycystinea, andForaminifera) GreenNoctiluca scintillans | |||
| aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm. | ||||||
Phaeocystis species are endosymbionts toacantharian radiolarians.[53][54]Phaeocystis is an important algal genus found as part of the marinephytoplankton around the world. It has apolymorphic life cycle, ranging from free-living cells to large colonies.[55] It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size duringblooms.[56] As a result,Phaeocystis is an important contributor to the marinecarbon[57] andsulfur cycles.[58]
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A number of forams are mixotrophic. These have unicellularalgae asendosymbionts, from diverse lineages such as thegreen algae,red algae,golden algae,diatoms, anddinoflagellates.[37] Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.[59] Some forams arekleptoplastic, retainingchloroplasts from ingested algae to conductphotosynthesis.[60]
By trophic orientation, dinoflagellates are all over the place. Some dinoflagellates are known to bephotosynthetic, but a large fraction of these are in factmixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).[61] Some species areendosymbionts of marine animals and other protists, and play an important part in the biology ofcoral reefs. Others predate other protozoa, and a few forms are parasitic. Many dinoflagellates aremixotrophic and could also be classified as phytoplankton. The toxic dinoflagellateDinophysis acuta acquire chloroplasts from its prey. "It cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates such as the redMyrionecta rubra, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".[51]
Free-living species in the crustacean classCopepoda are typically 1 to 2 mm long with teardrop-shaped bodies. Like all crustaceans, their bodies are divided into three sections: head, thorax, and abdomen, with two pairs of antennae; the first pair is often long and prominent. They have a toughexoskeleton made of calcium carbonate and usually have asingle red eye in the centre of their transparent head.[62] About 13,000 species of copepods are known, of which about 10,200 are marine.[63][64] They are usually among the more dominant members of the zooplankton.[65]
In addition to copepods the crustacean classesostracods,branchiopods andmalacostracans also have planktonic members.Barnacles are planktonic only during the larval stage.[66]
Ichthyoplankton are theeggs andlarvae of fish ("ichthyo" comes from the Greek word forfish). They are planktonic because they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow intojuvenile fish. Fish larvae are part of the zooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals.[67][68]
Gelatinous zooplankton includectenophores,medusae,salps, andChaetognatha in coastal waters. Jellyfish are slow swimmers, and most species form part of the plankton. Traditionally jellyfish have been viewed astrophic dead ends, minor players in themarine food web, gelatinous organisms with abody plan largely based on water that offers little nutritional value or interest for other organisms apart from a few specialised predators such as theocean sunfish and theleatherback sea turtle.[69][70]
That view has recently been challenged. Jellyfish, and more gelatinous zooplankton in general, which includesalps andctenophores, 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. It is difficult for scientists to detect and analyse jellyfish in the guts of predators, since they turn to mush when eaten and are rapidly digested.[69] But jellyfish bloom in vast numbers, and it has been shown they form major components in the diets oftuna,spearfish andswordfish as well as various birds and invertebrates such asoctopus,sea cucumbers,crabs andamphipods.[71][70] "Despite their low energy density, the contribution of jellyfish to the energy budgets of predators may be much greater than assumed because of rapid digestion, low capture costs, availability, and selective feeding on the more energy-rich components. Feeding on jellyfish may make marine predators susceptible to ingestion of plastics."[70] According to a 2017 study,narcomedusae consume the greatest diversity of mesopelagic prey, followed byphysonectsiphonophores,ctenophores andcephalopods.[72]
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The importance of the so-called "jelly web" is only beginning to be understood, but it seems medusae, ctenophores and siphonophores can be key predators in deep pelagic food webs with ecological impacts similar to predator fish and squid. Traditionally gelatinous predators were thought ineffectual providers of marine trophic pathways, but they appear to have substantial and integral roles in deeppelagic food webs.[72]
Grazing by single-celled zooplankton accounts for the majority oforganic carbon loss frommarine primary production.[73] However, zooplankton grazing remains one of the key unknowns in global predictive models of carbon flux, themarine food web structure and ecosystem characteristics, because empirical grazing measurements are sparse, resulting in poor parameterisation of grazing functions.[74][75] 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.[73]
Grazing is a central, rate-setting process in ocean ecosystems and a driver ofmarine biogeochemical cycling.[76] In all ocean ecosystems, grazing by heterotrophic protists constitutes the single largest loss factor of marine primary production and alters particle size distributions.[77] Grazing affects all pathways of export production, rendering grazing important both for surface anddeep carbon processes.[78] Predicting central paradigms of ocean ecosystem function, including responses to environmental change requires accurate representation of grazing in global biogeochemical, ecosystem and cross-biome-comparison models.[74] Several large-scale analyses have concluded that phytoplankton losses, which are dominated by grazing are the putative explanation for annual cycles in phytoplankton biomass, accumulation rates and export production.[79][80][75][73]
![Pelagic food web and the biological pump. Links among the ocean's biological pump and pelagic food web and the ability to sample these components remotely from ships, satellites, and autonomous vehicles. Light blue waters are the euphotic zone, while the darker blue waters represent the twilight zone.[81]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aExport_Processes_in_the_Ocean_from_Remote_Sensing.jpg)
In addition to linking primary producers to highertrophic levels inmarine food webs, zooplankton also play an important role as "recyclers" of carbon and other nutrients that significantly impactmarine biogeochemical cycles, including thebiological pump. This is particularly important in theoligotrophic waters of the open ocean. Through sloppy feeding, excretion, egestion, and leaching offecal pellets, zooplankton releasedissolved organic matter (DOM) which controls DOM cycling and supports themicrobial loop. Absorption efficiency, respiration, and prey size all further complicate how zooplankton are able to transform and deliver carbon to thedeep ocean.[77]
Excretion and sloppy feeding (the physical breakdown of food source) make up 80% and 20% of crustacean zooplankton-mediated DOM release respectively.[84] In the same study, fecal pellet leaching was found to be an insignificant contributor. For protozoan grazers, DOM is released primarily through excretion and egestion and gelatinous zooplankton can also release DOM through the production of mucus. Leaching of fecal pellets can extend from hours to days after initial egestion and its effects can vary depending on food concentration and quality.[85][86] Various factors can affect how much DOM is released from zooplankton individuals or populations. 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.[77] Depending on the feeding rate and prey composition, variations in AE may lead to variations in fecal pellet production, and thus regulates how much organic material is recycled back to the marine environment. Low feeding rates typically lead to high AE and small, dense pellets, while high feeding rates typically lead to low AE and larger pellets with more organic content. Another contributing factor to DOM release is respiration rate. Physical factors such as oxygen availability, pH, and light conditions may affect overall oxygen consumption and how much carbon is loss from zooplankton in the form of respired CO2. The relative sizes of zooplankton and prey also mediate how much carbon is released via sloppy feeding. Smaller prey are ingested whole, whereas larger prey may be fed on more "sloppily", that is more biomatter is released through inefficient consumption.[87][88] There is also evidence that diet composition can impact nutrient release, with carnivorous diets releasing more dissolved organic carbon (DOC) and ammonium than omnivorous diets.[85]
Zooplankton play a critical role in supporting the ocean'sbiological pump through various forms ofcarbon 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. The importance of fecal pellets can vary both by time and location. For example, zooplankton bloom events can produce larger quantities of fecal pellets, resulting in greater measures of carbon export. Additionally, as fecal pellets sink, they are reworked by microbes in the water column, which can thus alter the carbon composition of the pellet. This affects how much carbon is recycled in the euphotic zone and how much reaches depth. Fecal pellet contribution to carbon export is likely underestimated; however, new advances in quantifying this production are currently being developed, including the use of isotopic signatures of amino acids to characterize how much carbon is being exported via zooplankton fecal pellet production.[90] Carcasses are also gaining recognition as being important contributors to carbon export.Jelly falls – the mass sinking of gelatinous zooplankton carcasses – occur across the world as a result of large blooms. 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 forbenthic organisms.[77]
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