Zooplankton

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Zooplankton are the animal (or heterotrophic) component of the

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 are mixotrophs. A mixotroph is an organism that can behave sometimes as though it were a plant and sometimes as though it were an animal, using a mix of autotrophy and heterotrophy. Many marine microzooplankton are mixotrophic, which means they could also be classified as phytoplankton.

Overview

Zooplankton (

Through their consumption and processing of phytoplankton and other food sources, zooplankton play a role in aquatic food webs, as a resource for consumers on higher trophic levels (including fish), and as a conduit for packaging the organic material in the biological pump. Since they are typically small, zooplankton can respond rapidly to increases in phytoplankton abundance,^{[clarification needed]} for instance, during the spring bloom. Zooplankton are also a key link in the biomagnification of pollutants such as mercury.^[6]

• Typical models featuring zooplankton
• 
        Upper left: Biogeochemical models                        Right: Ecosystem models

  
  

        Lower left: Size-spectra models

  
  

  These models also have temporal and spatial components.^[7]

Body size has been defined as a "master trait" for plankton as it is a morphological 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, namely Bergmann'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 determining

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	${\displaystyle <}$2μm
nanozooplankton	2-20μm
microzooplankton	20-200μm
mesozooplankton	0.2-20 millimeters

Microzooplankton

Microzooplankton are defined as heterotrophic and

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 almost 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

Because plankton are rarely fished, it has been argued that mesoplankton abundance and

Radiolarian shapes

          Drawings by Haeckel 1904 (click for details)

• 
  Like diatoms, radiolarians come in many shapes
• 
  Also like diatoms, radiolarian shells are usually made of silicate
• 
  However

  acantharian radiolarians have shells made from strontium sulfate

  crystals
• 
  Cutaway schematic diagram of a spherical radiolarian shell

External videos
Radiolarian geometry
Ernst Haeckel's radiolarian engravings

Foraminiferans

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 called tests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) silica. Most forams are benthic, but about 40 species are planktic.^[36] They are widely researched with well-established fossil records which allow scientists to infer a lot about past environments and climates.^[35]

Foraminiferans

...can have more than one nucleus

...and defensive spines

Foraminiferans are important unicellular zooplankton protists, with calcium tests

• 
  section showing chambers of a spiral foram
• 
  Live Ammonia tepida streaming granular ectoplasm for catching food
• 
  Group of planktonic forams
• 
  The

  Egyptian pyramids were constructed from limestone that contained nummulites.^[37]

External videos
foraminiferans
Foraminiferal networks and growth

Amoeba

Shelled and naked amoeba

testate

) or naked

• 
  Naked amoeba sketch showing food vacuoles and ingested diatom
• 
  Shell or test of a

  testate amoeba, Arcella

  sp.
• 
  Xenogenic testate amoeba covered in diatoms

Ciliates

• 
  Stylonychia putrina
• 
  Holophyra ovum
• 
  Blepharisma japonicum
• 
  This ciliate is digesting cyanobacteria. The mouth is at the bottom right.

Dinoflagellates

See also: Predatory dinoflagellate

Dinoflagellates are a phylum of unicellular flagellates with about 2,000 marine species.^[38] Some dinoflagellates are predatory, and thus belong to the zooplankton community. Their name comes from the Greek "dinos" meaning whirling and the Latin "flagellum" meaning a whip or lash. 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.^[39]

Dinoflagellates

        Armoured

        Unarmoured

Traditionally dinoflagellates have been presented as armoured or unarmoured

• 
  Gyrodinium, one of the few naked dinoflagellates which lack armour
• 
  The dinoflagellate Protoperidinium extrudes a large feeding veil to capture prey
• 
  Nassellarian

  radiolarians can be in symbiosis with dinoflagellates

Dinoflagellates often live in

nassellarian radiolarians house dinoflagellate symbionts within their tests.^[40] The nassellarian provides ammonium and carbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders.^[41] There is evidence from DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with foraminifera.^[42]

• 
  Tripos muelleri is recognisable by its U-shaped horns
• 
  velvet disease in fish^[43]
• 
  Karenia brevis produces red tides highly toxic to humans^[44]
• 
  Red tide

Mixotrophs

See also: Mixotroph and Mixotrophic dinoflagellate

A

heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.^[45] There are two types of eukaryotic mixotrophs: those with their own chloroplasts, and those with endosymbionts—and others that acquire them through kleptoplasty or by enslaving the entire phototrophic cell.^[46]

The distinction between plants and animals often breaks down in very small organisms. Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic.^[47] They can take advantage of different environmental conditions.^[48]

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 radiolarian biomass was mixotrophic.^[49]

Mixotrophic zooplankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017 [50]
Description		Example		Further examples
Called nonconstitutive mixotrophs by Mitra et al., 2016.[51] 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			Most oligotrich 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	Myrionecta rubra
	2. Protists or zooplankton with algal endosymbionts of only one algal species or very closely related algal species		Noctiluca scintillans	Polycystinea, and Foraminifera) Green Noctiluca scintillans
aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.

Phaeocystis species are endosymbionts to acantharian radiolarians.^[52][53] Phaeocystis is an important algal genus found as part of the marine phytoplankton around the world. It has a polymorphic life cycle, ranging from free-living cells to large colonies.^[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.^[55] As a result, Phaeocystis is an important contributor to the marine carbon^[56] and sulfur cycles.^[57]

• Mixoplankton
• 
  Tintinnid ciliate Favella
• 
  Euglena mutabilis, a photosynthetic flagellate
• 
  Zoochlorellae (green) living inside the ciliate

  Stichotricha secunda
• 
  The dinoflagellate Dinophysis acuta

Mixotrophic radiolarians

Acantharian radiolarian hosts Phaeocystis symbionts

White Phaeocystis algal foam washing up on a beach

A number of forams are mixotrophic. These have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates.^[36] Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.^[58] Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.^[59]

By trophic orientation, dinoflagellates are all over the place. Some dinoflagellates are known to be

Myrionecta rubra, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".^[50]

Metazoa (animals)

Free-living species in the crustacean class Copepoda 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 tough exoskeleton made of calcium carbonate and usually have a single red eye in the centre of their transparent head.^[61] About 13,000 species of copepods are known, of which about 10,200 are marine.^[62][63] They are usually among the more dominant members of the zooplankton.^[64]

In addition to copepods the crustacean classes ostracods, branchiopods and malacostracans also have planktonic members. Barnacles are planktonic only during the larval stage.^[65]

• Metazoan
  zooplankton
• 
  Copepod with eggs
• 
  Segmented worm
• 
  Amphipod
• 
  Krill
• 
  Blue ocean slug

Holoplankton and meroplankton

Ichthyoplankton

larvae of fish ("ichthyo" comes from the Greek word for fish). 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 into juvenile 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.^[66][67]

• 
  Juvenile planktonic squid
• 
  Ocean sunfish larvae (2.7mm)
• 
  Boxfish

  larva

Gelatinous zooplankton

salps, and Chaetognatha in coastal waters. Jellyfish are slow swimmers, and most species form part of the plankton. Traditionally jellyfish have been viewed as trophic dead ends, minor players in the marine food web, gelatinous organisms with a body plan largely based on water that offers little nutritional value or interest for other organisms apart from a few specialised predators such as the ocean sunfish and the leatherback sea turtle.^[68][69]

That view has recently been challenged. Jellyfish, and more gelatinous zooplankton in general, which include

ctenophores and cephalopods.^[71]

• 
  Jellyfish
• 
  This free-floating

  bioluminescent

  tunicates
• 
  Salp chain

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 deep

pelagic food webs.^[71]

Role in food webs

Grazing by single-celled zooplankton accounts for the majority of

organic carbon loss from marine primary production.^[72] However, zooplankton grazing remains one of the key unknowns in global predictive models of carbon flux, the marine food web structure and ecosystem characteristics, because empirical grazing measurements are sparse, resulting in poor parameterisation of grazing functions.^[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.^[72]

Grazing is a central, rate-setting process in ocean ecosystems and a driver of

marine biogeochemical cycling.^[75] In all ocean ecosystems, grazing by heterotrophic protists constitutes the single largest loss factor of marine primary production and alters particle size distributions.^[76] Grazing affects all pathways of export production, rendering grazing important both for surface and deep carbon processes.^[77] 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.^[73] 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.^{[78][79][74][72]}

• Pelagic food web
• 
  euphotic zone, while the darker blue waters represent the twilight zone.^[80]

Role in biogeochemistry

In addition to linking primary producers to higher

deep ocean.^[76]

Sloppy feeding and release of DOM

See also:

Zooplankton grazing

particulate organic carbon
Adapted from Møller et al. (2005),^[81]
Saba et al. (2009)^[82] and Steinberg et al. (2017).^[76]

Excretion and sloppy feeding (the physical breakdown of food source) make up 80% and 20% of crustacean zooplankton-mediated DOM release respectively.[83] 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.^[84][85] 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.^[76] 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.^[86][87] 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.^[84]

Comparison of zooplankton-mediated carbon cycles 

epipelagic. The main export particles are diatom resting spores, which bypass the intense grazing pressure, followed by fecal pellets.^[88]

Southern Ocean waters
High nutrient, low chlorophyll

In Southern Ocean waters in summer, iron levels are relatively low and support a more diverse phytoplankton community, but with lower biomass, which, in turn, affects zooplankton community composition and biomass. The grazing pressure during summer is focused mostly on picoplankton, which leaves large particles for export.^[88]

Grazing and fragmentation of particles at both sites increases nutrient recycling in the upper water column