Zooplankton

Zooplankton sample including fish eggs, doliolids, several species of copepods, and gastropod and decapod larva

Zooplankton (/ˈzoʊ.əˌplæŋktən, ˈzuː(ə)-, ˈzoʊoʊ-/, /ˌzoʊ.əˈplæŋktən, -tɒn/) are heterotrophic (sometimes detritivorous) plankton (cf. phytoplankton). Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word zooplankton is derived from the Greek zoon (ζῴον), meaning "animal", and planktos (πλαγκτός), meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some (such as jellyfish) are larger and visible to the naked eye.

Ecology

Zooplankton is a categorization spanning a range of organism sizes including small protozoans and large metazoans. It includes holoplanktonic organisms whose complete life cycle lies within the plankton, as well as meroplanktonic organisms that spend part of their lives in the plankton before graduating to either the nekton or a sessile, benthic existence. 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.

• Typical models featuring zooplankton
• 
  Upper left: Biogeochemical models

  Right: Ecosystem models

  
  

  Lower left: Size-spectra models

  
  

  These models also have temporal and spatial components.

Ecologically important protozoan zooplankton groups include the foraminiferans, radiolarians and dinoflagellates (the last of these are often mixotrophic). Important metazoan zooplankton include cnidarians such as jellyfish and the Portuguese Man o' War; crustaceans such as copepods, ostracods, isopods, amphipods, mysids and krill; chaetognaths (arrow worms); molluscs such as pteropods; and chordates such as salps and juvenile fish. This wide phylogenetic range includes a similarly wide range in feeding behavior: filter feeding, predation and symbiosis with autotrophic phytoplankton as seen in corals. Zooplankton feed on bacterioplankton, phytoplankton, other zooplankton (sometimes cannibalistically), detritus (or marine snow) and even nektonic organisms. As a result, zooplankton are primarily found in surface waters where food resources (phytoplankton or other zooplankton) are abundant.

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 the mesopelagic, specific species of zooplankton are strictly restricted by salinity and temperature gradients; while other species can withstand wide temperature and salinity gradients. 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. 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.

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, for instance, during the spring bloom. Zooplankton are also a key link in the biomagnification of pollutants such as mercury.

Zooplankton can also act as a disease reservoir. Crustacean zooplankton have been found to house the bacterium Vibrio cholerae, which causes cholera, by allowing the cholera vibrios to attach to their chitinous exoskeletons. This symbiotic relationship enhances the bacterium's ability to survive in an aquatic environment, as the exoskeleton provides the bacterium with carbon and nitrogen.

Role in food webs

Sloppy feeding by zooplankton

DOC = dissolved organic carbon. POC = particulate organic carbon. Adapted from Møller et al. (2005), Saba et al. (2009) and Steinberg et al. (2017).

Further information: Marine food web

• Pelagic food web
• 
  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.

Role in biogeochemistry

DOM, POM and the viral shunt

Connections between the different compartments of the living (bacteria/viruses and phyto−/zooplankton) and the nonliving (DOM/POM and inorganic matter) environment

The viral shunt pathway facilitates the flow of dissolved organic matter (DOM) and particulate organic matter (POM) through the marine food web

Further information: Marine biogeochemical cycles

Microzooplankton

See also: Marine microorganisms § Zooplankton

Microzooplankton: major grazers of the plankton...

Protists

Further information: marine protists

Radiolarians

Radiolarian shapes

          Drawings by Haeckel 1904 (click for details)

Radiolarians are unicellular predatory protists 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 the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.

• 
  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) of silica. Most forams are benthic, but about 40 species are planktic. They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.

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.

External videos
foraminiferans
Foraminiferal networks and growth

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

Amoeba

Shelled and naked amoeba

Testate amoeba, Cyphoderia sp.

Naked amoeba, Chaos sp.

                  Amoeba can be shelled (testate) or naked

• 
  Naked amoeba 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 part of the algae group, and form a phylum of unicellular flagellates with about 2,000 marine species. The 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. Like other phytoplankton, dinoflagellates are r-strategists which under right conditions can bloom and create red tides. Excavates may be the most basal flagellate lineage.

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 symbiosis with other organisms. Many nassellarian radiolarians house dinoflagellate symbionts within their tests. 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. There is evidence from DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with foraminifera.

• 
  Tripos muelleri is recognisable by its U-shaped horns
• 
  Oodinium, a genus of parasitic dinoflagellates, causes velvet disease in fish
• 
  Karenia brevis produces red tides highly toxic to humans
• 
  Red tide

Mixotrophs

See also: Mixotroph and Mixotrophic dinoflagellate

Mixotrophic radiolarians

Acantharian radiolarian hosts Phaeocystis symbionts

White Phaeocystis algal foam washing up on a beach

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. It is estimated that mixotrophs comprise more than half of all microscopic plankton. 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.

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. They can take advantage of different environmental conditions.

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.

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. 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. As a result, Phaeocystis is an important contributor to the marine carbon and sulfur cycles. Phaeocystis species are endosymbionts to acantharian radiolarians.

Mixotrophic zooplankton that combine phototrophy and heterotrophy – table based on Stoecker et. al., 2017
Description		Example		Further examples
Called nonconstitutive mixotrophs by Mitra et. al., 2016. 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	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 algal endosymbionts Most mixotrophic Rhizaria (Acantharea, Polycystinea, and Foraminifera) Green Noctiluca scintillans
aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.

• Mixoplankton
• 
  Tintinnid ciliate Favella
• 
  Euglena mutabilis, a photosynthetic flagellate
• 
  Zoochlorellae (green) living inside the ciliate Stichotricha secunda
• 
  The dinoflagellate Dinophysis acuta

By trophic orientation dinoflagellates are all over the place. Some dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy). Some species are endosymbionts of marine animals and other protists, and play an important part in the biology of coral reefs. Others predate other protozoa, and a few forms are parasitic. Many dinoflagellates are mixotrophic and could also be classified as phytoplankton.

The toxic dinoflagellate Dinophysis acuta acquire chloroplasts from its prey. "It cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates such as the red Myrionecta rubra, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".

Zooplankton diversity

• Mixotrophs
• 
  Tintinnid ciliate

• Ichthyoplankton
• 
  Salmon eggs
• 
  Juvenile planktonic squid
• 
  Ocean sunfish larvae (2.7mm)
• 
  Boxfish larva

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

See also

• Bacterioplankton
• Biological pump
• Census of Marine Zooplankton
• Diel vertical migration
• Gelatinous zooplankton
• Ocean acidification
• Phytoplankton
• Plankton
• Primary production
• Thin layers (oceanography)

External links

• SAHFOS Sir Alister Hardy Foundation for Ocean Science
• Ocean Drifters Short film narrated by David Attenborough about the varied roles of plankton
• Sea Drifters BBC Audio slideshow
• Plankton Chronicles Short documentary films & photos
• COPEPOD: The global plankton database. A global coverage database of zooplankton biomass and abundance data.
• Guide to the marine zooplankton of south eastern Australia, Tasmanian Aquaculture and Fisheries Institute
• Australian Continuous Plankton Recorder Project
• An Image-Based Key to Zooplankton of North America