Monday, June 13, 2022

Zooplankton – Definition, types and examples – an overview


Zooplankton – Definition, types and examples – an overview, what do zooplankton eat, what eats zooplankton, what is zooplankton, zooplankton vs phytoplankton

Zooplankton – Definition, types and examples – an overview

Animals that inhabit the water column of oceans and lakes and lack the means to counteract transport currents. Zooplankton inhabit all layers of these water bodies to the greatest depths sampled, and constitute a major link between primary production and higher trophic levels in aquatic ecosystems. Many zooplankton are capable of strong swimming movements and may migrate vertically from tens to hundreds of meters; others have limited mobility and depend more on water turbulence to stay afloat. All zooplankton, however, lack the ability to maintain their position against the movement of large water masses.

Zooplankton can be divided into various operational categories. One means of classification is based on developmental stages and divides animals into meroplankton and holoplankton. Meroplanktonic forms spend only part of their life cycles as plankton and include larvae of benthic worms, mollusks, crustaceans, echinoderms, coral, and even insects, as well as the eggs and larvae of many fishes.


Representative zooplankton. (a) Foraminifera. (b) Dinoflagellate. (c) Tintinnid ciliate. (d) Ctenophore. (e) Cnidarian (scyphozoan jellyfish). (f) Rotifer. (g) Cladoceran. (h) Copepod. (i) Gastropod veliger larva. (j) Chaetognath. (k) Insect larva (Chaoborus). (Parts a and e from L. H. Hyman, The Invertebrates, vol. 3: Protozoa through Ctenophora, McGraw-Hill, 1940)

Holoplankton spend essentially their whole existence in the water column. Examples are chaetognaths, pteropods, larvaceans, siphonophores, and many copepods.

Size is another basis of grouping all plankton. A commonly accepted size classification scheme includes the groupings: picoplankton (<2 micrometers), nanoplankton (2–20 µm), microplankton (20–200 µm), mesoplankton (0.2–20 mm), macroplank-ton (20–200 mm), and megaplankton (>200 mm).

Systematic composition.

Nearly every major taxonomic group of animals has either meroplanktonic or holoplanktonic members. Some of the more common zooplankton groups (see illustration)are described below.

Protists are single-celled eukaryotes that are vital to planktonic systems. Photosynthetic protists form the base of planktonic food webs as the major primary producers in the plankton. Animal like protists (protozoa) constitute the largest proportion of the nano and microzooplankton and include amebas, nonphotosynthetic flagellates, and ciliates. Some of the ameboid Foraminifera and Actinopoda with tests (rigid skeletal supports) or hard skeletons are larger and reach sizes exceeding 2 mm. The skeletons of foraminiferans and actinopods form an important part of the deep-sea sediments, and foraminiferans are used asmarkers in oil exploration. Nanoplanktonic flagellates are usually the most important predators of bacteria; however, some dinoflagellates can capture and digest larger organisms using protoplasmic nets. Both photosynthetic and nonphotosynthetic dinoflagellates have been implicated in noxious red tides or fish kills. Various ciliates species capture bacteria, phytoplankton, and other protists. Many protists that do not fall neatly into a classification as animallike (such as protozoa) or plantlike organisms are termed mixotrophs. Mixotrophs bridge the plant/animal division because they have both photosynthetic and phagotrophic capabilities. Some flagellates with chloroplasts ingest bacteria, phytoplankton, or other protists. Conversely, some ciliates retain functional chloroplasts from phytoplankton that are ingested and otherwise digested.



The phylum Cnidaria contains a number of groups which are important in the marine plankton, including scyphozoans, the true jellyfish. Both scyphozoans and the colonial siphonophores are carnivores with tentacles bearing stinging nematocyst cells. They are found at all depths in the ocean but are most common in the upper waters. Cnidaria are rare in fresh water.

The ctenophores, or comb jellies, were once grouped with Cnidaria but are now treated separately because they lack nematocysts. All are exclusively carnivorous and are important predators of many zooplankton, especially copepods.



Rotifers probably arose in fresh water; only about 5% of the approximately 2500 described species are found in marine and brackish waters. Although about a hundred species are holoplanktonic, most are primarily sessile. Nevertheless, they are often a numerically important fraction of the zooplankton.

Most rotifers feed on bacteria, detritus, and algae, although some are predators of protozoa and other rotifers. Under favorable conditions, rotifers exhibit parthenogenesis (egg production without fertilization leading to female-only populations). Sexual reproduction does occur, but only after male eggs are produced as a response to environmental stresses. Cyclomorphosis (seasonal changes in morphology within a species) is common among rotifers.



Chaetognatha is a carnivorous marine group with worldwide distribution. They are mostly holoplanktonic and are found at all depths, although a given species may be restricted to certain water masses and depths. The elongate body is transparent with lateral fins and is usually less than 3 cm in length.



Veliger larvae of many benthic mollusks are frequently seen in coastal plankton. There are also marine gastropods, including heteropods and pteropods, which are adapted to a holoplanktonic life style. Another group represented in shallow marinewaters by larvae is Polychaeta. This annelid class also contains a few holoplanktonic families.



Copepods are almost always the most numerous members of the macrozooplankton community in marine systems, and are important in freshwater as well. They often migrate to deeper water in day time and move toward the surface at night, using the first antennae and the thoracic appendages to swim. Some are primarily suspension feeders and use appendages with hairlike setules to remove phytoplankton and detritus from the water. Others are primarily raptorial and feed on larger particles, including smaller zooplankton.



Cladocera are a second major group of planktonic crustaceans. They often outnumber copepods in fresh water and occasionally are abundant in coastal marinewaters.Most have bodies covered by a folded carapace that gives an appearance of being bivalved.

The enlarged second antennae are the primary swimming appendages. For most Cladocera the primary food source is phytoplankton, which are filtered from water passing through appendages within the carapace. A few species are predaceous on other zooplankton. Like copepods, cladocerans are well known for diel (diurnal) vertical migrations. Their occurrence tends to be seasonal, with a resting egg formed between periods of rapid parthenogenic reproduction. Cyclomorphosis is common.

Euphausids or krill, are relatively large marine crustaccans (up to 3cm) that are found worldwide at all depths. Individual species have more limited distribution. Some species, especially coldwater forms, demonstrate swarming behavior, and in high latitudes they are the major food source of baleen whales. The most important food of polar euphausids appears to be diatoms, while carnivorous species are more common in warmer waters.



Most aquatic insect species tend to be associated with the bottoms, shores, or surface layers of streams and lakes. The only major insect member of the plankton is the larvae of the phantom midge Chaoborus. This larvae is able to resist anoxia (lack of oxygen) and is often found at the bottom during day and in the plankton at night. Chaoborus species are important predators of other zooplankton in lakes.

Tunicates are all marine. Many are benthic and produce planktonic larvae, but two classes, Appendicularia and Thaliasia, are holoplanktonic. Appendicularia are neotenic and build a mucous house which acts as a filter to capture small food particles. The Thaliacea, including salps and doliolids, are also filter feeders.





A problem faced by all plankton is to maintain position in the water column. Flattened bodies and numerous lateral spines or plumose setae which increase surface-to-volume ratios are common in various zooplankters. This increases resistance to the passage of water and thus slows sinking. Other adaptations to a floating existence are positive buoyancy mechanisms, such as oil droplets, gas-filled floats, or regulation of ionic balance by replacement of heavy ions with lighter ones. Some zooplankton have gelatinous sheaths that appear to slow the sinking rate. For example, the gelatinous sheath produced by the cladoceran Holopedium gibberum reduces its sinking rate by about 50%.

Life in open water exposes zooplankton to heavy predation by visual predators, especially fishes. Many zooplankters are nearly transparent, which affords them some protection. Others, such as fresh-water mites, have conspicuous coloration to advertise their noxious taste, so that visual predators learn to avoid them. In the blue light that penetrates to deep water, reddish hues appear black and thus invisible; it is not surprising that many bathypelagic zooplankton are red. Another antipredation adaptation is cyclomorphosis. Kairomones (chemical messengers that are beneficial to the recepient) released by predators appear to induce cyclomorphosis in succeeding generations of rotifers and cladocerans. These changes in morphology, such as increased spine lengths or head shields, reduce predation. Some adaptations have multiple advantages; the gelatinous sheath of Holopedium reduces predation by invertebrates in addition to slowing the sinking rate.




Verticalmigration. Mostmajor zooplankton groups have at least some species that display dielmigrations which usually consist of downward movement during the day and upward movement at night. The distance traveled can be hundreds of meters. The same species may display the classical day-down, night-up movement in one area and display no migratory be havior in another location. The sex and age of the zooplankton, as well as the season, can affect their vertical position in the water column and the degree of migration observed. Diurnal light variation is the most likely mechanism triggering vertical migration. Many planktonic animals are positively phototactic at low light intensities and negatively phototactic at high intensities. Although the general consensus is that light is a major stimulus for the timing of migration, there are many explanations for its purpose. Zooplankton may sink to depths where illumination is insufficient for detection by visual predators in daytime, while at night, when visual predators do not hunt, zooplankton can return to the surface to feed on phytoplankton. Another explanation is that zooplankton remain in deeper, colder water during the day to reduce their metabolism, and return to the surface at night to feed on phytoplankton. The energetic advantage gained by remaining in colder water must exceed that expended in migration and lost due to lack of continuous feeding at the surface. Additionally, the potentially reduced food quantity and the cold temperature of the deeper water result in slower egg development and thus lower reproductive rates. Differential migration by ecologically similar species could also reduce competition for resources. It is unlikely that a single factor can explain all vertical migration; the above factors and others probably interact, or are important at different times.


Zooplankton, like all organisms, have a range of environmental conditions to which they are adapted. The optimum environment for one species may be barely tolerable to another. Physical and chemical boundary conditions, including turbulence, light, temperature, and salinity gradients, are important in determining species makeup of a zooplankton community. For example, plankton on the two sides of the Gulf Stream or within the upper and lower waters of a lake differ considerably. Some zooplankton have such clear-cut ecological demands that the presence of particular species can indicate the origin of the water mass. Narrow temperature tolerances limit some taxa to tropical or polar waters or to certain periods of the year in temperate zones. Differential salinity tolerances are reflected by changes in the composition of a community as estuaries become increasingly brackish downstream. Thus zooplankton populations are not distributed homogeneously, but tend toward both vertical and horizontal patchiness.

In addition to the physical and chemical boundary conditions mentioned above, biological interactions such as predation, food availability, reproduction, and social behavior affect the distribution of zooplankton and the resulting community structure. Trophic interactions, that is, “what eats what,” are of major importance in structuring zooplankton communities. The presence of fishes that feed selectively on larger zooplankton can limit the species composition to smaller-bodied animals. Lakes without size-selective vertebrate predators usually have a higher proportion of larger invertebrate plankton. Likewise, selective feeding by zooplankton on various algae and protozoa will alter the community structure of these groups.

Seasonal breeding of holoplanktonic organisms and entrance of larval meroplankton into the water column play a large part in changing community composition. Food supply, temperature, and other factors interact to determine seasonal breeding patterns in zooplankton. Increasing light and temperature in temperate and boreal areas leads to spring phytoplankton blooms, when many zooplankters release their young. Other groups have maximum abundances in summer or during a secondary peak of primary production in the fall.





Productivity is the amount of biomass generated per unit time. There is extensive geographic variation in the pattern and magnitude of productivity of the phytoplankton that are the base of planktonic food webs. This variation in primary production dictates a similar difference in the secondary productivity of zooplankton in both lakes and marine systems. Zooplankton tend to grow faster or produce more young when high-quality food is readily available. However, organisms are not 100% efficient at converting food to biomass. Some ingested food is not assimilated into the animals’ bodies, and some assimilated food is lost to maintenance metabolism and respiration; only what is left becomes available for secondary production (growth and reproductive output). Although conversion efficiencies as high as 90% have been determined for individual zooplankton species feeding on high-quality foods in laboratory experiments, 10–30% efficiencies are more common. Zooplankton production is difficult to determine for natural populations because of the diversity of zooplankton species, the differential digestibility and availability of their food sources, and temperature-induced alterations in their metabolism, among other reasons.




Planktonic  food web. The classic description of the trophic dynamics of plankton is a food chain consisting of algae grazed by crustacean zooplankton which are in turn ingested by fishes. This model may hold true to a degree in some environments such as upwelling areas, but it masks the complexity of most natural food webs. Zooplankton have an essential role in linking trophic levels, but several intermediate zooplankton consumers can exist between the primary producers (phytoplankton) and fish. Thus, food webs with multiple links to different organisms indicate the versatility of food choice and energy transfer and are a more realistic description of the planktonic trophic interactions.

Size is of major importance in planktonic food webs. Most zooplankton tend to feed on organisms that have a body size smaller than their own. However, factors other than size alsomodify feeding interactions. Within a range of ingestible sizes, dependent on species and age, zooplankton can track changes in the particle-size spectrum of phytoplankton and graze the most abundant size classes. Some phytoplankton are noxious and are avoided by zooplankton, and others are ingested but not digested. Furthermore, zooplankton frequently assume different feeding habits as they grow from larval to adult form. They may ingest bacteria or phytoplankton at one stage of their life cycle and become raptorial feeders later. Other zooplankton are primarily herbivorous but also ingest heterotrophic protists and can opportunistically become carnivorous. Consequently, omnivory, which is considered rare in terrestrial systems, is a relatively common trophic strategy in the plankton. In all foodwebs, some individuals die without being consumed and are utilized by scavengers and ultimately by decomposers (bacteria and fungi).



Robert W. Sanders

Bibliography. W. Lampert and U. Sommer, Limnoecology, 1997; M. Omori and I. Tsutomu, Methods in Marine Zooplankton Ecology, 1992; U. Sommer (ed.), Plankton Ecology, 1989; T. Tamminen and H. Kuosa (eds.), Eutrophication in Planktonic Ecosystems: Food Web Dynamics and Elemental Cycling, 19XX; J. Thorp and A. Covich (eds.), Ecology and Classification of North American Freshwater Invertebrates, 1991; C. D. Todd, M. S. Laverack, and G. Boxshall, Coastal Marine Zooplankton, 1996.

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