What is Zooplankton? Examples and Types

What is Zooplankton?

Zooplankton is a small aquatic animal of the planktonic community. They are heterotrophic aquatic microorganisms in the water column. The word “zoo” comes from the Greek word for “animal” and “planktos” which means “wanderer” or “drifter“. They are normally microscopic, even though some of them are larger and can be seen with the naked eye.

Zooplankton definition in biology

In biology, zooplankton can be defined as a type of heterotrophic plankton that ranges from microscopic organisms to large species. They are found within aquatic ecosystems and are drifting organisms that are ecologically important. These small animals are crucial components of the food chain that live in the water column of all water bodies. They are seen in lakes, ponds, oceans, and even freshwater bodies though they are rare in rivers and streams.

Zooplankton
Photo credit: https://extension.usu.edu

Nutrition

Zooplanktonic organisms cannot produce their own food. Thus, they feed on other plants or animals and also feed on phytoplankton. They are made up of both primary and secondary consumers that feed on free-floating algae and other zooplankton. Zooplankton are therefore known as heterotrophic plankton.

These zooplanktons similarly have a wide range in feeding behavior which include filter-feeding, predation, and symbiosis with phytoplankton like in corals. They feed on bacterioplankton, phytoplankton, detritus, nektonic organisms, and even other zooplanktons. Due to this, zooplanktons are mainly found in surface waters where phytoplankton and other zooplankton (food resources) are abundant.

Size

The sizes of these organisms range from a few millimeters to a few microns (1 micron = 1/1000 of a millimeter).

Movement

Zooplanktonic species are mainly transported by ambient water currents. However many of them have locomotion to either avoid predators or to increase prey encounter rate.

What is plankton?

Plankton is an organism that drifts in water bodies (oceans, seas, and freshwater). Generally, plankton consists of phytoplankton and zooplankton. They are typically tiny animals that are found near the surface in water bodies. Phytoplankton and zooplankton are usually weak swimmers and therefore, they usually just drift along with the currents. Plankton is grouped into two main groups: holoplankton and meroplankton.

The holoplankton is a permanent member of the plankton which includes diatoms, salps, radiolarians, krill, dinoflagellates, amphipods, foraminifera, copepods, and many others.

The meroplankton are temporary members of the plankton which include most larval forms of echinoderms, crustaceans, marine worms, most fish, some marine snails, and many others. Zooplankton and phytoplankton are key components of marine ecosystems. They actually form the base trophic levels of most marine food webs.

Protists are the most common plankton as well as nanoplanktonic flagellates, veliger larvae, cnidarians, rotifers, ctenophores, Chaetognatha, copepods, cladocera, euphausids, krill, and tunicates. These protists can produce energy by photosynthesis, therefore, they are primary producers and form the base of marine food webs.

The zooplankton community is a crucial element of the aquatic food chain. They are an intermediary species in the food chain. These organisms transfer energy from planktonic algae which are the primary producers to the larger invertebrate predators and fish. The fish and the larger invertebrates in turn feed on them.

Zooplanktons are very sensitive to changes in the aquatic ecosystem. This is why they are used as bioindicators because the effects of environmental disturbances can cause changes in their composition, abundance, and body size distribution. As a result, the kind of zooplankton seen in the water and the abundance of certain species of zooplankton serve as an indicator of a biological condition. They are good at indicating changes in nutrient pollution over time. This is because they are quick to respond to changes in nutrient input to water bodies.

What does zooplankton eat?

Zooplanktonic organisms are usually found in the surface water where their kind of food is abundant. They feed on a variety of bacterioplankton, phytoplankton, and even other zooplankton. The majority of zooplanktons are filter feeders that use their feeding appendages to filter particles from the water. They use their appendages to strain bacteria, algae as well as other fine particles in the water. Whereas, other zooplankton are predators that feed on smaller zooplankton.

They are the center of the open-water food web of most lakes. Bacteria and algae form the base of the food web and are eaten by zooplankton which is in turn eaten by insects, fishes, and other zooplankton. Many zooplankton possesses a clear shell that prevents them from being seen by visual feeders like fishes.

Zooplanktonic species use a variety of feeding strategies in order to keep their taxonomic diversity. They can eat algae, bacteria, and other zooplankton. Also, they may even be parasites while some are indiscriminate grazers such as many Cladocera.  Whereas, others like many Copepods are more selective by selecting their zooplankton prey or individual particles based on their size, shape, and taste.

Zooplanktonic organisms are also predators of algae or protozoa, which are phytoplankton. The population of zooplankton increases proportionately as the population of phytoplankton. Light affects the population of phytoplankton which indirectly affects that of zooplankton as well. Whenever there is an abundance of light such as in boreal or temperate areas, the population of phytoplankton increases which provides more food for the zooplankton which in turn also increases in their population. This phenomenon can be observed during the spring when the light concentration begins to increase.

Is zooplankton a producer?

The zooplanktons are not producers, rather, they are the animal-like primary consumers of the plankton community. Zooplanktonic organisms in turn are food for larger secondary consumers like fish. The phytoplankton, on the other hand, is the producer, not zooplankton, they are the tiny plant-like producers of the plankton community.

Phytoplankton includes bacteria and algae which form the trophic base of aquatic food webs. Typical examples of phytoplankton are diatoms, dinoflagellates, cyanobacteria, and green algae. These phytoplanktonic organisms through photosynthesis produce nutrients and oxygen for other organisms.

Zooplankton classification

The zooplankton can be grouped by size and by developmental stage.

Classification of zooplankton according to size

  1. Picoplankton: This includes zooplanktons that measure less than 2 micrometers in size.
  2. Nanoplankton: This category comprises zooplankton that measures between 2-20 micrometers in size.
  3. Microplankton: This includes zooplankton with sizes that ranges between 20-200 micrometers.
  4. Mesoplankton: This includes zooplankton with sizes that measure between 0.2-20 millimeters.
  5. Macroplankton: This includes zooplankton that measures between 20-200 millimeters in size.
  6. Megaplankton: This includes zooplankton with sizes that measure over 200 millimeters which is almost 8 inches.

Examples of microscopic zooplankton

  1. Protozoans
  2. Crustaceans
  3. Zooflagellates
  4. Foraminiferans
  5. Radiolarians
  6. Some dinoflagellates
  7. Marine micro animals
  8. Aquatic mites
  9. Rotifers
  10. Open water insect larvae

Examples of macroscopic zooplankton

  1. Tunicates
  2.  Pelagic cnidarians
  3. Ctenophores
  4. Mollusks
  5. Arthropods
  6. Planktonic arrow worms
  7. Bristle worms.

Classification of zooplankton based on their stages of development

  1. Meroplankton: These organisms are usually zooplankton at their larvae stage. They eventually later metamorphose into mollusks, coral, fishes, insects, echinoderms, or crustaceans.
  2. Holoplankton: These zooplanktons remain planktons during their entire life cycle. Examples are the pteropods, larvaceans, copepods, chaetognaths, and siphonophores.

Types of Zooplankton

  1. Radiolarians
  2. Dinoflagellates
  3. Foraminiferans
  4. Crustaceans
  5. Molluscs
  6. Cnidarians
  7. Chordates

Radiolarians

Radiolarians are also called radiozoa. They are small protozoans of diameter 0.1-0.2 mm that produce mineral skeletons that are made of silica. This group of zooplankton is found throughout the global ocean. The skeletal remains of these organisms can be found at the bottom of oceans. Radiolarians comprise a large part of the ocean floor cover as siliceous ooze. As a result of their rapid change and intricate skeletons, radiolarians represent an important diagnostic fossil found from the Cambrian to date.

Dinoflagellates

This zooplanktonic type is extremely small. They are considered mixotrophic species because they can be photosynthetic or ingest other species. Dinoflagellates represent a significant part of marine eukaryotes and are crucial for the health of coral reefs.

The nanoplanktonic flagellates are also zooplanktons that help keep bacteria populations in check. They either have a long tail for swimming (flagellates) or a hair-like structure referred to as cilia (ciliates). Some dinoflagellates have a protoplasmic net (net-like structure). They use this protoplasmic net to capture and eat their prey that is larger than bacteria in size. Also, some of these dinoflagellate species are responsible for fish kill and red tide.

Foraminiferans

This type of zooplanktonic organism can be seen in sediments. Also, they can be seen drifting about the upper surface of water bodies. Foraminiferan is a type of amoeboid protest and has an external shell and ectoplasm used to obtain food. The shell of foraminiferans is comprised of mainly calcium carbonate with some species having other minerals.

Crustaceans

These organisms are a type of arthropod. They include crabs, krill, shrimp, and barnacles. Ranging in terms of size, they comprise a significant part of the food chain. Amongst the crustaceans, krill and copepods especially, are crucial zooplanktonic species.

Molluscs

The mollusks are a highly diverse group of organisms. They include the octopus and squid species as well as sea snails and sea slugs. These organisms, however, are usually zooplanktonic at their larvae stage.

Cnidarians

These organisms belong to the phylum Cnidaria that consists of the colonial siphonophores and the scyphozoans (true jellyfish). They are both predators and possess stinging tentacles. These animals live in the ocean at the layers that are closer to the water surface and are seldom found in freshwater.

Cnidarians are gelatinous zooplankton that has specialized cells called cnidocytes. They use the cnidocytes to capture their prey. Also, their bodies have a jelly-like substance (mesoglea), a mouth, and tentacles that contain the cnidocytes.

Chordates

Animals that possess a notochord, pharyngeal slits, dorsal hollow nerve chord, and post-anal tail are called chordates. This is a highly diverse group of animals.

Zooplanktonic chordates include:

  1. Vertebrates such as larval fish and fish eggs
  2. Cephalochordates such as amphioxus, or lancelet larvae
  3. Urochordates such as tunicates. Zooplanktonic tunicates include the tadpole larvae of adult benthic tunicates as well as salps, doliolids, and appendicularians.

Zooplankton examples

  • Krill
  • Jellyfish
  • Aquatic sowbug or water louse
  • Segmented worm
  • Portuguese man o’ war
  • Calamus finmarchicus
  • Ostreopsis ovata
  • Blue button (Porpita porpita)
  • Acartia tonsa
  • Forams
  • Acartia clausi
  • Paracalanus parvus
  • Common pill-bug
  • Ligia exotica
  • Common rough wood louse
  • Giant isopod
  • Anchor worms
  • Air fern
  • Talitrus saltator
  • Common salp
  • Pacific sea nettle
  • Ligia oceanica
  • By-the-wind sailor
  • Flying spaghetti monster (Bathyphysa conifera)

Zooplankton adaptation

For plankton to be able to float in the water, they have to develop structural adaptations. Hence, all species of plankton have adaption for floating. Such adaptations include:

  • Flat bodies
  • Lateral spines
  • Oil droplets
  • Floats filled with gases
  • Sheaths made of gel-like substances
  • Ion replacement

The flat body and spines of some species of plankton resist them from sinking. This is achieved by increasing the surface area of their bodies while minimizing the volume. All the other adaptations also prevent plankton from sinking quickly to the bottom.

Also, zooplankton has adapted mechanisms to scare off or discourage fish which happens to be their heaviest predator. Such adaptive mechanisms include:

  • Transparent bodies
  • Bright colors
  • Bad tastes
  • Red coloring in deeper water
  • Cyclomorphosis: This occurs when predators release chemicals in the water. These chemicals thereby signal zooplankton like rotifers or cladocerans to increase their spines and protective shields.

Zooplankton Habitat

Certain zooplankton species occupy specific marine habitats. They are uniquely adapted to factors in their habitats such as light, turbulence, temperature, and salinity. The zooplankton species on one side of the Gulf Stream are different from the zooplankton on the other side. These characteristics of various zooplankton species can sometimes be used by scientists to distinguish one water body from another.

Also, zooplanktons are also sensitive to their habitat and a change in their concentration can indicate an environmental change. They respond highly to pollution, nutrient levels, temperatures, heightened predation, and levels of light. These organisms provide an essential link in the marine food chain of their habitat. The diversity of zooplankton species, amount of biomass and their abundance can be used as a determining factor to the ecosystem’s health.

Zooplankton can be limited within a geographical region and are not dispersed uniformly or randomly within a region of the ocean, there are patches of their species throughout the ocean. However, above the mesopelagic, few physical barriers exist. Certain zooplanktonic species are hindered strictly by temperature and salinity gradients. Whereas other species can tolerate wide salinity and temperature gradients.

Also, the patchiness of zooplankton can be influenced by biological factors and other physical factors. Such biological factors are vertical migration, breeding, predation, as well as the concentration of phytoplankton. The mixing of the water column is the physical factor that influences zooplankton distribution the most. Mixing of the water column affects nutrient availability as well as phytoplankton production.

The distribution of zooplankton in their habitat is influenced by other factors such as reproduction, predation, community interaction, and the number of available resources. Levels of pH, calcium, heavy metals, and aluminum in the habitat also affect zooplankton. Some nutrients such as phosphorous and nitrogen will affect the prey of zooplankton which will indirectly affect the zooplankton survival.

Zooplankton in the food chain

Zooplankton and phytoplankton are key components of marine ecosystems. They actually form the base of trophic levels of most marine food webs. The zooplankton community is a crucial element of the aquatic food chain. They are an intermediary species in the food chain. These organisms transfer energy from planktonic algae which are the primary producers to the larger invertebrate predators and fish. The fish and the larger invertebrates in turn feed on them.

As zooplankton feed on and process phytoplankton as well as other food sources, they play a role in aquatic food webs. They serve as a resource for consumers on higher trophic levels, and as a channel for packaging the organic matter in the biological pump. Zooplanktons are the animal-like primary consumers of the plankton community. They are in turn eaten by larger secondary consumers like fish. The phytoplankton, on the other hand, is the producer. They are the tiny plant-like producers of the plankton community. Once, there is an increase in phytoplankton abundance, zooplankton responds rapidly such as during the spring bloom.

Also, zooplanktonic organisms are a key link in the biomagnification of contaminants like mercury. They can also serve as a disease reservoir. For instance, the Crustacean zooplankton has been seen housing the bacterium Vibrio cholerae that causes cholera. Thus, Vibrio cholerae and zooplankton have a symbiotic relationship as zooplankton allow the bacterium to attach to their chitinous exoskeleton. This relationship enhances the ability of the bacterium to survive in waterbodies. The exoskeleton gives the bacterium nitrogen and carbon.

How do zooplankton reproduce

Zooplankton tends to reproduce rapidly. As a result, their population can increase a day by about 30% under favorable conditions. Their reproduction may be sexually or asexually depending on the species. However, asexual reproduction is more common among holoplankton. This can be done via cell division where one cell divides into two cells, and so on. Asexual reproduction takes place in planktonic organisms but tends to be less common in marine invertebrates.

Holoplanktonic species and some meroplankton possess specialized features that enable them to reproduce in water more efficiently. Sexual reproduction is seen in various planktons and benthic organisms. Some of them make use of specialized appendages. These appendages, during copulation, grasp onto their respective partners to prevent them from drifting away.

Other meroplanktonic and holoplanktonic organisms release sperm into the water column. The females then take up the sperm to fertilize their eggs. Some zooplanktons release eggs and sperm simultaneously into the planktonic habitat to increase the probability of fertilization. Environmental, mechanical, or chemical cues can trigger the release of this sperm and eggs.

Phytoplankton vs Zooplankton

The main difference between these two planktons is that phytoplanktons are plants whereas zooplanktons are animals.  Phytoplanktons are the primary producers in the aquatic food chains, whereas zooplanktons are consumers that feed on the phytoplankton.

Phytoplanktons as producers can manufacture food via chemosynthesis or photosynthesis, hence, they liberate large amounts of oxygen and are known as good indicators of ocean health.

Similarities

In terms of similarities, they both float on water surfaces and are similar in size. Phytoplankton and zooplankton are economically important.

Differences between Phytoplankton and Zooplankton

A table showing the differences between phytoplankton and zooplankton
PhytoplanktonZooplankton
LocationThey are plant-like and are found in fresh and marine waterThey are animal-like. They are found in water bodies and most of them are larval stages of other aquatic animals
AppearanceThey grow in a group forming cloudy patches that are brown in color.They are mostly translucent and can appear in various shapes and colors.
OxygenThey liberate lots of oxygenThey make use of oxygen
Mode of NutritionThey manufacture their own food via photosynthesis or chemosynthesisThey consume other zooplankton and phytoplankton
Trophic levelPhytoplanktons are primary producerszooplanktons are primary or secondary consumers in the aquatic food web.
ExamplesTypical examples of phytoplankton include cyanobacteria, Blue-green algae, diatoms, dinoflagellatesExamples of zooplankton include crustaceans like krill, meroplankton, protozoans, and holoplankton.

 

Why is zooplankton important?

Zooplankton are important due to their crucial position in the food webs of water bodies. They can strongly affect the quality of the water, fish production, algal densities, nutrient and contaminant cycling.

Since the density and composition of zooplankton species are sensitive to environmental changes, they are commonly included in biomonitoring programs. Also, some zooplankton like Mysis has been introduced deliberately to lakes for fish production enhancement.

Since zooplankton feeds on algae, it has been proposed that by increasing zooplankton grazing, it may be possible to control algal blooms. This method is referred to as biomanipulation. It is done usually by reducing predation on zooplankton; their predators such as planktivorous fish are either removed directly or by adding a fish predator such as a pike. Algal blooms in water bodies cause eutrophication and zooplankton grazing can prevent it.

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