Zooplankton are small animals in the planktonic community. These microorganisms are heterotrophic aquatic organisms. In Greek, "zoo" comes from "anima" and "plankton", which means wanderer, drifter, or wanderer. Even though some of them are larger, they are typically microscopic.

Biochemically, zooplankton are heterotrophic planktonic organisms ranging in size from microscopic organisms to large creatures. These organisms drift in aquatic ecosystems and are ecologically significant drifting organisms. The presence of these small animals in the water column of all water bodies is crucial to maintaining the food chain. Although they are rare in rivers and streams, they can be found in lakes, ponds, and oceans.

Size

A few millimetres to a few microns is the size of these organisms (1 micron is 1/10000 of a millimetre).

Movement

The main transport mechanism for zooplankton species is ambient water currents. In some cases, locomotion is used to either avoid predators or increase the rate at which prey is encountered.

Zooplankton Classification

There are two ways to group zooplankton: by size and by developmental stage.

Zooplankton Classification by Size

Picoplankton: Zooplankton that is no larger than 2 micrometres.
Nanoplankton: This category consists of zooplankton that is between 2 and 20 micrometres in size
Microplankton: A form of zooplankton with a size range of 20-200 microns.
Mesoplankton: This category includes zooplankton between 0.2 and 20 millimetres in size.
Macroplankton: Animals between 20 and 200 millimetres in size.
Megaplankton: Populations of zooplankton that measure over 200 millimetres in size, or about 8 inches in length.

Zooplankton Classification Based on Their Developmental Stages

Meroplankton: Meroplankton is zooplankton when it is in larval form. Over time, they metamorphose into molluscs, corals, fishes, insects, echinoderms, and crustaceans.
Holoplankton: Zooplanktons that remain planktonic throughout their lives. Pteropods, larvaceans, copepods, chaetognaths, and siphonophores are examples of these organisms.

Distribution of Zooplankton

Radiolarians
Dinoflagellates
Foraminiferans
Crustaceans
Molluscs
Cnidarians
Chordates

Radiolarians: As well as radiolarians, there are radiozoa. Protozoans that produce mineral skeletons made of silica are small, 0.1-0.2 mm in diameter. Globally, this type of zooplankton is found in all oceans. Ocean bottoms often contain the skeletal remains of these organisms.

Dinoflagellates: In the distribution of zooplankton, these are very few in number. Because they are photosynthetic or ingest other species, they are considered mixotrophic species. Dinoflagellates account for a significant proportion of marine eukaryotes and are essential to coral reef health.

Crustaceans: Organisms of this type are arthropods. Barnacles, crabs, krill, and shrimp are among them. A significant part of the food chain they are a variety of sizes. The zooplanktonic species krill and copepods are especially important among crustaceans.

Molluscs: A wide variety of organisms makes up the molluscs. Octopuses, squids, sea snails, and sea slugs are some of the species found in these waters. The larvae stage of these organisms is usually zooplanktonic.

Cnidarians:They are members of the phylum Cnidaria, which includes colonial siphonophores and scyphozoans (true jellyfish). Their stinging tentacles make them predators. Most of these creatures live in the ocean nearer to the surface of the water and are seldom found in freshwater.

Chordates:An animal that has a notochord, pharyngeal slits, a hollow nerve chord on the dorsal side, and a post-anal tail is called a chordate. This distribution of zooplankton has a large variety of forms and functions.

Zooplankton Diagram

(Image Will be Updated Soon)

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 woodlouse
Giant isopod
Anchor worms
Air fern
Talitres saltator
Common salp
Pacific sea nettle
Ligia oceanica
By-the-wind sailor
Flying spaghetti monster (Bathyphysa conifer)

What are the Benefits of Zooplankton?

Zooplankton plays an important role in the food web of water bodies due to their position in the food chain. These factors can have a significant impact on the quality of the water, fish production, algal densities, and nutrient and contamination cycling.

Biomonitoring programs frequently include zooplankton species due to their sensitivity to environmental change. Furthermore, some zooplankton species, such as Mysis, have been intentionally introduced to lakes to increase fish production.

Algal blooms may be controlled by increasing zooplankton grazing because zooplankton feed on algae. Biomanipulation is the process of doing this.

Normally this is accomplished by reducing predation on the zooplankton diagram; their predators, such as planktivorous fish, are either removed directly, or a fish predator, such as a pike, is added. Grazing by zooplankton can prevent eutrophication caused by algal blooms in water bodies.

Difference Between Zooplankton and Phytoplankton

	Phytoplankton	Zooplankton
Location	In fresh and marine water, they look like plants	They resemble animals. The majority of them live in water bodies.
Appearance	A group of them forms cloudy patches of brown colour.	Most of them are translucent and come in various colours and shapes.
Oxygen	Lots of oxygen is liberated by them	Oxygen is consumed by them
Mode of Nutrition	Photosynthesis or chemosynthesis is how they make their food	Zooplankton and phytoplankton they consume
Trophic level	The phytoplankton is a primary producer	Zooplankton are primary or secondary consumers in aquatic food webs.
Examples	Cyanobacteria, Blue-green algae, diatoms, and dinoflagellates are typical phytoplankton examples	Krill, meroplankton, protozoa, and holoplankton are all zooplankton species.

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FAQs on What Are Zooplankton? Types, Functions & Examples

1. What are zooplankton and where are they found?

Zooplankton are a diverse group of small, free-floating animals or animal-like organisms that drift in the water column of oceans, seas, and freshwater bodies. Unlike phytoplankton, which are producers, zooplankton are primary consumers because they cannot produce their own food. They form a critical link in aquatic food webs, consuming microscopic algae and in turn being eaten by larger animals.

2. What are the main types of zooplankton based on their life cycle?

Zooplankton are broadly classified into two main types based on their life stages:

Holoplankton: These are organisms that spend their entire life cycle as plankton. Common examples include copepods, krill, and cladocerans.
Meroplankton: These are organisms that are planktonic for only a part of their life, typically the larval stage. Examples include the larval forms of larger animals like fish, crabs, lobsters, and starfish.

3. What do zooplankton typically eat?

The diet of zooplankton is varied, but their primary food source is phytoplankton (microscopic marine algae). Many zooplankton are herbivores that graze on these algae. However, their diet can also include:

Bacterioplankton (bacteria)
Detritus (organic waste material)
Other, smaller zooplankton (making them carnivorous or omnivorous)

4. What are some common examples of zooplankton?

Zooplankton include a vast range of organisms, from single-celled protists to larger animals. Some well-known examples are:

Copepods: Small crustaceans that are often the most abundant members of the zooplankton community.
Krill: Small, shrimp-like crustaceans that are a key food source for whales, seals, and birds.
Cladocerans: Often called water fleas, such as Daphnia.
Jellyfish: Macroscopic gelatinous zooplankton.
Larval Fish and Crabs: Temporary members of the plankton (meroplankton) before they mature.

5. How are zooplankton different from phytoplankton?

The primary difference lies in their role within the ecosystem and how they obtain energy. Phytoplankton are microscopic plants (autotrophs) that perform photosynthesis to produce their own food, forming the base of the aquatic food web. In contrast, zooplankton are microscopic animals (heterotrophs) that must consume other organisms, like phytoplankton, for energy, making them primary consumers.

6. What is the ecological importance of zooplankton?

Zooplankton play a crucial role in aquatic ecosystems. Their importance stems from being the primary link between producers and higher trophic levels. They transfer energy, captured by phytoplankton from the sun, to larger organisms like small fish, which are then eaten by larger fish, marine mammals, and birds. This makes them an indispensable part of the aquatic food chain and the overall health of marine and freshwater environments.

7. What special adaptations help zooplankton survive and float in water?

To avoid sinking and to escape predators, zooplankton have evolved several key adaptations:

Increased Buoyancy: Many possess oil droplets, flat bodies, or long spines to increase their surface area, which helps them float and resist sinking.
Camouflage: Many species are transparent, making them nearly invisible to predators in the water column.
Diel Vertical Migration: This is a behavioural adaptation where zooplankton migrate to deeper, darker waters during the day to avoid visual predators and return to the surface at night to feed on phytoplankton.