Zooplankton

Zooplankton are tiny animals that float in water. They cannot swim against strong currents and move wherever the water carries them. Their name means "animal drifter," because they wander with the water flow.

Unlike phytoplankton, which are plant-like and can make their own food using sunlight, zooplankton need to eat other small organisms to survive. Most zooplankton are very small and can only be seen with a microscope. But some, like jellyfish, are big enough to see without tools.

Many different tiny creatures are zooplankton. These include single-celled organisms and small animals. Some zooplankton can sometimes make their own food and also eat other things, showing how flexible they are.

Overview

Zooplankton are tiny animals that float in water. The name means "animal drifter." They cannot swim against strong currents, so they move with the water.

Zooplankton eat other small organisms, like tiny plants called phytoplankton, to survive. They come in many sizes and types, from very small to larger ones like tiny crustaceans. They are important because they help move energy through the food chain, providing food for bigger animals like fish.

Some zooplankton live their whole lives drifting, while others only spend part of their lives this way before moving to other areas. They are found in patches throughout the ocean, influenced by water temperature, salt levels, and other factors.

Size classification

Size is important for plankton because it affects how they grow, eat, and survive. Bigger animals often live in colder places, called Bergmann's rule. In the ocean, size helps decide who eats whom and how carbon moves through the water.

Zooplankton come in different sizes. Microzooplankton are tiny animals that eat many of the tiny plants in the ocean. They help keep the ocean’s food chain going. Mesozooplankton are bigger and include animals like copepods, which are important food for fish. Because these small animals live for less than a year, scientists can watch how they change to learn about the ocean and climate.

Sampling methods

See also: Plankton net

Research vessels gather zooplankton from the ocean using special fine nets. They pull the nets through the water or bring seawater onboard to pass through the nets.

There are many ways to collect zooplankton:

• Neuston net tows are done near the surface using a net attached to a frame.
• The PairoVET tow is used for collecting tiny fish eggs by dropping a net deep into the water and pulling it back up.
• Ring net tows use a net fitted to a round frame, but these are now often replaced by bongo nets, which have two nets and give duplicate samples.
• The bongo tow pulls nets shaped like bongo drums from a moving vessel. The net is usually lowered deep and then allowed to rise to the surface, collecting samples from the photic zone where many tiny fish live.
• MOCNESS and BIONESS tows and Tucker trawls use several nets that open and close at different depths to study how plankton are spread vertically.
• The manta trawl pulls a net along the water's surface from a moving vessel, collecting tiny fish like grunion, mahi-mahi, and flying fish.

After collecting, the zooplankton is rinsed into the bottom of the net and placed in a special liquid before being studied in a lab.

Another way to collect tiny fish is by using plankton pumps. Water from about three metres deep is brought onboard and filtered through a net while the vessel moves.

Taxonomic groups

Protozooplankton

Protozooplankton are tiny animals that float in water and eat other tiny plants or animals. They are a type of zooplankton, which are small creatures that drift in oceans and lakes.

Protozooplankton include groups like zooflagellates, foraminiferans, radiolarians, and some dinoflagellates. These creatures help control the numbers of other tiny ocean plants and animals.

Radiolarians

Radiolarians are tiny sea creatures with beautiful glass-like shells. They catch food by reaching out through holes in their shells. When they die, their shells sink to the ocean floor and help scientists learn about past ocean conditions.

Foraminiferans

Foraminiferans, often called "forams," are tiny sea creatures with shells that have holes. Their shells can be made from different materials, like calcium or tiny bits of sediment. They are important for studying past climates because their shells are well-preserved in ocean sediments.

Amoeba

Amoeba are tiny creatures that can either have shells or move around without them. They eat by surrounding and swallowing other small organisms.

Ciliates

Ciliates are tiny animals covered in hair-like structures called cilia. They use these cilia to move and eat. Some ciliates eat tiny blue-green algae.

Dinoflagellates

Dinoflagellates are tiny sea creatures with two whip-like parts that help them move. Some of them eat other tiny organisms, while others live inside other sea creatures and help them.

Mixoplankton

Mixoplankton are special tiny sea creatures that can both make their own food from sunlight and eat other tiny organisms. They are very common in the ocean and can survive in different conditions by changing how they get their food.

Planktonic metazoa (animals)

Some tiny animals, like copepods, also float in the ocean. Copepods are tiny crustaceans, usually about 1 to 2 mm long, with teardrop-shaped bodies. They have tough outer coverings and are very important in the ocean because they eat tiny plants and are food for bigger animals.

Other tiny sea animals include ostracods, branchiopods, and malacostracans. Barnacles are only planktonic when they are very young.

Holoplankton and meroplankton

Ichthyoplankton

Ichthyoplankton are the eggs and young stages of fish. They cannot swim well and drift with ocean currents. Fish eggs carry their own food, while young fish eat tiny plants and animals. Both are important food for bigger sea creatures.

Gelatinous zooplankton

Gelatinous zooplankton include creatures like jellyfish, salps, and ctenophores. They are soft and float in coastal waters. Jellyfish were once thought to be unimportant, but scientists now know they are important food for many bigger sea animals, like tuna and swordfish. They are also eaten by birds and other sea creatures.

Mixotrophic zooplankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017
Description		Example		Further examples
The term nonconstitutive mixotrophs was defined by Mitra et al. in 2016), and refers to zooplankton capable of photosynthesis. These include microzooplankton or metazoan zooplankton that gain phototrophic abilities by retaining chloroplasts∆ (often through kleptoplasty) acquired from ingested algal prey or by maintaining photosynthetic endosymbiotic algae within their cells. They are photosynthetic zooplankton that do not inherently possess any genetic machinery for photosynthesis but acquire it secondarily from their prey or symbionts. This contrasts with constitutive mixotrophs, which have their own built-in photosynthetic capabilities.
Generalists	Protists that sequester chloroplasts (occasionally alternate organelles) from a diverse array of algal species			Most oligotrich ciliates with plastid retention ∆
Specialists	1. Protists that sequester chloroplasts (occasionally alternate organelles) either from a single algal species or from several strongly related strains		Dinophysis acuminata	Dinophysis spp. Myrionecta rubra
	2. Protists or zooplankton that maintain endosymbiosis by hosting either a single algal species or several closely related strains as their internal photosynthetic partners		Noctiluca scintillans	Metazooplankton with algal endosymbionts Most mixotrophic Rhizaria (Acantharea, Polycystinea, and Foraminifera) Green Noctiluca scintillans
∆ Chloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.

Role in food webs

Zooplankton are important in the ocean. They eat tiny plants called phytoplankton and help move energy through the food web. When they eat, they help control how much plant life grows and how carbon moves through the ocean.

Their eating habits affect many parts of the ocean, including both the surface and deeper areas. Learning about zooplankton helps us understand how ocean ecosystems work.

Role in biogeochemistry

Zooplankton are important in the ocean because they connect small plants and animals to bigger animals in the food chain. They also help recycle carbon and other important nutrients.

When zooplankton eat, they sometimes leave food behind. This leftover food and their waste release tiny bits of organic material. These bits feed tiny ocean organisms and affect how carbon moves through the ocean.

In some places, like the Kerguelen Plateau, lots of tiny plants grow because of iron in the water. Zooplankton there eat these plants and help control how much carbon moves to deeper parts of the ocean. In other areas, like the Southern Ocean, where there is less iron, zooplankton affect carbon movement in different ways.

Zooplankton also help move carbon to the deep ocean by producing waste pellets, mucous, and even their own bodies when they die. These particles sink and provide food for animals living deep in the ocean.