Zooplankton refers to tiny animals that typically exist near the surface in aquatic environments in a drifting state. Together with phytoplankton (tiny sea plants), they form the plankton family, which refers to aquatic organisms that can’t swim well due to their small body size or weak body structures. That’s why currents, tides, and other forces carry zooplankton into the ocean and other water bodies.

Zooplankton plays a central role in the aquatic food chain. They mainly serve as an intermediary in the food chain by accumulating and transferring energy from the primary producers (phytoplankton) to the fish and larger invertebrate predators that feed on them. Moreover, they serve as a conduit for packaging carbon in the biological pump.

From this brief introduction, you can see that zooplankton are vital parts of our ocean ecosystem. Unfortunately, most people hardly know about them. This article explores these intermediary species in the food chain with the view of helping every reader appreciate their critical contribution to the environment.


Since zooplankton refers to an extensive collection of aquatic organisms, no image of a single creature can illustrate its appearance. However, we know they drift along currents and tides because they cannot swim independently.

MMixed zooplankton sample
Mixed zooplankton sample. Copepods: 1 Calanidae 2 Temora stylifera 3 Calocalanus 4 Clausocalanus 5 Oncaeidae 6 gastropod larva 7 doliolid 8 fish egg 9 decapod larva

Also, most zooplankton has developed effective camouflage to help them survive in the ocean ecosystem. Remember, many marine animals depend on them for food, and they can’t take that for granted.

 Since these animals live in clear, blue water, the best way to camouflage themselves is to be as transparent as possible.

If that’s not enough, we can consider the size of the zooplankton. Studies show that they range from a few millimeters to a few microns. Note that a micron equals 1/1,000 of a millimeter. Some are microscopic, meaning you can’t see them with your naked eye. However, some, like jellyfish, are larger and are classified as macroscopic, meaning you can see them without the help of any device.  

Here are three examples of animals that belong to the zooplankton family:

1. Scale worm larva

Scale worms are covered in a layer of very brightly colored and patterned scales
Scale worms are covered in a layer of very brightly colored and patterned scales

2. Mantis shrimp larva

The second larval stage of a Mantis shrimp, with its shield-like carapace and spines
The second larval stage of a Mantis shrimp, with its shield-like carapace and spines

3. Copepod candacia

Copepods are tiny crustaceans and the most abundant multicellular animals in the sea
Copepods are tiny crustaceans and the most abundant multicellular animals in the sea


Unlike phytoplankton, which is self-feeding, zooplankton is heterotrophic, meaning they can’t manufacture food. These sea animals eat other tiny animals and plants. 

In most instances, zooplankton is a secondary consumer. In other words, these animals eat phytoplankton, which is often smaller than them.

Some zooplankton species are primary consumers, meaning they eat free-floating algae. Some others, like many Cladocera, are indiscriminate grazers. They use their feeding appendages like rakes to filter various types of foods from the water. At the same time, others, like copepods, are more selective. They pick out individual food items based on size, shape, and taste.


Zooplankton inhabits the water column of almost all water bodies. They exist in large numbers in oceans, ponds, and lakes, but you can find a few in some rivers and streams.

Since zooplankton is semi-transparent, it can be difficult to tell whether or not there is a large population of these animals in a particular body of water. However, if the water is brownish or greenish, you can tell that there are plenty of phytoplankton, their primary food source.


Zooplankton primarily reproduces sexually. However, depending on the species, asexual reproduction is also possible.

Asexual reproduction is common among zooplankton that spends their entire life drifting along with currents and tides, known as holoplankton. They primarily accomplish this through cell division. One cell divides into two to increase the population of these animals.

Some zooplankton uses specialized appendages during copulation to prevent their partners from drifting away for a while. Others release sperm and eggs into the water simultaneously to increase the chances of fertilization. At the same time, some zooplankton store the fertilized eggs in a special sac for the entire gestation period. 



Zooplankton belongs to the group of living things that are the most sensitive to changes in aquatic ecosystems. Scientists agree that you can detect the effects of climate change, ocean acidification, rising sea temperature, and the like through variations in species composition, body size distribution, and abundance.

Many marine animals, like fish and larger invertebrate predators, feed on plankton. However, they aren’t among the endangered species as of now.

Facts About Zooplankton

  1. Zooplankton range in size from a few millimeters to a few microns.
  2. These freshwater and marine animals form a central part of the food chain in the aquatic ecosystem, but most people don’t know about them.
  3. Zooplankton uses various feeding strategies and may feed on zooplankton, parasites, algae, bacteria, and more. Some are indiscriminate grazers.
  4. These tiny animals are the best food for many marine animals, so they camouflage themselves by being transparent to survive in aquatic environments.
  5. Some zooplankton house disease-causing organisms and can cause serious health problems in the community.


Is zooplankton harmful to humans?

Yes, zooplankton can be dangerous to humans. Some of them, like crustacean zooplankton, can act as disease reservoirs. They typically host the bacteria that causes cholera, known as Vibrio cholera. They allow the bacteria to attach to their external skeletons, enhancing this disease-causing organism’s ability to survive in aquatic environments and putting the lives of millions of people at great risk.

What happens if there is too many zooplankton?

If too many zooplanktons are in the water, the population of harmful algae may increase significantly. Excess nutrients in the water, referred to as eutrophication, are often responsible for this. In most cases, the excess zooplankton covers the surface of the water, which significantly reduces the ability of sunlight and oxygen to penetrate the water. This can disrupt the natural ecosystem.

Do insects eat zooplankton?

Aquatic insects often feed on these invertebrates as they swim or drift. Some can see these microscopic organisms with their naked eyes. Others target the species classified as macroscopic, which are also at the base of the food chain. Other than aquatic insects, other animals, like fish and salamanders, also eat zooplankton.

What can zooplankton tell you about the condition of water?

Zooplankton can help you know the degree of change in nutrient pollution over time. These animals respond fast to changes in nutrient input since their population can increase significantly within a day or two after the nutrients are added. However, for accurate analysis, you should be able to know the types of zooplankton available in the water and the abundance of specific species relative to others.

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