Zooplankton

Zooplankton is a collective term for a vast and diverse group of small, often microscopic, animals that drift or weakly swim in aquatic environments. It encompasses a wide range of organisms with different scientific classifications. 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.

They play 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. This article explores these species in the food chain with the view of helping every reader appreciate their critical contribution to the environment.

Appearance

The term “plankton” is a broad category that encompasses all organisms—both plant-like and animal-like—that float and drift in water columns and are largely incapable of swimming against currents. This includes both phytoplankton and zooplankton, among other microscopic organisms.

Zooplankton, in particular, comprises a diverse array of organisms like tiny copepods, krill, and fish larvae, that drift with ocean currents, spending either part of their life or their entire life as plankton. Many, including rotifers, mollusks, and polychaetes, possess transparent bodies to evade predators.

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.

Also, most zooplankton have developed effective camouflage to help them survive in the ocean ecosystem. Remember, many marine animals depend on them for food.

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

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.  

Zooplankton plays a crucial role in the marine ecosystem, serving as an important source of food for various animals, from filter feeders like corals and baleen whales to seabirds and larger fish.

Here are three examples of animals that belong to this family:

1. Scale worm larva

2. Mantis shrimp larva

3. Copepod candacia

Habitat

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 they are 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. The brownish or greenish color of the water indicates the presence of plenty of phytoplankton.

Diet

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

In most instances, zooplankton is a secondary consumer. These animals eat phytoplankton, which are often smaller than them.

Some 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.

Reproduction

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

Asexual reproduction is common among zooplankton that spend their entire life drifting along 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 use 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. 

 

Threats

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.

Studies suggest that by focusing on these key areas and utilizing advanced methodologies, future zooplankton research can significantly enhance our understanding of biological processes in the ocean. This knowledge is crucial for comprehending community and ecosystem functioning, as well as predicting the impacts of environmental change.

6 Interesting Facts About Zooplankton

1. Zooplankton play a significant role in regulating biogeochemical cycling and biomass in marine ecosystems. 
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. Comb jellies, with their long tentacles, capture prey particles from the water column.
4. Meroplankton, such as crab larvae, spend only part of their life as plankton before developing into bottom-dwelling adults.
5. Baleen whales migrate to warm waters for breeding and rely on dense krill swarms as their primary food source.
6. Sea ice provides a crucial habitat for some species, offering protection and food sources, making them vulnerable to the effects of less iced marine environments.

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