What Is Zooplankton?

What is Zooplankton?

Zooplankton refers to the small, often microscopic, animals that drift in the water columns of oceans, seas, and freshwater bodies. They form a critical component of aquatic food webs, serving as the primary food source for many marine animals, particularly young fish. Zooplankton can range from tiny organisms like copepods and rotifers to larger forms like jellyfish. Unlike phytoplankton, which are photosynthetic and produce their food, zooplankton primarily consume other planktonic organisms or organic detritus. They play a vital role in marine ecosystems by transferring energy from primary producers (like phytoplankton) to larger predators in the food chain.

Background on Plankton

Plankton are largely at the mercy of the ocean’s currents, winds and waves and don’t have much (if any) mobility. Zooplankton are either too small to compete against the ocean currents or large (like many jellyfish) but have relatively weak propulsion systems. The word plankton comes from the Greek word planktos meaning “wanderer” or “driter.” The word zooplankton incorporates the Greek word zoion, for “animal.

Species of Zooplankton

There are thought to be over 30,000 species of zooplankton. Zooplankton can live in fresh or salt water, but this article focuses on marine zooplankton.

Types of Zooplankton

Zooplankton may be classified according to their size or by the length of time they are planktonic (largely immobile).  Some terms that are used to refer to plankton include:

• Microplankton: organisms that are 2-20 µm in size, including some copepods and other zooplankton.
• Mesoplankton: organisms that are 200 µm-2 mm in size, which includes larval crustaceans.
• Macroplankton: organisms 2-20 mm in size, which includes euphausiids (e.g., krill) – an important food source for many organisms, including baleen whales.
• Micronekton: organisms 20-200 mm in size. Examples include some euphausiids and cephalopods.
• Megaloplankton: planktonic organisms greater than 200 mm, including jellyfish and salps.
• Holoplankton: organisms that are planktonic throughout life, such as copepods.
• Meroplankton: organisms that have a planktonic stage but grow out of it at some point, like fish and crustaceans.

You can see a list of marine zooplankton groups with examples on the Census of Marine Zooplankton website.

Varieties of Plankton

Amphipod: Small, laterally compressed crustaceans, amphipods thrive in both marine and freshwater habitats. While they might resemble shrimp, they lack a carapace.
Zooplankton: These are tiny animals drifting in aquatic environments. As primary consumers, they feed on phytoplankton and are consumed by larger marine creatures.
Moina: A relative of Daphnia, Moina are small freshwater crustaceans, often used in aquaculture and aquariums as food due to their high nutritional value.
Artemia: Often referred to as brine shrimp, Artemia thrive in salt lakes and serves as a popular live feed in aquaculture, especially for larvae.
Copepod: Vital to the aquatic food web, copepods are tiny crustaceans in marine and freshwater settings. They feed on phytoplankton and become prey for larger organisms.
Daphnia: Often referred to as “water fleas” due to their jerky movements, Daphnia are freshwater crustaceans and are important in aquatic food chains, serving as food for many small fishes.
Brachionus: A genus of rotifers, Brachionus thrives in aquaculture settings. They actively feed on phytoplankton and serve as a primary food source for fish larvae.
Krill: Small shrimp-like crustaceans found in cold ocean waters. Krill are a major food source for many marine animals, including whales, seals, and penguins.
Atlanta: A genus of small, planktonic marine snails, Atlanta is part of the larger mollusc family and is known for their transparent, spiral shells.
Phyllosoma: The larval stages of spiny and slipper lobsters are transparent with long, spindly legs and predominantly found in oceanic waters.
Rotifer: Microscopic aquatic animals, rotifers are essential to freshwater food webs. They consume detritus and phytoplankton and serve as food for larger zooplankton and small fish.
Phytoplankton: These microscopic plants and algae float in the water column. As the base of the aquatic food web, they perform photosynthesis, producing a significant amount of the world’s oxygen and serving as primary producers in the food chain.

What Does Zooplankton Eat?

Zooplankton and the Food Web

Zooplankton are the second step of the oceanic food web.  The food web starts with the phytoplankton, which are primary producers. They convert inorganic substances (e.g., energy from the sun’s nutrients such as nitrate and phosphate) into organic substances. The phytoplankton, in turn, are eaten by zooplankton, who smaller fish and even gigantic whales eat.

What eats zooplankton?

In the intricate web of aquatic life, zooplankton serve as a crucial link in the food chain. These tiny organisms, while abundant and diverse, are a primary source of nutrition for various aquatic creatures. Numerous species rely on zooplankton as a vital component of their diet.

1. Fish: Many species of fish, both marine and freshwater, depend on zooplankton as a primary food source during their larval stages. Fish like herring, anchovies, and juvenile cod are known to feed on copepods and other plankton, ensuring their growth and survival.

2. Invertebrates: Various invertebrates, such as jellyfish and comb jellies, are filter feeders that actively consume plankton. These gelatinous creatures extend their tentacles or cilia to capture zooplankton drifting in the water.

3. Whales and Baleen Sharks: Some of the largest animals in the ocean, including baleen whales like humpbacks and fin whales, rely on plankton as a significant part of their diet. These massive filter feeders use their baleen plates to strain zooplankton from the water.

4. Birds: Many seabirds, such as puffins and shearwaters, forage for zooplankton on the ocean’s surface. They employ various feeding strategies, including surface-skimming and plunge-diving, to capture these tiny organisms.

5. Cnidarians: Corals and sea anemones often extend their stinging tentacles to capture plankton drifting nearby. These organisms supplement their diet with the nutrients provided by zooplankton.

6. Other Zooplankton: Some larger plankton species are cannibalistic and feed on smaller plankton. This intra-plankton predation is common among copepods, for instance.

Understanding the predators of plankton is essential in comprehending the dynamics of marine and freshwater ecosystems. The availability of plankton can influence the distribution and abundance of numerous species, highlighting the interconnectedness of life in aquatic environments.

How Does Plankton Reproduce?

Phytoplankton may reproduce sexually or asexually, depending upon the species.  Asexual reproduction occurs more often and can be accomplished through cell division, in which one cell divides in half to produce two cells.

Sources

• Harris, R., Wiebe, P., enz, J., Skjoldal, H-R., and M. Huntley.  ICES Zooplankton Methodology Manual. Accessed May 30, 2014.
• Marine Education Society of Australasia. Zooplankton. Accessed May 30, 2014.
• Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life, Tenth Edition. Jones & Bartlett Learning, LLC.  467pp.

Woods Hole Oceanographic Institution.  Jellyfish and Other plankton.  Accessed May 30, 2014