Zooplankton Fact Sheet

Zooplankton are microscopic invertebrate animals that swim or drift in water. They are at the base of the food chain, feeding on microscopic plants and being fed upon by aquatic insects, fish and salamanders. Their sizes usually range from one-tenth of a millimeter to four millimeters, which is smaller than the head of a pin.

At the Savannah River Site, zooplankton are found in freshwater reservoirs, ponds and streams. They are abundant in
Carolina bays, wetlands that often dry in the summer and fill with water in the fall.

Because the animals are wholly aquatic but often live in habitats that dry temporarily, they are faced with problems related to the unpredictability of their watery existence. Different types of zooplankton respond to these challenges in various ways. Adult females may lay different types of eggs, depending on the season and whether the pond is likely to dry soon.

Some eggs are resistant and do not hatch until the pond fills. Some emerge within 24 hours of the pond filling; others may take days or weeks to emerge. Some juveniles go down in the mud and rest in a protective case when the bay dries up. Some species produce resting stages when the pond dries, food supply declines or the temperature changes.

Because zooplankton have low migration rates, zooplankton found in Carolina bays provide a good system to study for population and community processes. The community is fairly well contained, and researchers know a direct link exists between what is found in the habitat from year to year.

Most zooplankton are filter feeders, using their appendages to strain bacteria and algae and other fine particles in the water. Others are predators, feeding on smaller zooplankton.

Zooplankton can reproduce rapidly, and populations can increase by about 30 percent a day under favorable conditions. Zooplankton reach maturity quickly and live short, but productive lives. For example, adult females of a zooplankter called Daphnia can produce their body mass in eggs every two to three days. Daphnia live an average of one month.

Researchers at the Savannah River Ecology Laboratory are studying how zooplankton respond to the extreme variability of water levels in Carolina bays. This information is being used to develop mathematical models that could predict the effects of climatic change, or global warming, on biological communities. Also, scientists are developing a computer-based model to examine how population variation is related to environmental variation and what adaptations zooplankton have that enable them to survive and leave "the best" number of offspring in different kinds of environments.

Results from the models can be used to evaluate ecological consequences of decisions that affect the water levels of Carolina bays and similar wetlands, thus supporting the
U.S. Department of Energy's land management and conservation activities on the Savannah River Site.


  • The total weight of zooplankton produced annually in Rainbow Bay, a Carolina bay on the Savannah River Site, is about twice the weight of the thousands of salamanders found there.
  • If environmental factors did not regulate the population growth of zooplankton, a variety called Filinia could cover the entire world to a depth of 1 meter in 130 days.

Although food, temperature and water chemistry all are important in determining what kinds of zooplankton can live in a particular lake or pond, the most important factor is predators, particularly fish. Fish prefer to eat the larger and more visible kinds of zooplankton. Thus, zooplankton that coexist with fish are typically small (less than 1 to 1.5 millimeters) and transparent.

In contrast, zooplankton that live in ponds without fish, such as temporary ponds, often are much larger (up to 3 to 4 millimeters). Some kinds of zooplankton in fishless ponds are quite colorful: a variety called copepods can be bright orange, blue or blue with red antennae. The very largest zooplankton, called fairy and clam shrimps, live only in fishless ponds.

On the Savannah River Site and throughout the Upper Coastal Plain, three varieties of freshwater zooplankton are most common. They are rotifers, cladocerans and copepods. Also, fairy and clam shrimps live in ponds that dry out seasonally.

Rotifers are called such because some species -- scientists have identified more than 1,700 species -- have a disc-shaped anterior end, covered with hairlike protrusions, that resembles a pair of revolving wheels. The movement is actually the synchronized beating of the hairlike protrusions, called cilia.

Rotifers are small animals with simple body forms. They have no legs, although some have a single foot at the end of the body. Some species are protected by a shield or shell-like structure called a lorica; some have spines or paddles for protection; others live in colonies encased in a jelly-like substance.

An organ called the mastax is unique to the digestive system of rotifers, and no comparable device is known elsewhere in the animal kingdom. It consists of a complicated arrangement of muscles that activate a set of translucent jaws used to seize, tear and grind food.

Cladocerans, commonly called water fleas, have been favorite objects of observation by both amateur and professional biologists since the invention of the microscope. Their bodies are not clearly segmented, and many species are covered by a shell-like material. The head is a compact structure and bends downward. The most conspicuous internal structure of the head is the cladocerans' large compound eye. Cladocerans have two antennae, which help them move, and five or six pairs of lobed, leaf-like legs. Cladocerans live in nearly all types of freshwater habitats; they are most abundant in the spring.

Adult copepods have long, cylindrical or torpedo-shaped bodies with a single eye and a pair of long antenna at the front, five pairs of legs along the middle and a paddle-like tail at the end. They use their legs for swimming. Copepods use small appendages near their mouths to capture food. Copepods lay eggs that hatch into tiny nauplii that have only three pairs of appendages. Nauplii gain legs and change form as they grow. Unlike other kinds of freshwater zooplankton, copepods also are very abundant and important in the plankton communities of the oceans.

Fairy and clam shrimps move along the bottom or swim about and glide about gracefully in temporary ponds. They are distinctly segmented and have 10 to 71 pairs of delicate, flat, swimming and respiratory appendages. These zooplankton are rarely found in lakes; temporary ponds, such as Carolina bays, are one of their main habitats.