Zooplankton, form the Greek word zoon (animal) and planktos (wanderer or drifter), are small animal-like creatures inhabiting the pelagic zone of oceans, and fresh water bodies, such as lakes, rivers and ponds. Most zooplankton are microscopic in size, although, some species, such as the jellyfish may grow into larger and varied sizes. Zooplankton’s primary sources of food include bacterioplankton, phytoplankton, marine snow (detritus) and other zooplankton. Zooplankton are usually found on the surface of the ocean and freshwater bodies, where these sources of food abound. Zooplankton, along with phytoplankton, form the base of most marine and freshwater food webs.
Zooplankton comprises a wide range of organisms with varied sizes, classified within two distinct groups. Holoplanktonic organisms, including, foraminiferans, dinoflagellates and radiolarians, among others, which spend most of their lifecycles as microscopic plankton. Meroplanktonic organisms, including starfish, sea urchins, crustaceans, mollusks, marine worms and fish, among others, form the second group of zooplankton. Meroplankton remain as plankton during their larval stage before evolving into larger creatures.
Zooplankton, like all plankton, are gently carried in the upper layer of the ocean by ocean currents. Although, they’re carried primarily by oceanic currents, some species of zooplankton are able to displace along the ocean. One such species are ciliates, which use cilia, usually found in rows over their bodies, to swim; foraminiferans capture prey with pseudopodia (movable false foot) and they also use pseudopodia for locomotion. Many zooplankton species use locomotion to avoid predators, as well as to increase their chances of meeting their prey. The abundance of zooplanktonic species depends on the availability of nutrient concentrations in the water column and abundance of other plankton.
Certain species of zooplankton have developed a wide range of feeding habits, including filter feeding, suspension feeding and raptorial feeding. Zooplankton are also predators of phytoplankton and other zooplanktonic species. They also feed on decayed organisms falling from the upper layer of the ocean (marine snow). This is the main reason why they’re found in the upper ocean layer, where their principal source of nourishment abounds. The main aspect determining the distribution of zooplankton is the availability of nutrients in the water column due to upwelling of water-rich nutrients. This affects the phytoplankton community and as a consequence the survivability of zooplankton.
Many species of zooplankton descend deeper into the water column and migrate to the surface at night. It is believed that the displacement of zooplankton to the ocean’s deeper waters is to avoid being eaten by predators which depend on the lighted ocean waters for food. At night, zooplankton will swim to the surface ocean water to feed on phytoplankton. The displacement of zooplankton from a zone of high food availability to one of limited food accessibility, where low temperatures and low oxygen levels reduce respiration and helps conserve energy, is thought to influence zooplankton movement along the water column.
Importance in marine food webs
Zooplankton represents, along with phytoplankton, the base of many ocean and freshwater food webs. The availability of zooplankton supports many fisheries around the world. Zooplankton adds to the biological pump by grazing on phytoplankton. By grazing on the primary producers (phytoplankton), zooplankton obtains carbon, which in turn is delivered to other consumers in the food web. Upon death of zooplankton and other marine creatures, organic material sinks down into the deep ocean, carrying organic carbon. In this way, zooplankton plays an important role in the carbon cycle.
Like phytoplankton, zooplankton are significantly sensitive to changes in their environment. A change in zooplankton concentrations can indicate a variation in the environment. Zooplankton has developed special adaptations to avoid their main predator, the fish. To avoid being eaten by fish, zooplankton have developed transparent bodies. According to marinebio.org, zooplankton species have evolved to occupy specific marine habitats. Certain species are particularly adapted to external features, including temperature, light, salinity, turbulence, and others. These characteristics can sometimes help scientists differentiate between masses of water.