Zooplankton
Zooplankton are small, free-floating aquatic organisms found in both freshwater and marine environments, playing a crucial role in aquatic ecosystems. They belong to various animal phyla, including Protozoa, Cnidaria, and Arthropoda, and can be broadly classified into holoplankton, which spend their entire lives as plankton, and meroplankton, which include larval stages of benthic organisms. Their sizes range from nanoplankton (2 micrometers) to megaplankton (up to 200 centimeters), with some species, like jellyfish, classified as zooplankton despite their larger sizes.
Zooplankton are essential for nutrient cycling as they feed on phytoplankton, thus preventing overgrowth and facilitating energy transfer up the food chain to larger predators, including fish and mammals. They adapt to their environments by adjusting buoyancy and employing various body forms to resist sinking, often utilizing oils for buoyancy and engaging in small movements to maintain their position in the water column. Many species also exhibit diel vertical migration, moving deeper during the day to avoid predation and rising at night to feed. Their diverse habitats and life cycles reflect their ecological significance in regulating marine and freshwater ecosystems.
Zooplankton
Zooplankton Facts
Classification:
Kingdom: Animalia
Major Phyla: Protozoa, Cnidaria, Ctenophora, Rotifera, Arthropoda, Annelida, Mollusca, Chordata, Ctenophora
Geographical location: Large bodies of freshwater and saltwater
Habitat: Mainly in surface waters
Gestational period: Variable
Lifespan: Variable
Special anatomy: Highly variable between different phyla; generally small with a flattened body and many projections; zooplankton may also store oils used for buoyancy
Zooplankton is found in almost all major fresh and marine water bodies. Many species, however, are restricted to specific territories depending on light conditions, temperature, salinity, chemical composition, and turbulence. Their range also varies geographically and vertically within the water column.
Zooplankton Types
The diversity of zooplankton makes classification difficult. Holoplanktonic zooplankton are free-floating their entire lives and are mostly invertebrates. Flagellated, ciliated, and amoeboid protozoa, as well as several species from the animal phyla Cnidaria, Ctenophora, Chaetognatha, Mollusca, Annelida, Urochordata, Echinodermata, and Arthropoda, make up the majority of collected organisms. Meroplanktonic zooplankton are the free-floating forms of organisms with swimming nekton or attached benthic stages as part of their life cycle. They consist mostly of egg and larval stages of marine benthic invertebrates that include worms, snails, clams, barnacles, crabs, starfish, and sea urchins, as well as many marine fish, such as tuna. Body forms, life cycles, diet, and location vary among species.
Zooplankton are often divided according to size. The most common divisions and sizes for zooplankton are nanoplankton (2 to 20 micrometers), microplankton (20 to 200 micrometers), mesoplankton (0.2 to 20 millimeters), macroplankton (2 to 20 centimeters), and megaplankton (20 to 200 centimeters). Although generally small, even jellyfish several meters in size are considered zooplankton because they cannot swim effectively against the current.
Copepods are one of the most studied forms and are classified as holoplanktonic zooplankton. Copepods have a segmented body with three distinct sections. Several pairs of legs and large antennae are used to create feeding currents that capture phytoplankton, especially diatoms and other small zooplankton. Copepods are classified as mesoplankton and are nearly 5 millimeters in size. They are the most important herbivore in the ocean. Advanced collection techniques, however, have uncovered the presence of smaller zooplankton classifications that may be an important ecological first link.
Ecological Importance
Zooplankton diet varies greatly among species. There are carnivorous and herbivorous zooplankton, as well as those that feed on nonliving organic material. Herbivorous forms feed on phytoplankton through a process called grazing. This process prevents damaging blooms of phytoplankton from increasing in numbers. The link between phytoplankton and zooplankton is also important in passing the energy fixed by autotrophs to higher trophic levels of the food chain. Carnivorous zooplankton, other invertebrates, and fish eat herbivorous zooplankton. The food chain may terminate in the top carnivores, including large fish, birds, mammals, and humans.
Adaptations
Because of their small size, zooplankton are very sensitive to water conditions. Maintaining position in the water column is an important adaptation for many zooplankton species because they cannot swim strongly enough to counteract the water current. Their density is often slightly greater than that of seawater. In addition, high salinity and low temperatures increase the viscosity of surrounding water, making it harder for zooplankton to move. A larger surface area and flattened body form covered by many projections or spines counteract these obstacles and prevent the zooplankton from sinking. Several species of zooplankton also increase their buoyancy by storing low-density materials, such as certain oils. Small body movements, including the action of flagella and cilia, also counteract the sinking motion.
Vertical Migration
The depth at which zooplankton exist is variable. While some float at the surface of the water, others may inhabit depths greater than four thousand meters. The concentration, or biomass, of zooplankton, decreases with increasing depth. Many species of zooplankton exhibit a daily rhythmic vertical movement called diel vertical migration. They may migrate downward from the water surface during the day and upward at night. These movements may avoid predation and conserve energy by slowing metabolism in colder, deeper waters. Seasonal migrations from deeper waters in the winter to surface waters in the spring are common. This migration may decrease metabolism and conserve energy throughout the winter when food is lacking.
Principal Terms
Autotroph: an organism that has the ability to make its own food from inorganic substances
Benthic: organisms that live at or near the bottom of the ocean
Heterotroph: an organism that requires the ingestion of food for survival
Nekton: an aquatic organism that has the ability to swim
Phytoplankton: small, aquatic free-floating plants
Bibliography
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