microcrustaceans

Microcrustaceans are a diverse group of small crustaceans, typically less than a few millimeters in length, that inhabit freshwater and marine environments. They include copepods, cladocerans, ostracods, and related branchiopods. Because of their abundance and rapid reproduction, they form a major component of zooplankton and play a central role in aquatic food webs.

The major groups commonly referred to as microcrustaceans are copepods (including Calanoida, Cyclopoida, and Harpacticoida), cladocerans (water fleas such as Daphnia and Bosmina), and ostracods (seed shrimp). Copepods are among the most abundant and ecologically versatile planktonic crustaceans; cladocerans are widespread in freshwater; ostracods are tiny crustaceans with a calcified two-valved carapace found in both marine and freshwater habitats. Branchiopods such as fairy shrimps occur in temporary ponds and moist soils. These groups display a range of lifestyles, from free-swimming plankton to benthic and littoral forms.

Habitat and distribution: Microcrustaceans occur from open oceans to rivers, lakes, and wetlands, including temporary ponds. Planktonic species drift with currents, while benthic forms live on or within substrates. Some species tolerate brackish conditions; there are also terrestrial or semi-terrestrial forms in moist soils and leaf litter.

Biology: They are generally small and possess diverse feeding strategies, including filter feeding, detritivory, and omnivory. Most undergo nauplius larvae, and life cycles vary. Reproduction is diverse: many cladocerans can reproduce parthenogenetically during favorable conditions and produce resting eggs under stress; copepods and ostracods typically reproduce sexually with multiple developmental stages.

Ecology and significance: As a major component of zooplankton, microcrustaceans transfer energy from phytoplankton to higher predators such as fish and invertebrates, influence algal populations and nutrient cycling, and serve as bioindicators in water quality assessments. They are used in ecotoxicology studies and, in some cases, as a nutritious feed in aquaculture.