ostracoden

Ostracods, or Ostracoda, are a class of small crustaceans distinguished by a hinged, two-valved carapace that encloses most of the body. Individual shells (tests) are typically a few tenths to several millimeters long, though some species reach larger sizes. The body inside the shell has a head, a simple thorax, and abdominal segments, with paired appendages used for swimming, feeding, and respiration. The carapace is often ornamented with ridges or pits and can be calcareous or, in some lineages, more fragile.

Habitat and distribution: Ostracods are found in nearly every aquatic environment, from the deepest oceans to freshwater lakes and temporary pools, and in some moist terrestrial habitats. They are among the most abundant and diverse microcrustaceans, contributing to benthic and pelagic communities. Pelagic species typically inhabit the water column, while most benthic forms live on or in sediments.

Biology and reproduction: Reproduction is primarily sexual, with many species displaying direct development and a range of life cycle strategies. In numerous ostracods, eggs mature within the female’s brood pouch and hatch as juveniles, bypassing a free-swimming larval stage; other species lay eggs that hatch into free nauplius larvae. Parthenogenetic reproduction is common in many groups, allowing rapid population growth in favorable conditions.

Fossil record and uses: The calcified carapace preserves well in sediments, yielding a rich fossil record dating back to the Cambrian. Ostracods are widely used in biostratigraphy and paleoenvironmental reconstruction because shell morphology and chemistry reflect salinity, temperature, and depth.

Taxonomy and diversity: Ostracoda comprises several major groups, including the predominantly marine Podocopida and the largely pelagic Halocyprida and Myodocopida. Modern ostracods inhabit a broad range of lifestyles, from minute detritivores to swift swimmers.