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Dana 1852


Phylum Arthropoda
Subphylum Crustacea
Class Ostracoda


  • Pelagic ostracods are small, usually less than 2mm.

Distinguishing characteristics

  • On first appearance have some resemblance to cladocerans. However, ostracods differ in that the body is completely enclosed in a clam-like bivalve shell that is hinged dorsally.
  • A transverse adductor muscle, similar to that in clams and mussels, closes the shell.
  • The carapace (shell) can have various shapes but is typically circular, elliptical, or almost rectangular.
  • There is often an anterior antennal notch (not clearly visible on image).
  • The carapace can be smooth, thin, and transparent in planktonic ostracods, while benthic forms have more robust, ornate shells.
  • The trunk is reduced and unsegmented with 2 or less appendages, much fewer than other adult crustaceans. The abdomen is also reduced.
  • Both the first and second antennae are well developed, extend beyond the carapace, and are the main swimming appendages.


  • They are not particularly common in coastal plankton samples.
  • Benthic samples may be collected when sampling close to the bottom or close in to the shore.

Example of an ostracod