Copepod. Scanning electron micrograph (SEM) of a copepod (subclass Copepoda). This microscopic aquatic crustacean is found in waters around the world. Copepods may either be free living or parasitic on other organisms. They are the main constituent of zooplankton and therefore are very important as food organisms for fish, whales and other crustaceans. Copepods feed on other microscopic animals and plants. Magnification: x250 when printed 10 centimetres wide.