Copepods. Macro photographs of a selection of copepods (subclass Copepoda). All, except the one at lower left, are carrying eggs (small spheres). These microscopic aquatic crustaceans are 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.