Copepod, << KOH puh pod, >> is the name of a large group of tiny animals that live either in the ocean or in fresh water. There are thousands of species of copepods. They form part of the plankton, the mass of small organisms usually found at or near the water surface (see Plankton). Copepods can swim, but many of the smaller species cannot swim strongly enough to avoid being carried about by water currents. They feed on floating microscopic organisms and plant life. Copepods are an important source of food for many fish, especially herring.