Copepod

Copepods (meaning "oar feet") are small, shrimp-like crustaceans that swim in seas, lakes, and ponds. Copepods are very important in the food web, and many animals eat them.
There are 10 orders of copepods and over 4500 species; a few orders are free-swimming, but many are parasites (of fish). The free-swimming copepods move through the water in jerky motions by moving their swimming legs.