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.

Anatomy: Copepods have a hard exoskeleton, many legs (used for swimming and gathering food), a segmented body, and jointed appendages. Most copepods are under 1 mm long, but a few oceanic species are over 1/4 inch (1 cm) long. Although they lack compound eyes, these arthropods have a single simple eye in the middle of the head (sometimes it is only present in the larval stage); this simple eye can only differentiate between light and dark. There are two pairs of antennae; one pair is long and one pair is short. Like all crustaceans, copepods molt their exoskeleton as they grow.

Diet: Copepods eat bacteria, diatoms, and other tiny, single-celled organisms in the water. Maxillae, maxillipeds and antennae push food towards the mandibles (jaws), which process the food.

Predators: Free-swimming copepods are a component of zooplankton and are eaten by many organisms, including mussels, fish and fish larvae, squid, sea birds, and mammals (like baleen whales and some seals).

Reproduction: The female copepod produces clusters of eggs that she carries in one or two egg sacs that are attached to her adbdomen.