A copepod is any of the roughly 7,500 species of marine or freshwater crustacea of the subclass Copepoda. Copepods are possibly the most numerous
type of animal in the world and are a major component of the ocean food
chain. Some live as parasites, especially
on fish. Their segmented, cylindrical bodies
have a single median eye and no carapace.
Copepods are typically 0.5–2 mm (0.02–0.08 in.) in length, although
some parasitic forms may be more than 30.5 cm (1 ft.) long.
Copepod. Image credit: NASA/JPL