Copepod. Image credit: NASA/JPL.
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 millimeters (0.02–0.08 inch) in length, although some parasitic forms may be more than 30.5 centimeters (1 foot) long.