An order of small copepod Crustacea. The abundant fresh-water and salt-water species form an important link in the food chains of aquatic life, consuming tiny plants, animals, and detritus and, in turn, furnishing food for small fish, some large fish, and other organisms. Some species are important intermediate hosts for human parasites. Females of the family Ergasilidae are parasitic on aquatic animals, while the males are free-swimming.