Fish lice are ectoparasites that infest marine and freshwater fishes. Unlike other blood-sucking invertebrates bearing the common name of louse, the fish louse is more closely related to lobsters, shrimp, and water fleas than they are to insects such as the human head or body louse. However, in an analogous fashion to true lice, members of the crustacean genus Argulus pierce the skin and inject digestive enzymes into the host with a highly modified mouthpart known as a stylet. Subsequently, the parasitic insects then suck out blood, mucus, and partially digested body fluids.

Although the fish louse poses no health threat to humans, it is of economic importance because their presence makes marketing a fish catch nearly impossible. Famous for their tenacious hold on a host fish's skin or gills, fish louse use small spines, hooked appendages, and a pair of large ventral suckers to latch onto their prey. Argulus species are known to travel thousands of miles during massive fish migrations, sometimes at impressive swimming speeds. Fish lice are also capable of moving very quickly on hosts and can often be observed without magnification crawling over their skins.

The life cycle of the fish louse is near perfectly adapted to the episodic or seasonal appearance of migratory fishes. Male and female lice mate while clinging on the host fish. After mating the fertilized female drops off to lay her eggs on aquatic plants and other submerged substrates. With 500 eggs in a strip and a life cycle that lasts anywhere from 40 to 100 days, populations of fish lice can rapidly escalate. This is particularly worrisome for aquaculturists and others managing contained fish populations in ponds, aquaria, or hatchery raceways.