Argulus is a genus of many parasites commonly known as fish lice; along with shrimps, prawns and water fleas, they are members of the subphylum Crustacea. Fish lice are capable of moving quickly and can be seen crawling over the skin of migratory marine and freshwater fishes, which are their primary target hosts.

These crustacean parasites are infamous for their aggressive attachment and feeding behavior, grasping tightly onto their hosts using small spines, hooked appendages, and a pair of large suckers. A feeding louse will pierce and inject digestive enzymes into the skin or gills of a fish using a stylet -- a pointed, needlelike structure -- and then suck blood, mucus and enzyme-liquefied body fluids from the host. Constant piercing of the fish skin or gills can cause inflammation and leave the damaged site vulnerable to infection by opportunistic bacteria.

The life cycles of the louse revolve around its fish host. Male and female lice mate while clinging onto their host, after which the female detaches to lay strips of eggs on plants or other submerged objects. Each strip may contain up to 500 eggs. In suitably temperate environments, the eggs produce hungry juvenile lice about 0.5 mm in length. In order to survive, a newly hatched parasite must very quickly locate a host on which it will feed and rapidly develop, undergoing several molting stages as it reaches adulthood. Depending on the environment, a fish louse may complete its life cycle in anywhere from 40 to 100 days' time. Because of their rapid procreative turnaround, if left untreated, fish lice may quickly infest an aquarium, pond, or fishery and are therefore of major concern to the aquaculture industry.