Argulus or fish lice as they are commonly called are a very major threat to your fish’s health. They can cause significant morbidity and mortality when heavily infesting your fish and they have been known to be the vehicle for other fish diseases. Argulus are a crustacean parasite in the subphylum Crustacea, which groups them together with prawns, shrimp, water fleas and others. The animals that are in the Crustacea group have semi-rigid to rigid chitin exoskeleton. The chitin exoskeleton will molt, or shed, as they get larger. Argulus are in the Branchiura class, which is group of Crustaceans that all have similar features. All of the Branchiura are fish parasites.
Argulus another crustacean parasite, round and up to 1cm wide. They have a sucker to hold on to the Koi with needle-like mouth parts which they stick into the Koi and inject a toxin. This causes intense irritation to the Koi and they scratch and jump and can cause bacterial infection.
Argulus have a direct life cycle using only the fish as hosts. They prey upon freshwater fish and marine fish. Argulus can spend a large amount of time swimming around and mating occurs when the male and female Argulus are swimming. The eggs clusters are dropped on any convenient submerged item. After hatching the Argulus makes several metamorphic changes as it goes towards adulthood. The whole cycle takes between 30-100 days depending on the temperature. After hatching they must find a host within a around 4 days or they will not survive.
If they infect the gills they cause severe damage and often death. Most antiparasite remedies will not kill fish lice, a strong chemical is needed which is not freely on sale. Ask a professional dealer or vet.