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 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.
Argulus are one of the biggest parasites and are seen with the naked eye. They range in size from 5 to 10 mm in length. While they are easy to spot if you know what you are looking for, they are quite easy to miss when doing a skin scrape. You may see small dark spots on your fish and not realize that they are Argulus until they move. You can usually find them located behind the fins or around the head, in sheltered locations. You can spot them easier on the fins than on other parts of the body because they show up better against the transparent background.
The fish lice are oval in shape and flat. They can move quickly and you can sometimes spot them in the tank when they move from one host to another. If you try to net your fish, they will abandon the host and move into the free water.
Affected fish have patches of hemorrhagic and edematous affected skin, gills or fins. The parasite causes these injuries by attaching to the fish with its curved hooks and sucker. Its feeding apparatus further injures the host fish when it inserts the stylet into the epidermis and underlying host tissue causing hemorrhage. Argulus feed on the host’s blood and body fluids. The feeding apparatus also releases digestive enzymes which can cause systemic illness.
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