Fish Lice: Branchiura

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FISH LICE: Branchiura



Most fish lice are external parasites of freshwater fishes. They live on the outside of the bodies of their hosts and feed on blood and other body fluids. Fish lice are flat, egg-shaped crustaceans and have bodies divided into three regions: head, thorax, and abdomen. A well-developed, shieldlike carapace (CARE-eh-pes) covers the head. In some species the carapace covers the sides of the body and legs, and sometimes covers part of the abdomen. The carapace has special organs inside that help the fish louse to digest its food and control the quality of fluids inside the body. The compound eyes are distinct and are not set on stalks. Each compound eye has multiple lenses. Both pairs of antennae are very short and have claws. The claws are used to help them attach to their host. The tubelike mouthparts have jaws, or mandibles, at the tip. The mandibles are used to scrape loose tiny chunks of skin. These bits of tissues, along with body fluids, are sucked into the mouth. A second pair of mandibles, or maxillae (mack-SIH-lee), has spines and claws. There are no maxillipeds (mack-SIH-leh-pehds), or thoracic limbs, that work together with the mouthparts. Instead, all four pairs of branched, or biramous (BY-ray-mus), limbs on the thorax are used for swimming. In males, the third and fourth pairs of limbs are used to transfer sperm to the female during mating. The unsegmented abdomen does not have any appendages underneath. It ends in two, rounded projections, or lobes, separated by a notch.


Fish lice are found in freshwater habitats on all continents.


Fish lice parasitize freshwater fishes. A few species attack fish living along the coast or in estuaries, but they are never found out in the open sea. They are sometimes found on amphibians.


Fish lice eat the skin and blood of their host.


Only the behavior of Argulus is well known. They use their mandibles to scrape skin into their mouth. They use their needle-like mouthparts to inject chemicals into the host's body. These chemicals may help turn nearby tissues into liquid so that they can be sucked into the mouth.

After taking a meal, mature females leave their hosts to lay eggs. They glue their eggs in rows on hard surfaces and leave them to hatch on their own. The newly hatched larvae (LAR-vee) do not resemble the adults at all. Their antennae, mouthparts, and first two pairs of thoracic limbs are bristly and used for swimming. They grow by molting, or shedding their external skeletons (exoskeletons). After the second molt, fish lice replace the bristles on the antennae with strong claws in preparation for their new lives as parasites. The claws are used to grab on to their first host. As they grow and develop, fish lice will change hosts several times. As the larvae mature, they develop thoracic limbs, sucking mouthparts, and reproductive organs.


Does your fish scratch against rocks in the tank? It may have an itchy infestation of fish lice (A. foliaceus). These tiny crustaceans look like clear discs (0.75 inches, 19.05 millimeters) with eyes attached to their scaly skin. If you find them, put the infested fish(es) in a separate tank. Check with a pet shop for treatment options for both fish and tank. Parasites can be pulled off with tweezers and their wounds treated with special medicine.

Both males and females are required for reproduction. In most species, males transfer their sperm directly to the reproductive organs of the female with their third and fourth pairs of thoracic limbs. In Dolops, however, males transfer their sperm inside a packet.


Fish lice are important pests in fish culture facilities. They usually occur in freshwater facilities, but occasionally infest marine fish farms. Aquarium fish may be killed by infestations of fish lice.


No species of fish lice is considered threatened or endangered.


Physical characteristics: The abdominal lobes of fish lice are broadly rounded. The notch between the lobes is not very deep and less than half the length of the lobes. The first three pairs of thoracic limbs are dark at their bases. Adult females measure 0.39 inches (10 millimeters) in length, while the males are 0.35 inches (9 millimeters).

Geographic range: They are found in Europe, east to central Asia and Siberia.

Habitat: Fish lice are external parasites on many kinds of freshwater fishes.

Diet: Fish lice eat fish skin and blood.

Behavior and reproduction: After hatching, the larvae swim in open water for two or three days before attaching themselves to a host.

Up to four hundred eggs are laid at a time in two to six rows. The eggs hatch after twenty-five days at temperatures of 59°F (15°C). Depending on temperature, the first larval stage lasts about six days, and the animal molts every four to six days until reaching adulthood.

Fish lice and people: They sometimes infest fish hatcheries and aquariums. Their feeding activities spread disease and weaken or kill fish.

Conservation status: Fish lice are not considered threatened or endangered. ∎



Overstreet, R. M., I. Dyková, and W. E. Hawkins. "Branchiura." In Microscopic Anatomy of Invertebrates. Vol. IX, Crustacea, edited by F. W. Harrison. New York: J. Wiley and Sons, 1992.


Gresty, K. A., G. A. Boxshall, and K. Nagasawa. "The Fine Structure and Function of the Cephalic Appendages of the Branchiuran Parasite,Argulus japonicus Thiele." Philosophical Transactions of the Royal Society of London B339 (1993): 119-135.

Web sites:

Arguloidea (Branchiura, Maxillipoda). (accessed on March 16, 2005).

Introduction to Branchiura. Fish Lice! (accessed on March 16, 2005).