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Today on Animal Fact Files we’re discussing fish lice. Fish lice, like head lice, are external parasites, specifically on fish, hence the common name! These arthropods are crustaceans like crabs and lobsters. Fish lice are also known as carp lice because they're frequently observed on carp and ornamental koi fish, but these aren't the only fish they parasitize. They can also be found on salmon, trout, goldfish, sunfish, minnows, and many more. They've even been observed on frogs and toads!

Fish lice spend most of their lives on their host, thus they live where their hosts live in fresh, brackish, and saltwater. They're generally found throughout the world and are believed to have hitched rides on fish from the fish trade to reach non-native areas. They're often seen on fish scales and skin and can be observed with the naked eye, but they also live in fish gills and on fins. In size, they're typically less than a quarter inch (7mm) long and about half as wide. They look kind of like tadpole shrimp with a round, hard shell. They have dark compound eyes and a sharp, needle-like stylet for piercing their host's skin and sucking out bodily fluids like blood. Adults have suction disks for holding onto their host while juveniles use hooks.

Fish lice breed on their host but the female, who is slightly larger than the male, leaves the host to lay her eggs in vegetation before returning. Adults can survive over a week away from their host, but must return in order to survive. They're dependent on their host to live. Females lay anywhere from four to over 250 eggs in strands or clutches. These take from under two weeks to over two months to hatch depending on the temperature. The colder the water, the longer they take to develop. They can even overwinter if needed. After hatching, the young must find a host within a few days or perish. Once attached, they'll grow and shed their exoskeleton until they reach reproductive maturity about one to two months later. Unlike copepods who stop shedding, fish lice continue to shed throughout their lives. As adults, they may live a few months.

Fish infected with fish lice may or may not show signs of their parasitic neighbors. A bite from a fish louse can lead to infection. Some fish will also try to rub the crustaceans off their body by swimming against hard surfaces. A few fish lice don't normally cause too much harm, but can quickly grow into an infestation which can lead to scale loss, lethargy, anorexia, and more. Often only one or a few individuals within a fish population are affected with fish lice. This is because the invertebrates congregate on a single host where they have easy access to food and mates. This in turn can impact the fish's behavior to move slower, thus making it easier to access for the parasites. While free-swimming and as eggs, the fish lice may be eaten by fish and other invertebrates in the water column.

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