Sea lice (singular: sea louse) are copepods (small crustaceans) of the family Caligidae within the order Siphonostomatoida. They are marine ectoparasites (external parasites) that feed on the mucus, epidermal tissue, and blood of host fish. The roughly 559 species in 37 genera include around 162 Lepeophtheirus and 268 Caligus species. The genera Lepeophtheirus and Caligus parasitize marine fish, in particular those species that have been recorded on farmed salmon. Lepeophtheirus salmonis and various Caligus species are adapted to salt water and are major ectoparasites of farmed and wild Atlantic salmon. Several antiparasitic drugs have been developed for control purposes. L. salmonis is the best understood in the areas of its biology and interactions with its salmon host. Caligus rogercresseyi has become a major parasite of concern on salmon farms in Chile. Studies are under way to gain a better understanding of the parasite and the host-parasite interactions. Recent evidence is also emerging that L. salmonis in the Atlantic has sufficient genetic differences from L. salmonis from the Pacific to suggest that Atlantic and Pacific L. salmonis may have independently co-evolved with Atlantic and Pacific salmonids respectively. Diversity The family Caligidae is estimated to contain around 559 species in 37 genera. The largest of these are Caligus, with around 268 species, and Lepeophtheirus with around 162 species. Wild fish Most understanding of the biology of sea lice, other than the early morphological studies, is based on laboratory studies designed to understand issues associated with sea lice infecting fish on salmon farms. Information on sea lice biology and interactions with wild fish is sparse in most areas with a long-term history of open net-cage development, since understanding background levels of sea lice and transfer mechanisms has rarely been a condition of tenure license for farm operators. Many sea louse species are specific with regard to host genera, for example L. salmonis, which has high specificity for anadromous fish including sticklebacks and salmonids including the widely farmed Atlantic salmon (Salmo salar). Lepeophtheirus salmonis can parasitize other salmonids to varying degrees, including brown trout (sea trout: Salmo trutta), Arctic char (Salvelinus alpinus), and all species of Pacific salmon. In the case of Pacific salmon, coho, chum, and pink salmon (Oncorhynchus kisutch, O. keta, and O. gorbuscha, respectively) mount strong tissue responses to attaching L. salmonis, which lead to rejection within the first week of infection. Pacific L. salmonis can also develop, but not complete,