Sea lice are copepos which parasitise fish. There are two species of sea lice commonly found on cultured salmonids, Caligus elongatus, a species of parasite that infests over fifty different species of marine fish, and Lepeophtheirus salmonis, which infests only salmon and closely related species such as rainbow trout (but current research indicates it may be spread via the three-spined stickleback). Both species are also found on wild salmon.