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. 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 countries including Chile and Scotland. 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, its full lifecycle on the three-spined stickleback (Gasterosteus aculeatus). This has not been observed with Atlantic L. salmonis. How planktonic stages of sea lice disperse and find new hosts is still not completely known. Temperature, light, and currents are major factors and survival depends on salinity above 25 ‰. L. salmonis copepodids migrating upwards towards light and salmon smolt moving downwards at daybreak have been hypothesized to facilitate finding a host. Several field and modeling studies on L. salmonis have examined copepodid populations and have shown that planktonic stages can be transported tens of kilometres from their source, including how their behaviour results in their being moved towards the coastline and mouth of estuaries. The source of L. salmonis infections when salmon return from fresh water has always been a mystery. Sea lice die and fall off anadromous fish such as salmonids when they return to fresh water. Atlantic salmon return and travel upstream in the fall to reproduce, while the smolts do not return to salt water until the next spring. Pacific salmon return to the marine nearshore starting in June, and finish as late as December, dependent upon species and run timing, whereas the smolts typically outmigrate starting in April, and ending in late August, dependent upon species and run timing. Sea lice possibly survive on fish that remain in the estuaries or they transfer to an as yet unknown alternate host to spend the winter. Smolt get infected with sea lice larvae, or even possibly adults, when they enter the estuaries in the spring. How sea lice distribute between fish in the wild also is not known. Adult stages of Lepeophtheirus spp. can transfer under laboratory conditions, but the frequency is low. Caligus spp. transfer quite readily and between different species of fish, and are regularly found in the plankton.
Morphology L. salmonis tends to be about twice the size of most Caligus spp. (e.g. C. elongatus, C. clemensi, etc.). The body consists of four regions: cephalothorax, fourth (leg-bearing) segment, genital complex, and abdomen. The cephalothorax forms a broad shield that includes all of the body segments up to the third leg-bearing segment. It acts like a suction cup in holding the louse on the fish. All species have mouth parts shaped as a siphon or oral cone (characteristic of the Siphonostomatoida). The second antennae and oral appendages are modified to assist in holding the parasite on the fish. The second pair of antennae is also used by males to grasp the female during copulation. The adult females are always significantly larger than males and develop a very large genital complex, which in many species makes up the majority of the body mass. Two egg strings of 500 to 1000 eggs (L. salmonis), which darken with maturation, are roughly the same length as the female's body. One female can produce 6–11 pairs of egg strings in a lifetime around seven months.
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