| Summary: | The collective term “sea lice” is colloquially used to refer to numerous species of copepod crustaceans of the family Caligidae that are externally parasitic on the skin of marine and anadromous fishes. The most intensively studied species ‐ Lepeophtheirus salmonis ‐ is, as its specific name implies, a specialist parasite of salmonid fishes. It is commonly associated with a total of 12 host salmonid fish species of the genera Salmo, Oncorhynchus and Salvelinus in the Pacific and Atlantic Oceans. Along the Pacific coasts of Alaska and British Columbia, L. salmonis as well as Caligus clemensi and Lepeophtheirus cuneifer (both host generalist lice species) have been recorded on wild and farmed salmonids. In British Columbia, Chile and Tasmania Atlantic salmon (Salmo salar) is the principal salmonid species in culture. While the Tasmanian industry apparently suffers no especial problems from caligid infestation, the Chilean industry has been heavily impacted by Caligus species, initially C. teres but more recently and significantly C. rogercresseyi (both of which are host generalists). In Japan, Caligus orientalis is the most pathogenic sea louse on cultured Pacific salmon, although L. salmonis also remains a problem. L. salmonis is associated with wild chum and pink salmon in Japan, but also infests cultured coho salmon and rainbow trout. C. orientalis – like C. elongatus in the North Atlantic – is a host generalist; C. orientalis occasionally impacts salmonids, and it is an especial problem to cultured rainbow trout. The current scientific literature refers to Lepeophtheirus salmonis Krøyer as occurring on salmonids in both the North Pacific and North Atlantic Oceans. At first sight it might appear curious that the same species should occur in two separate and geographically distant oceans, but there is convincing geological, molecular and ecological evidence of past trans‐Arctic connectivity of the marine fauna of the North Pacific and North Atlantic basins — and specifically of Pacific species having tended to ...
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