Friday, September 17, 2010
September 17, 2010 : Sea Lice
Sea louse (plural sea lice) are copepods within the order Siphonostomatoida, family Caligidae. There are 36 genera within this family which include approximately 42 Lepeophtheirus and 300 Caligus species. Sea lice are marine ectoparasites (external parasites) that feed on the mucus, epidermal tissue, and blood of host marine fish. This article focuses on the genera Lepeophtheirus and Caligus which parasitize marine fish, in particular those species that have been recorded on farmed salmon. Lepeophtheirus salmonis and various Caligus species are adapted to saltwater and are major ectoparasites of farmed and wild Atlantic salmon. Several antiparasitic drugs have been developed for control purposes. Since L. salmonis is the major sea louse of concern and has the most known about its biology and interactions with its salmon host, this review will focus on this species. Caligus rogercresseyi has become a major parasite of concern on salmon farms in Chile, and 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, suggesting that Atlantic and Pacific L. salmonis may have independently co-evolved with Atlantic and Pacific salmonids, respectively.
Most of our 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 unfortunately sparse in most areas with a long-term history of open net-cage development, since understanding background levels of sea lice and transfer mechanisms have rarely been a condition of tenure license for farm operators.
Many sea louse species are specific with regards to host genera, for example L. salmonis which has high specificity for 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 (O. 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 life cycle on the three-spined stickleback (Gasterosteus aculeatus L.). This has not been observed with Atlantic L. salmonis.