The Differential Secretion of Proteolytic Salivary Enzymes by the Salmon Louse, Lepeophtheirus salmonis (Copepoda: Caligidae) in Response to the Skin Mucus of Various Salmonid and Non-Salmonid Fish Hosts
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Lepeophtheirus salmonis is a salmonid-specific marine ectoparasite but has also been documented on white sturgeon, Acipenser transmontanus, and three-spine stickleback, Gasterosteus aculeatus in coastal British Columbia (BC). Sea lice graze on the mucus and epithelial cells of their host and this initial interaction provides an ideal medium for examining the host-parasite relationship. Previous studies determined that “trypsin-like” enzymes in the mucus of L. salmonis-infected fish originated with the parasite and may be differentially secreted in response to various fishes as an indication of host-specificity. Geographic variation between east and west coast lice was also noted. Thus, the main objective of the present study was to establish proteases in the saliva of BC L. salmonis and then determine whether secretion was differential in response to pink salmon, Oncorhynchus gorbuscha, chum salmon, Oncorhynchus keta, Atlantic salmon, Salmo salar, white sturgeon, and three-spine stickleback. Mucus was removed from naïve fishes, combined to form pooled species solutions, and then exposed to L. salmonis. A “trypsin-sensitive” azocasein assay detected increased enzyme activity in louse-incubated mucus from pink (0.12 ΔOD450), chum (0.08 ΔOD450), and Atlantic salmon (0.15 ΔOD450), white sturgeon (0.13 ΔOD450), and three-spine stickleback (0.15 OD450) in comparison with non-incubated mucus. Some variation in protease activity was evident between species but the differences were not significant (Kruskal Wallis 1-way ANOVA: χ2 = 10.32, df = 5, P = 0.07). Although not statistically significant, these results inspire new questions. For example, could the indiscriminate release of enzymes confer the trophic status of generalist upon BC L. salmonis in contrast with its more specialized east coast relative? Local descriptions of the louse on various marine fish species indicate an expansion of host range with the potential inclusion of paratenic hosts.