Background. Arctic charr, Salvelinus alpinus (L.), are known to harbour significantly more sea lice, Caligus elongatus von Nordmann, 1832, than do Atlantic salmon, Salmo salar L. This research investigated whether this is due to differences in initial infection or to resettlement/loss of mobile adults. Skin mucus protein profiles and epidermal histology were also studied as two characteristics that might help explain interspecific differences in host susceptibility. Materials and Methods. Atlantic salmon and Arctic charr were sampled from the sea-pen after 12 weeks of exposure to natural infection. Fishes were examined for sea lice counts and mucus was analysed for protein profiles using standard methods of poly-acrylamide gel electrophoresis. Epidermal tissue of fishes around chalimus attachment sites was also examined by transmission electron microscopy. Results. Interspecific differences in host susceptibility were determined not to be due to differences in initial infection rate, as charr and salmon had similar numbers of attached chalimi. Adult parasites were significantly more numerous on charr as a result of resettlement of adults lost by other fishes in the area. Gel electrophoresis indicated that salmon had a greater abundance of skin mucus proteins than charr, particularly in the 30–42 and 67–94 kDa range. Ultrastructural analysis of epidermal tissue indicated that charr had more mucous cells and showed evidence of possible osmotic stress. Interspecific differences also existed in intercellular adhesion characteristics. Salmon and charr showed a similar absence of inflammation around chalimus attachment sites. Conclusion. Artic charr had a more intense infection with Caligus elongatus than did salmon corresponding to fewer potentially antagonistic proteins in the mucus. Perhaps also, the epidermis of charr provides more easily accessible food for the sea lice. Higher levels of stress in charr in sea water may also have predisposed them to higher sea lice infections.
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