Host identification and settlement of the infective copepodid stage of the salmon louse, Lepeophtheirus salmonis (Kroyer, 1837)
Settlement of Lepeophtheirus salmonis copepodids on salmonid and non-salmonid fish hosts was investigated at a variety of time points post-infection. Infection was also evaluated following previous exposure to L. salmonis to determine if cues of attached lice stimulated further settlement. A new method for estimating the surface area of a fish was devised. Consequently, surface area was examined for effect on louse burden in all fish species. The external surface of the copepodid was surveyed for potential receptors that may be used in host location and recognition. The behaviour of L. salmonis copepodids was studied on a collagen matrix containing host cues. An increase in time post-infection did not significantly affect louse burden. L. salmonis copepodids settle on fish in a hierarchy dependant on several factors including the availability of settlement sites, the species being settled and the fitness of the host. Sea trout and salmon were preferred to other hosts including rainbow trout, however all salmonid species investigated were more attractive than the non-salmonid species. In all trials, only a marginal surface area effect was seen suggesting surface area alone is not a significant factor influencing settlement. It was shown that non-host fish may be temporary reservoir hosts for the louse, but do not sustain an infection. Copepodids leave and re-attach to fish early in the settlement process, which suggests host recognition occurs early in the settlement cascade and this process involves olfaction and taste. Previously un-described mechanoreceptors have been identified at the base of the 2 nd antennae of the copepodid. Their arrangement and sensitivity also imply a significant role in host identification and settlement.