The chemical ecology, physiology and infection dynamics of the sea louse copepodid, Lepeophtheirus salmonis Kroyer
This study examined three aspects of the processes of host location and initial attachment of the infective copepodid stage of the sea louse, Lepeophtheirus salmonis Krøyer 1838, to its host Atlantic salmon, Salmo salar (L.). The role of host and non-host odours and derived chemicals in modifying copepodid behaviour was identified in laboratory bioassays. Seasonal and stage-specific changes in the energetics of the eggs and the free-living larval stages were investigated through respiration experiments and chemical analyses. The influence of realistic physical factors, light intensity, salinity and host swimming speed, on determining the sites of initial copepodid attachment were identified in flowing water conditions, by the use of flume studies. Exposure to original odours and extracts of host odours resulted in significant changes in non-oriented behaviour and movement patterns which would increase the probability of host encounter, whilst exposure to original and extracts of host odour gave near-significant levels of oriented movement to the odour source. The host-derived chemicals isophorone and 1-octen-3-ol appeared to induce activated and limited arrested behaviours in the copepodid, respectively. During all stages of embryonic and larval development a gradual reduction in weight and energy content with developmental stage of larvae was noted, as lipid and protein stores were preferentially catabolised to provide energy for morphological changes and increased locomotion of the later larval stages, as revealed by an observed increase in respiration rates with developmental stage. Seasonal changes in embryonic dry weight, energy and biochemical composition were masked by the presence of seasonally variable proteinaceous egg cases. The free living stages showed a mixed trend in reproductive strategy, similar to previous studies of L. salmonis eggs in response to seasonal changes in temperature and photoperiod, but also similar to trends observed in food-unlimited holopelagic copepod species. Light intensity, salinity and host swimming speed independently and interactively altered the distribution and total initial attachment of L. salmonis copepodids on host Atlantic salmon smolts.