A study of the biology and behaviour of the copepodid larva of the salmon louse Lepeophtheirus salmonis (Kroyer, 1837) (Copepoda; Caligidae)

Abstract

The salmon louse Lepeophtheirus salmonis (Krøyer, 1837) (Copepoda; Caligidae) is a serious pathogen of wild and farmed salmonids in the marine environment. A study has been carried out to investigate the biology and behaviour of the infective copepodid stage of this parasite. Copepodids showed a positive phototactic response to a wide spectrum of light wavelengths (300 - 700 nm) with a peak response at 550 nm. Light response was positively correlated to light intensity between 2.4 - 240 lux with immobilsation occurring at 20,000 lux. Copepodids demonstrated a negative geotactic response to increasing pressure. No positive chemotactic or rheotactic behaviour was demonstrated, although copepodids showed a rapid swimming and fast turning response to directed water flow that was considered to be the principal mechanism of host contact. Light and electron microscope studiesshowed copepodids to be equipped with a wide range of sensory organs including integumental organs, a lensed nauplius eye, an extra-optic photoreceptor (Organ of Gicklhorn), an Organ of Bellonci and antennular elements comprising mechanoreceptive and chemoreceptive components. A new receptor termed the "cauliflower organ" was also described and is suggested to function in host recognition. Primary attachment was shown to be accomplished through grappling of the host epidermis by the hooked antennae. The maxillpeds were used to further embed the antennae and anterior cephalothoracic shield. Secondary attachment was accomplished through the use of a frontal fiament. This was formed internally within the copepodid and may be renewed at each moult. Morphological and histochemical studies demonstrated the frontal filament to be a cuticular extension and showed it to comprise an adhesive "basal plate", a fibrous proteinaceous "stem" and an "external lamina" which was continuous with the cuticle of the anterior cephalothoracic shield. An "axial duct" ran through the stem and terminated at the basal plate. An external "filament duct" protruded from the copepodid rostrum and was continuous with the axial duct. Three major organs, A-, B- and C-glands, were described and their role in filament production discussed. A "ventral fiament organ" equating to the "frontal organ" of earlier studies was also present in the copepodid and subsequent stages. The copepodid alimentary canal comprised a cuticle-lined foregut, a midgut comprising anterior midgut caecum, anterior midgut and posterior midgut and a cuticle-lined hindgut terminating in a slit-like anus. The oral cone conformed to the normal caligid pattern although the labium and labrum were less tightly opposed than those of adults. The strigil was covered by a "labial bar". The midgut comprised three cell types corresponding to the B, RIF and E cells of earlier studies. The posterior midgut comprised only RIF cells. Faeces were enclosed in a peritrophic membrane. A number of glandular systems were present which were associated with the oral cone. These were termed "mucoid glands" "proximal gland complex" "labial gland" and "distal labral organ" respectively. No glands were observed to interface to the fore- mid- or hindgut. The free-living copepodid is believed to be lecithotrophic and was demonstrated to carry large vesicles within the epithelial cells of the midgut which are believed to comprise lipid reserves. The attached copepodid was seen to feed on mucus, epidermal I dermal material and other available host components. Host pathology was limited and comprised principally mechanical damage and tissue compression resulting from attachment and feeding. Host tissue response was limited to an increase in mucous cells and occasional influx of eosinophilic granular cells. Maximum survival of copepodids under optimal conditions in vitro was 23 days following moulting from the second nauplius stage. Survival was not increased by provision of fibrous substrates. Experimental host infections demonstrated that copepodids were host specific and failed to attach to non-salmonid species. Copepodids were demonstrated to infect both marine and freshwater host stages. In experimental infections, larvae were attached principally to the fins and gils. Farmed salmon showed attachment mainly to fins. Spatial distribution was suggested to be principally the consequence of local hydrodynamics. The negative correlation of relative density of infection with size in smolts and positive correlation in larger farmed fish was considered to result principally from differences in swimming-speed and surface area. Disadvantaged, mature and diseased hosts showed higher infection levels. Overdispersal of parasites was considered to be principally the result of host heterogeneity in experimental and farm infections. Attempts to determine the spatial and numerical distribution of copepodids in the wild were unsuccessful and this was suggested to result from the temporally discrete "waves" of infection seen on farmed fish. The results of this study were discussed with reference to studies of other parasitic and free-living copepod species and suggestions were made as to the implications of these findings for aquaculture.