Studies on the reproductive biology of Lepeophtheirus salmonis (Kroyer, 1838) on Atlantic salmon (Salmo salar, L.)
The salmon louse (Lepeophtheirus salmonis) is recognised as a major pathogen of farmed Atlantic salmon (Salmo salar). With the growth in salmon farming in Scotland and Norway the problem of lice and the need to understand more about their biology and population dynamics has grown. The aims of this study were to examine various aspects of the reproductive biology of L. salmonis and increase fundamental knowledge of this area. It was discovered that the male reproductive system consisted of a pair of testes, vasa deferentia, spermatophore sacs and cement glands and the female reproductive system consisted of a pair of ovaries, oviducts, cement glands, and a single receptaculum seminis. The processes of spermatogenesis, cogenesis and spermatophore formation were examined and compared with those of other species of copepod. Mating in L. salmonis began with the establishment of a precopulatory pair, between an adult male and predominantly a preadult II female. Following the final moult of the female, copulation proceeded and a pair of spermatophores were transferred by the male onto the female. Details of the mating process and associated reproductive behaviour are described. The possible involvement of sex pheromones in mate location was examined. Both adult males and preadult II females were extremely mobile and freely changed host individual. Some evidence was found to suggest preadult II females produced a chemical stimulus to attract conspecifics. Reproductive output and investment by adult female L. salmonis were examined. Distinct seasonal variation in reproductive output existed in the field and under laboratory conditions. Summer generation females produced 6 batches of eggs whereas winter generation females may have produced 4 or 5 batches. Egg strings produced in summer were shorter and had fewer, larger, eggs compared to egg strings produced in winter. Summer generation females invested more protein per egg than winter generation females, which invested more protein per string. It appeared that total reproductive investment by winter and summer generation females may have been equal. Differences in reproductive output and investment were influenced by changes in environmental parameters, particularly temperature. Preliminary studies into the effect of egg size on hatching and nauplius survival were carried out. These results suggested different reproductive strategies were adopted by winter and summer generations of adult female L. salmonis under different environmental conditions. The causes and implications of these different reproductive strategies are discussed.