a1 Wellcome Marine Laboratory and Department of Pure and Applied Zoology, University of Leeds, Leeds LS2 9JT
The ectoparasitic copepod Lepeophtheirus pectoralis exhibited a regular annual cycle of abundance on its hosts. Both the incidence and intensity of infection were greatest in August or September and both declined over winter to their minimum values in April. Very similar cycles were observed in both 1972 and 1973. Analysis of the age structure of the parasite population showed that the increase in abundance coincided with the onset of the breeding season. Although females always outnumbered males the sex ratio approached unity during the breeding season, which extended from May to October. Copulating pairs were most commonly observed during this period.
There was a marked bimodal peak of egg production each year, with the first mode occurring around May and the second mode around August or September. This pattern was produced by a system of alternation of generations operating within the parasite population, with a summer type generation (distinguishable by its rapid maturation rate and reduced longevity) alternating with an overwintering type of generation.
(Received July 10 1974)
p1 Department of Zoology, British Musieum, (N.H.), Cromwell Road, London SW7 5BD.