Studies on the Life History of Schistocephalus solidus: Field Observations and Laboratory Experiments

Abstract

Field and laboratory investigations of the interactions between Schistocephalus solidus (Cestoda: Pseudophyllidea) and its hosts were carried out using: the copepod, Acanthocyclops viridis; the three-spined stickleback, Gasterosteus aculeatus and the chicken, Gallus gallus. The first part of the dissertation is concerned with the epidemiology and impact of the plerocercoid stage of S. solidus on a natural population of three-spined sticklebacks. Samples of about 60 sticklebacks were collected at roughly monthly intervals between August 1988 and September 1989 from an urban pond in Inverleith, Edinburgh. A study was made of uninfected fish in the 0+ cohort in order to evaluate the base level biology of the population (Chapter 2). Growth of the sticklebacks was confined to their first autumn, spring and summer of life when temperatures were high and daylight hours long. The normal life span of sticklebacks in this population is 12-18 months. However, some large 0+ fish were lost from the population over winter perhaps as a result of predation by black-headed gulls (Larus ridibundus). In previous studies lateral plate counts of 4-5 have been associated with a risk of avian predation. As these counts were observed most frequently in the stickleback population of Inverleith this may also reflect the action of avian predators. There is a single opportunity to reproduce between April and June and those that have bred succumb soon afterwards, but some 1+ sticklebacks (presumably non-breeders) manage to survive until their second winter. Analysis of stomach contents showed seasonal variation in stomach fullness. Overall, stomachs were less full during the winter than at any other time during the survey. Diet composition in uninfected sticklebacks also varied with season. In Inverleith pond, sticklebacks are largely benthic feeders, relying most heavily on chironomids and to a lesser extent on ostracods and free-living nematodes particularly during the spring and summer. Of the zooplankton consumed cladocerans were the most common and present in stomachs throughout the year. A greater reliance on plant material during winter may have been the result of food scarcity and could partly explain the poor growth at this time. The composition of the parasite population changed markedly during the life span of the 1988-1989 cohort of sticklebacks (Chapter 3). Logistic regression revealed that time of the year was the single most important determinant of the prevalence of S. solidus infection. The intensity of infection was also largely predicted by the time of year but in addition, host size was important, with the largest sticklebacks harbouring the most plerocercoids. By far the greatest proportion of infected fish and the highest infection intensities were recorded in autumn 1988. This increase in both the number of plerocercoids (which was accompanied by an increase in the abundance of small plerocercoids), strongly suggests that a wave of new infections was occurring at this time. Additional features of this wave of infection were: a tendency towards over-dispersion of parasite numbers, large relative weights of parasites and the presence of plerocercoids large enough to be infective to a definitive host. During winter, many infected fish were lost, notably those harbouring the highest parasite burdens, in terms of plerocercoid numbers, plerocercoid size and the relative weight of the plerocercoids. Such heavily infected sticklebacks may have died because of pathological consequences of their infections. Alternatively, the behaviour of the heavily infected host sticklebacks may have been altered such that they were rendered more susceptible to predation by black-headed gulls, a known definitive host of S. solidus. If avian predation is responsible for the loss of infected (and some uninfected) fish, then eggs will be released from the rapidly maturing adults into the pond at this time. The prevalence of infection was found to be fairly stable through the remainder of the survey period, but a further decrease in intensity was observed in fish sampled during summer; again this may have been the result of parasite-induced host mortality. The growth rate of plerocercoids from single infections parallelled fish growth, being high during autumn, spring and summer. In double infections, however, individual plerocercoid growth was reduced, suggesting that limited nutrients were available for parasite growth and this induced competition between two plerocercoids. Infection with S. solidus plerocercoids was accompanied by changes in the diet, growth, reproduction and longevity of sticklebacks from Inverleith pond (Chapter 4). (Abstract shortened by ProQuest.)