Differences in the parasite infections in three-spined sticklebacks (Gasterosteus aculeatusL.) of different sex, age and size

Abstract

A population of Gasterosteus aculeatus from Priddy pool in Somerset was infected with three species of parasites: Schistocephalus solidus, Diplostomum gasterostei and Echinorhynchus clavula. The variations in the incidence and intensity of the infections with the sex, age and size of the sticklebacks are described.The infections with the three parasites were very similar in male and female sticklebacks. The weights of Schistocephalus were greater in the females, however;the mean weight of the plerocercoids was larger suggesting that they grew more rapidly in female fish.The Schistocephalus infection increased in small fish, under 1 year of age, but in older larger ones the infection decreased. These changes can be related both to the feeding habits of the fish and to the weight relationships of host and parasite.It was found that the weight of Schistocephalus did not greatly exceed that of the fish, and also that the combined weight of fish plus Schistocephalus was rarely greater than 3 g. Therefore the proportional weight of Schistocephalus, or parasite index, was lower in sticklebacks heavier than about 1·5 g.The Diplostomum and Echinorhynchus infections both increased with age and size of fish, but the largest sticklebacks were not the most heavily infected. These increases can be related to the feeding habits of the fish in the case of Echinorhynchus and to accumulation with time in the case of Diplostomum.In all three species the largest and oldest fish were not the most heavily infected.It is suggested that the old fish are becoming senile and are therefore less able to support large parasite burdens.I am very grateful to Dr H. D. Crofton for his constant advice and encouragement during this study, and to Dr R. A. Avery for useful discussions. This work was supported by a Science Research Council research studentship and by a NATO studentship.