The effect of Schistocephalus solidus infection on meal size of three-spined stickleback

The effect of infection with the pseudophyllidean cestode Schistocephalus solidus on the meal size of individually housed three-spined stickleback Gasterosteus aculeatus was quantified. Infected fish harboured plerocercoid loads that contributed from 1.1 to 33.9% of their total mass. Across this range of infection levels, the presence of S. solidus infection had no significant directional effect on standard length (L_S) corrected meal size of host three-spined sticklebacks. Amongst multiply infected fish there was a significant negative relationship between L_S-corrected meal size and the proportion of host mass contributed by S. solidus parasites. This relationship, however, did not hold for singly-infected fish. Furthermore, the data suggest that multiply-infected fish that harbour a combined mass of parasites contributing <c. 15% to host body mass might exhibit meal sizes that exceed those of length-matched uninfected fish. The results suggest that although heavy infections can significantly reduce the meal size of heavily infected three-spined sticklebacks, in the early stages of multiple S. solidus infections host food intake may increase. The probable causes of these differential effects on meal size and their consequences for the host-parasite system are discussed.