Virulence of the cestode Schistocephalus solidus and reproduction in infected threespine stickleback, Gasterosteus aculeatus.
The relationship between reproduction in Gasterosteus aculeatus and parasitism by plerocercoids of Schistocephalus solidus was investigated in Walby Lake, Alaska, USA, by quantifying stickleback reproduction and parasite infection using 1655 fish from 4 samples collected in 1990-1996. The stickleback mostly spawned during May and June as 2-year-olds in the 2nd spring-summer after hatching, as was the case with other stickleback populations studied in south-central Alaska. Contrary to an earlier hypothesis that S. solidus has been selected to delay its deleterious effects on threespine stickleback by limiting its infection levels until after the stickleback have reproduced, substantial levels of infection coincided with the stickleback reproductive period. Chi-squared analyses of individual samples suggested that infected females were as capable of producing clutches of eggs as uninfected females in May, but S. solidus inhibited clutch production in June. An overall analysis, however, failed to support the hypothesis that the effect of S. solidus on clutch production differed between early and late periods of the spawning season. It was concluded that S. solidus inhibits the ability of female stickleback to produce a clutch, and that there was no differential effect on clutch production with season. Nevertheless, 77% of all infected females produced clutches. The results contrast with those from another study in which it was found that only 9% of infected females became gravid and another in which it was reported that 23% of infected females were able to mature. Hypotheses are offered for the co-occurrence of stickleback reproduction and substantial parasitism at the population level, and for the ability of a large proportion of infected females to produce clutches. The data suggest that the host parasite relationship is more complex than was previously assumed.