Effect of the cestode macroparasite Schistocephalus pungitii on the reproductive success of ninespine stickleback, Pungitius pungitius

We quantified the relationship between reproductive status in the ninespine stickleback, Pungitius pungitius (L., 1758), and parasitism by plerocercoids of the cestode Schistocephalus pungitii Dubinina, 1959 in Airolo Lake, Alaska, using fish specimens from the 1994, 1998, and 2000 reproductive seasons. Infected females were inhibited from producing clutches of eggs and infected males showed much-reduced testicular condition. These deleterious effects on reproduction appear to involve nutrient theft. In addition, there appears to be a greater physical effect of the parasites crushing the gonads of host fish. The results of this study support the theoretical prediction that S. pungitii is a parasitic castrator, as the fitness of the majority of infected female ninespine stickleback was apparently reduced to zero, and there may be a similar effect among the majority of male ninespine stickleback. The symptoms of infection may represent adaptive parasite manipulations resulting in larger, more fecund parasites or an extended parasite-transmission period, although the possibility that the symptoms represent nonadaptive side effects cannot be excluded.