Abstract
Parasites are intimately connected to the host in which they live, and some may be affected by the polluted environment of their host. The present study describes the effect of a steroid hormone (11-ketotestosterone) on the sex ratio of the invasive hematophagous nematode Anguillicola crassus Kuwahara, Niimi & Itagaki, 1974, when experimentally injected to European eels, Anguilla anguilla, Our results showed that this steroid induced a significant male-biased ratio in the nematode A. crassus infrapopulations, suggesting that the presence of endocrine disruptors in the environment may lead to skewed sex ratios among parasites.