Parasitism, space constraints, and gonad asymmetry in the pollan (Coregonus autumnalis)

Abstract

Both sexes of Lough Neagh pollan (Coregonus autumnalis) show directional asymmetry in gonad mass, the left being larger than the right in 70% of individuals. The frequency and degree of asymmetry are size independent in males but become progressively more marked in females of >120 g somatic mass. There is little evidence to support the hypothesis that the size of the body cavity constrains gonad size. Pollan are heavily and size-dependently infected by metacercariae of the trematode Ichthyocotylurus erraticus. Total gonad mass in both sexes of pollan is negatively correlated with infection intensity of this parasite. In females, mean egg size shows no trend but fecundity declines and the degree of asymmetry in ovary mass increases with infection intensity up to a threshold, beyond which it remains constant. While directional asymmetry in gonad size is strongly developmentally controlled in many fish species, variation in the degree of gonad asymmetry in pollan provides a potentially sensitive indicator of stress.