Populations of Acanthocephalus anguillae and Pomphorhynchus laevis in rivers with different pollution levels

The distribution of two acanthocephalan species (Pomphorhynchus laevis and Acanthocephalus anguillae) in the chub (Leuciscus cephalus) was studied in four river reaches characterized by different levels of pollution: the Rive r Ticino near Abbiategrasso (unpolluted), the Naviglio Grande Canal, in Mil ano (slightly polluted), the River Lambro near Merone village (polluted) an d the River Lambro near Monza (severely polluted). Pomphorhynchus laevis was restricted to the unpolluted and the slightly pol luted sites, while the intensity of A. anguillae increased proportionally t o water pollution. These differences were partially explained by the variat ion in abundance of their intermediate hosts (Echinogammarus stammeri for P . laevis and Asellus aquaticus for A. anguillae). Data on the occurrence of P. laevis and A, anguillae showed a significant negative binomial frequenc y distribution, suggesting their tendency to be aggregated within the host populations of L. cephalus.