Associations and interactions among intestinal helminths of the brown trout, Salmo trutta, in northern Italy.

Species co-occurrences and interspecific associations between intensity of infection were studied in helminth communities of three populations of brown trout, Salmo trutta, from northern Italy. Of the eight helminth species, only four were common enough to be included in the analyses: Pomphorhynchus laevis, Acanthocephalus anguillae, Echinorhynchus truttae and Cyathocephalus truncatus. The observed frequencies of co-occurrences of the different species, based on presence/absence data, did not differ from those predicted by a null model derived from prevalence data. However, the intensity of infection (number of individuals per fish) of different helminth species were generally, but not always significantly, negatively correlated in tests of pairwise associations. Variation in fish sizes and its effect on infection levels, and whether or not two helminth species used the same or different intermediate hosts, had no influence on these findings. Of the few significant negative associations found between pairs of helminth species, none was found in more than one fish population. This suggests that interspecific associations may be condition-dependent: even in apparently similar localities, the same combinations of helminth species show different associations. Without evidence of replicability, it is almost impossible to conclude to the consistent role of competition between any pair of helminth species in the field.

Copulation of Acanthocephalus anguillae (Acanthocephala).

A total of 114 specimens of brown trout, Salmo trutta Linnaeus, 1758, was sampled by electrofishing between October 1998 and July 1999 from the Ceresone stream (North of Padua, Italy). Over 63% of fish were infected with three species of helminth: a tapeworm, Cyathocephalus truncatus (Pallas, 1781), and two acanthocephalan species, Echinorhynchus truttae (Schrank, 1788) and Acanthocephalus anguillae (Muller 1780). On three occasions, in the fore-gut of three brown trout, three pairs of A. anguillae were found in copula. In all three cases, the male's bursa was fully extruded and firmly attached to the posterior region of the female. Cement was found inside the bursa suggesting that insemination had probably occurred. Eight female A. anguillae recovered in five other infected trout had attached copulatory caps.