The amphipod Gammarus fossarum Koch (Crustacea) as intermediate host for some helminth parasites, with notes on their occurrence in the final host

Monthly samples of Gammarus fossarum have been taken in the Rhône river system near Lyon, to determine the size composition and reproductive activity throughout the year. The acanthocephalan parasites of this gammarid species were also regularly collected. Furthermore, the intestinal tracts of a number of fishes (amongst others Thymallus thymallus), feeding mainly on Gammaridae, have been investigated for the presence of helminth parasites. G. fossarum reproduces throughout the year in the area near Lyon, but fluctuations in the rate of ovigerous females appear. A peak in the infestation of the gammarids by larvae of Pomphorhynchus laevis (Acanthocephala) occurred in autumn and a second peak, but a lower one, during spring. The fish hosts showed maximal infestations during the same periods, while during summer the incidence of Pom. laevis was low, both in the intermediate and in the final host. From the stomach contents of the fishes it seems that gammarids harbouring the invasive acanthocephalan larva (cystacanth) are selectively ingested. Reproduction of G. fossarum seems not always to be inhibited at the presence of acanthocephalan larvae, since several infested female gammarids were found carrying eggs or juveniles in their brood pouch. The occurrence of some other helminth parasites is recorded for the region near Lyon.