Do environmental changes influence the occurrence and effect of parasites?
External stressors such as environmental perturbations and habitat loss, can have a significant impact on fish communities and survival, and consequently on parasite communities. Relatively little is known about how external stressors can impact the occurrence of all endo- and exo-parasites and their potential effect on their host. In 1993, a centennial flood in the Vaccarès lagoon (Rhône delta, France) produced an influx of 110 million m3 of freshwater, providing us the opportunity to investigate the effects of possible intermediate or defnitive host-loss on a two-host parasite. In this host–parasite system composed of the goby P. microps and the trematode Aphalloïdes cœlomicola, the potential disappearance of the first intermediate Hydrobia ventrosa (Gastropoda, Hydrobiidae), which is an euryhaline species living in Brackish water environment (Montagu 1803), may prevent the life-cycle of the parasite to be completed. We found that, despite the disappearance of A. cœlomicola’s first intermediate host in the lagoon, the parasite occurrence did not differ for both male and female P. microps from 1993 to 1996; only in 1997 did we observe a reduction in parasitism. This suggests that the influx of fresh water during the flooding of 1993 did not increase P. microps susceptibility to A. cœlomicola, but also that A. cœlomicola was not directly affected by the loss of a primary intermediate host as a result of environmental changes resulting from the influx of fresh water during the flooding of 1993. This unexpected result may be explained by a resilient but small first intermediate population in the complex Vaccarès lagoon system.