en fish disease fish disease پژوهشي Orginal research papers The impact of two principal parasitic digenetic trematodes, <em>Meiogymnophallus minutus</em> and <em>Himasthla</em> spp., on the growth (weight and length) of the cockle, <em>Cerastoderma edule</em>, in Arcachon Bay (France) was studied. The experiment was conducted in the land-based mesocosms. The velocity of infestation with both trematodes was far more rapid during summer in the cockles already parasitized with these trematodes (0.35 to 0.45 metacercariae day^-1) than those of healthy ones (approximately 0.01 metacercariae day^-1). The parasitism had also an effect on the growth rates of cockles. During 190 days of the experiment, the daily growth speed varied from 0.001mm.da/ to 0.021mm.day^-1. On average, the growth was slightly more important in healthy cockles after the start of experiments (0.014mm.day^-1) than those initially infested cockles (0.011 mm.day^-1). The experiment also showed that infestation of hosts with trematodes could perhaps disturb their immunity system, as the intensification with trematode parasites had been favored by already infested cockles. Mesocosm, Cerastoderma edule, Parasitism, Digenea, Growth, Host parasite interactions 33 58 http://jifro.ir/browse.php?a_code=A-10-1150-3&slc_lang=en&sid=1 A. Javanshir arashjavanshir@hotmail.com 100319475328460024381 100319475328460024381 Yes S.J. Seyfabadi 100319475328460024382 100319475328460024382 No G. Bachelet 100319475328460024383 100319475328460024383 No J. Feghhi 100319475328460024384 100319475328460024384 No