Fitness of the marine parasitic nematode Anisakis simplex s. str. in temperate waters of the NE Atlantic

In temperate waters of the NE Atlantic, third-stage larvae of Anisakis simp lex sensu stricto collected from 3 paratenic host species were identified b y restriction fragment length polymorphisms. The condition of wild larval i nfrapopulations was assessed by examining morphometric and growth character istics. The differentiation patterns and the excretory/secretory products o f larvae grown in Vitro were also examined. An extensive morphometric, grow th and differentiation variability was found between parasite larvae collec ted from different paratenic host sources. Nematode infrapopulation larvae from the squid comprise those smaller individuals with the lowest values of survival rates and moult success. It may be then concluded that the fitnes s of A. simplex s. str. larvae is not the best possible in the squid, which impaired the competitiveness of the parasite and its chances of developing into an adult. This suggests that the microenvironments impaired by the pa ratenic host may provide larval infrapopulations with unique ecological fac tors probably influencing its recruitment to the final host populations.