Direct access to intermediate and final hosts is critical for successful trematode life cycle completion. The harsh environmental conditions of the wave-exposed rocky shores of the Swedish west coast seriously hamper trematode transmission. Limited numbers of potential host species are another obstacle for survival. In this case, paraxenia, or the ability of parasites to simultaneously use multiple host species, is of utmost importance in promoting distribution in a wide range of microhabitats. Our purpose was to examine trematode spatial distribution and life cycle strategies under these severe conditions in order to assess specific life history patterns linked to their transmission. Estimation of overall parasitological load was carried out by extensive sampling of all available potential hosts in a wide range of microhabitats (rock pools, rock surfaces, cracks and crevices) from lower to upper littoral levels on the rocky shores of two islands in the Skagerrak Strait, Saltö and Ursholmen. Log-linear analysis was performed separately for sporocysts and metacercariae to evaluate parasite prevalence and intensity values for the second intermediate snail host Littorina saxatilis. A detailed study of the spread of Renicola spp. metacercariae using randomly amplified polymorphic DNA technique revealed two additional previously unreported second intermediate hosts (L. littorea and L. saxatilis) for R. thaidus. Several interesting combinations of transmission patterns were noted demonstrating species plasticity in terms of host use.
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