Hyperparasitism among larval stages of Digenea in snail hosts: sophisticated life strategy or pure randomness? The scenario of Cotylurus sp

The hyperparasitism of tetracotyle metacercariae (Cotylurus sp.; Strigeidae) in trematode asexually multiplicating larval stages (sporocysts and/or rediae) within snail intermediate hosts have been perceived for years as a highly evolved mode of life strategy within trematodes, enhancing their development and transmission success. Here, we verified these data and the potential implications for the co-evolution of life strategies within the host–parasite–hyperparasite system. During complex research on the taxonomy and life cycles of trematodes of the family Strigeidae, the results showed that the phenomenon of metacercariae hyperparasitism on sporocysts/rediae within snail hosts is not uncommon in natural conditions in central Europe. However, the relatively low rate of cases of hyperparasitism within the population of snails and the lack of visible trends favouring such a life strategy suggest the accidental nature of this phenomenon. Simultaneously, we found a significantly higher prevalence and mean intensity of tetracotyles in snails without sporocysts/rediae than in snails co-infected by these larval stages and tetracotyles. These distinctions suggest negative relationships between tetracotyles and trematode asexually multiplicating larval stages within snails, with a potential impact on the structure and functioning of trematode larval infracommunities in snail intermediate hosts.