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The fauna of digenean daughter-sporocysts, rediae, cercariae and metacercariae infecting molluscs Littorina spp., Onoba aculeus. Nucella lapillus and Epheria vincta has been studied in the Skerjafjordur and Grindavik regions of SW Iceland. In total, intramolluscan stages of 19 digenean species were recorded; 14 of them are new for Iceland and one of them, a microphallid named Cercaria islandica I, was unknown. A description of this new microphallid cercaria is provided. In addition, the identification and separation of the intramolluscan stages of some microphallid, renicolid and echinostomatid species are discussed. Consideration is given to difficulties encountered when identifying digenean species found on the coasts of European countries. Problems have arisen largely because larval and adult stages have been described and named independently and in isolation, synonyms are common and many "definitive" descriptions are inadequate or incomplete.

Abstract:Trematode sporocysts and rediae reproduce by parthenogenesis, forming clonal groups in the molluscan host. It has recently become popular to consider these groups as eusocial colonies, with division of labour between rediae morphs: small 'soldiers' incapable of reproduction defend the colony, while large rediae reproduce. Alternatively, clonal groups can be considered as self-sustaining infrapopulations. We tested these two hypotheses in the light of new data on rediae of Himasthla elongata (Mehlis, 1831) from snails Littorina littorea (Linnaeus) concerning ultrastructure, growth character and composition of their groups. Clonal groups under study contained rediae of different age and maturity stages: small (young) rediae, rediae with early cercarial embryos, rediae with late embryonic cercariae, rediae with fully formed motile cercariae, rediae with redial embryos and degenerating rediae. Small rediae had a reproductive organ, the germinal mass, whereas most large rediae with developing cercariae did not, which contradicts the eusocialconcept. Overall distribution of rediae by size and by gut to body length ratio was bimodal, which agrees with the eusocial concept ('soldiers' and 'reproductives' as modal size classes). On closer inspection, however, the bimodal size-frequency distributions (SFD) turned out to be the sum of unimodal SFD of rediae at various stages of maturity. The overall bimodality was determined by the character of redial growth resulting in a relatively low occurrence of intermediate morphs and by the developmental arrest in young rediae. The facts that small rediae can attack other rediae and concentrate in the anterior parts of the mollusc can be explained by age-related feeding preferences and niche segregation. They are unlikely to be associated with the 'colony' defence against invaders. To sum up, clonal groups of H. elongata rediae in our study represented self-sustaining infrapopulations. We failed to find any arguments in favour of their eusocial organisation.