The digenean species of Echinostoma Rudolphi, 1809 (Echinostomatidae) with 37 collar spines that comprise the so-called Echinostoma ‘revolutum’ complex, qualify as cryptic (sensu Bickford et al.; see also Pérez-Ponce de León and Nadler  for a recent review) due to the interspecific homogeneity of characters used to differentiate species. Only five species, the Eurasian Echinostoma revolutum (Frölich, 1802), E. echinatum (Zeder, 1803) and E. jurini (Skvortsov, 1924), the North American E. trivolvis (Cort, 1914) and the African E. caproni Richard, 1964, were considered valid in the most recent revision of the group using for species delimitation a single morphological feature of the larval stages (the number of pores of the para-oesophageal gland-cells in the cercaria), the specificity towards the first intermediate host (at the familial level), the ability to infect avian or mammalian hosts (or both) and geographical range on a global scale (continents) [3–5] (but see Kostadinova and Gibson  for a critical review). It is worth noting that E. echinatum has not been formally described and justified in a taxonomic publication and is not recognised as valid [see 6 for details]. However, recent molecular studies have demonstrated a higher diversity within the ‘revolutum’ species complex. Thus one African species, Echinostoma deserticum Kechemir et al., 2002, and a yet unidentified species from New Zealand were distinguished based on molecular data  (see also ), and E. trivolvis was found to represent a species complex . Additional data on the geographical distribution of the Echinostoma spp. have also been obtained. E. revolutum was recorded in Australia  and North America [10, 11], Echinostoma paraensei Lie & Basch, 1967 in Australia and South America , and E. cf. robustum in North and South America .
The pioneer molecular studies, predominantly based on laboratory strains, have revealed that the mitochondrial nad 1 gene provides a better resolution for investigating relationships within the problematic Echinostoma ‘revolutum’ species complex in comparison with the nuclear rRNA spacers and the mitochondrial cox 1 gene [12, 13]. The subsequent DNA-based studies [7, 9–11, 14] have provided a framework for investigating genetic variation in natural Echinostoma spp. populations and revealed novel data on the cryptic variation, identification and geographical distribution of the species of the ‘revolutum’ complex.
However, in contrast with the wealth of sequences gathered recently from North America, which have revealed high diversity (six cryptic lineages) within the ‘revolutum’ complex of Echinostoma[9, 11], data from European natural populations are virtually lacking. Thus, of the eight species described and/or recorded from Europe, i.e. E. revolutum, E. paraulum Dietz, 1909, E. jurini (Skvortsov, 1924), E. miyagawai Ishii, 1932, E. robustum Yamaguti, 1935, E. bolschewense (Kotova, 1939), E. nordiana (Baschkirova, 1941), E. friedi Toledo et al., 2000 [3, 5, 15–22], sequence data are available only for E. revolutum[7, 12–14] and E. friedi (GenBank AJ564379).
In a study of the digeneans parasitising molluscs in central and northern Europe we found that Radix auricularia (Linnaeus, 1758), Radix peregra (Müller, 1774) and Stagnicola palustris (Müller, 1774) were infected with larval stages of two species of the Echinostoma ‘revolutum’ complex of cryptic species, one resembling E. revolutum sensu stricto (s.s.) and one undescribed species (see also ). Here we describe the cercariae of these two species and provide morphological and molecular evidence for their delimitation. Further, we extend the approaches of Morgan and Blair [7, 13], Kostadinova et al. and Detwiler et al. to the relationships within the ‘revolutum’ species complex inferred from the nad 1 gene with the newly-generated sequence data from natural infections in snails in Europe. Phylogenetic analyses revealed the presence of additional cryptic lineages of the Echinostoma ‘revolutum’ species complex.