Taxonomy of Echinostoma revolutum and 37-Collar-Spined Echinostoma spp.: A Historical Review

Echinostoma flukes armed with 37 collar spines on their head collar are called as 37-collar-spined Echinostoma spp. (group) or 'Echinostoma revolutum group'. At least 56 nominal species have been described in this group. However, many of them were morphologically close to and difficult to distinguish from the other, thus synonymized with the others. However, some of the synonymies were disagreed by other researchers, and taxonomic debates have been continued. Fortunately, recent development of molecular techniques, in particular, sequencing of the mitochondrial (nad1 and cox1) and nuclear genes (ITS region; ITS1-5.8S-ITS2), has enabled us to obtain highly useful data on phylogenetic relationships of these 37-collar-spined Echinostoma spp. Thus, 16 different species are currently acknowledged to be valid worldwide, which include E. revolutum, E. bolschewense, E. caproni, E. cinetorchis, E. deserticum, E. lindoense, E. luisreyi, E. mekongi, E. miyagawai, E. nasincovae, E. novaezealandense, E. paraensei, E. paraulum, E. robustum, E. trivolvis, and Echinostoma sp. IG of Georgieva et al., 2013. The validity of the other 10 species is retained until further evaluation, including molecular analyses; E. acuticauda, E. barbosai, E. chloephagae, E. echinatum, E. jurini, E. nudicaudatum, E. parvocirrus, E. pinnicaudatum, E. ralli, and E. rodriguesi. In this review, the history of discovery and taxonomic debates on these 26 valid or validity-retained species are briefly reviewed.

Recent Advances in the Biology of Echinostoma species in the “revolutum” Group

This review examines the significant literature on the biology of Echinostoma species in the "revolutum" group. We have considered 10 species belonging to this group. There is a considerable body of literature for four of the species, i.e. Echinostoma caproni, E. trivolvis, E. paraensei and E. revolutum. For these species we have arranged coverage to include the following headings: (1) systematic and descriptive studies; (2) experimental, manipulative and ecological studies; (3) physiological and biochemical studies; (4) immunological and molecular studies. For the remaining six species, i.e. E. friedi, E. miyagawai, E. echinatum, E. parvocirris, E. luisyrei and E. jurini, the literature is not very extensive, and headings were not used. Considerable information in various areas of modern parasitology can be obtained from species in the "revolutum" complex for which the entire life cycle is maintained in the laboratory. The review includes a list of researchers and their addresses who currently maintain such life cycles.

The life-cycle of Echinostoma friedi n. sp. (Trematoda: Echinostomatidae) in Spain and a discussion on the relationships within the 'revolutum' group based on cercarial chaetotaxy

The morphology of the different stages and life-cycle of Echinostomna friedi n. sp. are described and figured. The freshwater snail Lymnaea peregra (Gastropoda: Lymnaeidae) serves as the natural and experimental first intermediate host and L. corvus and Gyraulus chinensis (Gastropoda: Planorbidae) as experimental first intermediate hosts. These, and Physella acuta (Gastropoda: Physidae), also serve as second intermediate hosts. Adult worms, possessing 37 collar spines, were obtained from naturally infected Rattus norvegicus and experimentally from albino rats, golden hamsters and chickens. Mice were not suitable experimental definitive hosts. E. friedi differs from the most closely related species in the 'revolutum' group mainly in terms of several morphological and biological features of the life-cycle stages and in its cercarial chaetotaxy. The chaetotaxy patterns of the species of the 'revolutum' group are analyzed and the results show that a taxonomic comparison of these species may be carried out on the basis of the number of sensilla in the clusters CIII VI, CIII V2 (or CIII V1 + CIII V2), CIV DL and UVb. These clusters appear adequate to establish taxonomic relationships between different species within the 'revolutum' group.

Cercarial chaetotaxy of Echinostoma miyagawai Ishii, 1932 (Digenea: Echinostomatidae), with a review of the sensory patterns in the 'revolutum' group

The chaetotaxy of cercaria of the echinostome Echinostoma miyagawai, found in naturally infected Planorbis planorbis in a brackish water lake in Bulgaria, is described and figured. Comparisons with sensory patterns of cercariae of E. revolutum, E. echinatum, Echinostoma sp.? echinatum, E. audyi, E. lindoense, E. trivolvis, E. caproni and E. jurini, described by Kanev et al. (1987), Lie (1966), Dimitrov et al. (1997), Richard (1971), Richard & Brygoo (1978) and Dimitrov & Kanev (1992), showed the distinct species status of E. miyagawai which exhibits a unique number and distribution of sensilla in 10 groups (CII4, CIV3, CIV4, CIVD, ventral A, M and P: dorso-lateral A and M, and lateral P). A comparison of the chaetotaxy of cercariae of E. revolutum and two descriptions of E. audyi revealed that the sensory pattern in E. revolutum was incompletely described and that the three descriptions differ in 9 of the 16 groups on the head collar and in all groups along the body which casts doubt on their synonymy (sensu Kanev, 1985, 1994). Comparisons of chaetotaxy of cercariae of E. echinatum and its synonym E. lindoense (sensu Kanev, 1985, 1994) showed differences in the CII4, CIII3, CIII4, CIV4, CIVD, ventral A and dorsal A and P groups, and indicated that incomplete and contradictory information has been provided for E. echinatum by Kanev et al. (1987). Examination of the descriptions of the sensilla in cercariae of E. jurini and E. caproni suggested that the two species are indistinguishable with respect to their chaetotaxy. Comparative analysis of the variability of a total of 27 groups of cercarial sensilla reveals three sets of characters which are not useful in discrimination between the species of the 'revolutum' group because they exhibited either constancy (CI1, CII1, CII2, CIV1, MD, PDL), intraspecific variability (CIII1, CIII2) or overlapping ranges (CII3, CIII3, CIV2, tail sensilla).