The life cycle and transmission dynamics of the larval stages of Hypoderaeum conoideum C. Mun ˜oz-Antolı´, R. Toledo and J.G. Esteban* Departamento de Parasitologı ´a, Facultad de Farmacia, Universidad de Valencia,Av. Vicente Andre ´s Estelle ´s s/n, 46100 Burjassot, Valencia, Spain Abstract The morphology of the different larval stages and life cycle of Hypoderaeum conoideum (Trematoda: Echinostomatidae) are described. The freshwater snail species Lymnaea peregra (Gastropoda: Lymnaeidae) serves as the natural first intermediate host and this and L. corvus serve as experimental first intermediate hosts. These and other freshwater snails, such as Physella acuta and Gyraulus chinensis, in turn serve as second intermediate hosts. Adult worms were obtained from chicks and ducks, but not from rats, mice and golden hamsters. The morphology of the larval stages is compared with previous work on H. conoideum. Several aspects of the biology of the life history stages are described with emphasis on the transmission dynamics of the free-living stages. Differential suitability of the snail species that may act as first and/or second intermediate hosts is studied and discussed. Introduction The genus Hypoderaeum Dietz, 1909 (Trematoda: Echinostomatidae) comprises species characterized by a poorly developed head collar that may be differentiated on the basis of the body dimensions and sucker ratio (Toledo et al., 1996). Hypoderaeum conoideum (Bloch, 1782) is the type species and is the most frequently occurring species. Although a number of descriptions of the adult (see Toledo et al., 1996 and references therein) and cercarial stages (Dubois, 1929; Rees, 1932; Wesenberg- Lund, 1934; Wikgren, 1956; Ginetsinskaya & Dobrovolsky, 1964; Meyer, 1964; Jurlova, 1987; Grabda-Kazubska & Kiseliene, 1990; Niewiadomska et al., 1997; Toledo et al., 1998a,b) of H. conoideum from Europe exist, the complete life-cycle has only been studied by Mathias (1924, 1925). Nevertheless, detailed information on the morphology of the larval stages has not been published since then. During the course of studies on the larval digeneans of freshwater snails of the Albufera Nature Park of Valencia (Spain), Lymnaea peregra (Mu ¨ ller, 1774) (Gastropoda: Lymnaeidae) was found to be naturally infected with larval echinostomes and shedding cercariae of H. conoideum (Toledo et al., 1998b). Experimental studies carried out in the laboratory have allowed completion of the life cycle of this species and adult worms, recovered from experimentally infected chicks and ducks, were redescribed by Toledo et al. (1996). Recently, several papers adressing biological aspects of this species have revealed an interesting pattern in the behaviour of the free-living larval stages (Haas et al., 1995; Toledo et al., 1999a,b,c). In the present study, the complete life cycle of H. conoideum is established for the first time in Spain and studied with emphasis on the morphology, population dynamics and behavioural aspects of the larval stages. Moreover, experiments were carried out to determine the differential suitability of potential first and second intermediate hosts. Material and methods Parasite and hosts Lymnaea peregra, L. truncatula (Mu ¨ ller, 1774) and L. corvus (Gmelin, 1791) (Gastropoda: Lymnaeidae), Physella acuta (Draparnaud, 1805) (Gastropoda: Physidae) and Gyraulus chinensis (Dunker, 1848) (Gastropoda: Journal of Helminthology (2000) 74, 165–172 165 *Author for correspondence. Fax: 34 963864769 E-mail: Jguillermo. Esteban@uv.es