A consideration of infraspecific categories in the genus Echinococcus Rudolphi, 1801 (Cestoda: Taeniidae).

Echinococcus granulosus (Batsch, 1786) and E. multilocularis Leuckart, 1863, have been considered polytypic species, nine and three subspecies, respectively, having been recognized mainly on typological grounds. However, limits of taxonomically significant morphological and physiological variation have not been defined, nor do we know the degree to which such variation is influenced by species of host. With the probable exception of E. g. canadensis Cameron, 1960, the recognized subspecies of E. granulosus occur sympatrically with the nominate subspecies, E. g. granulosus, of European origin. Because there are no well-defined predator-prey relationships which would ensure their ecological isolation, seven of these subspecies are not reproductively segregated and therefore must be considered taxonomically invalid. Two subspecies of E. multilocularis are allopatric, but may represent two extremes of a cline; the status of the third is uncertain. Different populations (variants or physiological strains) of Echinococcus spp. might be more appropriately designated informally as strains or forms. Attempts have been made in recent years to demonstrate the occurrence of polytypic species in the genus Echinococcus Rudolphi, 1801. Such studies have been complicated by two factors: the undefined limits of taxonomically significant morphological and physiological variation in cestodes, and the inadequate knowledge of the degree to which such variation is influenced by species of host. Also bearing upon this difficult problem is the basic taxonomic concept of the subspecies. This has been defined by Mayr (1963, p. 348): "A subspecies is an aggregate of local populations of a species, inhabiting a geographic subdivision of the range of the species, and differing taxonomically from other populations of the species." For species of Echinococcus and other cestodes whose life cycles are similar, different host assemblages may be considered the equivalent of different subdivisions of the range. Therefore, in defining subspecies of such cestodes, evidence must be presented to show that populations occurring in assemblages of sympatric hosts are indeed segregated. Proceeding on this basis, the present study evaluates criteria utilized in the definition of subspecies of Echinococcus spp. and reviews the taxonomic status of subspecies that have