ABSTRACT
From an evolutionary viewpoint, the Schistosomatidae is a fascinating group within the Digenea due to its two-host life-cycle, the presence of separate sexes, and the colonization of the vasculature of birds, mammals and crocodiles. Together with the hermaphrodite families the Sanguinicolidae (parasites of fishes) and the Spirorchiidae (parasites of turtles), they collectively represent the 'blood-flukes', and gain entry to their definitive hosts by direct penetration of the body surface. The family contains the genus Schistosoma, the most intensively studied of trematode parasites, well known for the widespread diseases they cause in man and domestic animals. Thirteen genera are currently recognized in the family (Table 18.1), comprising around 85 species (Bãsch 1991). However, the relationships between Schistosoma and the 12 remaining genera of schistosomatids have received scant attention. The genera differ markedly in the degree of sexual dimorphism shown (Carmichael 1984; Bãsch 1991; Morand and Müller-Graf 2000). Most noticeably, some genera possess males which lack a gynaecophoric canal (e.g., Dendritobilharzia), whereas in the majority of other genera the canal is well developed, with the larger, muscular male grasping the more slender female. Striking differences in the number of testes in the adult male worms are also apparent; for example, in the case of Trichobilharzia there can be more than 500 testes, whereas two to seven occur in Schistosoma. Recently, Morand and Müller-Graff (2000) conducted a comparative analysis of genera within the family. They suggested that there is a trade-off between investment in muscle (in relation to sexual dimorphism) and number of testes (other variables being controlled for).
