Previous work has shown that karyology may be a valuable technique for discriminating morphologically similar species. In the present study, chromosome number and morphology were described for three strigeid species, Ichthyocotylurus erraticus (Rudolphi, 1809), I. variegatus (Creplin, 1825) and Apatemon gracilis (Rudolphi, 1819). The results show that these species possess the same diploid number of chromosomes (2n = 20) in somatic cells. The relative length and the position of the centromere were determined, and significant interspecific differences in the karyotypes are demonstrated, enabling the ready discrimination of I. erraticus and I. variegatus cercariae. A review of known karyotypes of the family Strigeidae suggests that possible routes of evolutionary changes involve pericentric inversions within chromosomes rather than Robertsonian translocations between elements.
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