Chaetotaxy of members of the Gyrodactylidae (Monogenea), with comments upon their systematic relationships with the Monopisthocotylea and Polyopisthocotylea

This study investigates intra- and inter-generic variation in the chaetotaxy pattern of members of the Gyrodactylidae and compares these data with those from other monogenean groups elucidated by other workers. The staining of surface sensillary structures has revealed areas of homology between the two genera Gyrodactylus and Ooegyrodactylus, which have markedly contrasting modes of reproduction. Although species specific maps have been elucidated for members of the genus Gyrodactylus, no evolutionary trends in their sensillary patterns could be determined. Members of the family Gyrodactylidae are shown to possess a cluster of 3 × 2 sensilla, similar in their size and arrangement to the post-ocular (spo) cluster of sensilla in polyopisthocotyleans, although on the opposite surface. A comparable cluster of sensilla is lacking in monopisthocotyleans. This information gathered from chaetotaxy supports the proposal that the order Gyrodactylidea is close to the polyopisthocotylean line of monogenean evolution.