Swimming in a sub-adult monogenean of the genus Entobdella.

Swimming in a sub-adult monogenean parasite is reported for the first time. When detached from the substrate, a specimen of an undescribed species of Entobdella from the ventral skin surface of the cow-tailed ray, Dasyatis sephen, was found to propel itself vigorously through the water, head-first, by rapid dorso-ventral body undulations travelling in an antero-posterior direction. These waves pass in the opposite direction to the slower breathing (?) undulations exhibited by the attached parasite. Benedeniella macrocolpa from another elasmobranch host and benedeniines from teleost fishes merely made uncoordinated wriggling movements when detached from the substrate. The possible function of swimming in monogeneans is discussed.