ULTRASTRUCTURE OF SPERMIOGENESIS IN ACANTHOCOTYLE AND MYXINIDOCOTYLE (PLATYHELMINTHES, MONOGENEA, ACANTHOCOTYLIDAE)

Spermiogenesis was studied by transmission electron microscopy in the acanthocotylid monogeneans Myxinidocotyle californica (from Eptatretus stoutii) and Acanthocotyle lobianchi (from Raja clavata). In Myxinido cotyle and Acanthocotyle, the zone of differentiation shows two 9+'1' axonemes, the elongating nucleus and mitochondrion, and a single corti cal cytoplasmic microtubule. This single microtubule is found in the m ature spermatozoon of both species and was also noted in capsalids. Th is requires a modified definition of 'pattern 2' of spermatozoa which becomes: 'spermatozoa with two axonemes and no cortical microtubules, except one single element much shorter than the spermatozoon'. A very unusual structure was found in Myxinidocotyle, but not in Acanthocotyl e: the centriolar derivative of one of the 9+'1' axonemes is made up o f 18 diverging singlets of unequal length associated with electron-den se cytoplasm. This seems to be the first case of a centriolar derivati ve without nine-fold symmetry associated with an axoneme with nine-fol d symmetry.