Abstract
The present paper deals with the secondary sexual characters observed in the dimorphic radulae of the genus Drupella by scanning electron microscope with a taxonomic revision of this characteristic group based on the samples collected from the Okinawan waters. The radulae of D. cornus (Roding), D. fraga (Blainville), D. dealbata (Reeve) and D. concatenata (Lamarck) were revealed to undergo sexually dimorphism in adult stage. The rachidian teeth of the males are larger in size, thicker, more massive in shape and darker in color than those of the females of the same species, as was first described by Arakawa (1958a) on two species. On the other hand, it was shown that the juveniles exhibit no radular sexual dimorphism, having thin and fragile rachidians as the adult females. In the present study it was also demonstrated that the male radular characters are acquired as the secondary sexual characters at certain growth stages, mainly in one of the four species, D. fraga.
