Abstract
A hypothesis is presented concerning the speciation and/or splitting processes of four species (P. cygnus, P. japonicus, P. marginatus, P. pascuensis) from a parent species 'P. longipes'. It is suggested that 'P. longipes' had occupied a much wider distribution, probably equivalent to the present distribution range of the whole P. japonicus group, in the Pleistocene before the last glacial age, and then peripheral populations, adapting to open rocky shore waters, were isolated geographically and reproductively through glacial ages of the Pleistocene.
