Bellies as dark as the one shown in these two pics are few and far between. I used to wonder if such differences were regional until I saw with my own eyes that there is wide variation in belly colors and patterns even among popei found in the same location.
The one pictured above is 2nd generation captive-bred, parents of which were wild-caught in the northern part of Okinawa Island while the two below are WC from Tokashiki Island, where they are reputed to have have darker bellies.
