Abstract
Satsuma ferruginea (Pilsbry, 1900) has been known to display geographic variation in epiphallic flagellum morphology. Around the type locality of the species, i.e., Kagawa and Okayama Prefectures, rod-like and hook-like flagellum forms are recorded. Our field survey and molecular phylogenetic analysis showed that the two forms have parapatric distributions and likely have limited gene flow at the present. The hook-form populations correspond to S. ferruginea, and rod-form species is here described as S. akiratadai n. sp. The new species had likely been an endemic of the islands around the Bisan Straits, because the current distribution is limited to small islands and hilly terrain areas in the Okayama Plain that had been separate islands before an increase in sedimentation activity and land reclamation since the 6th Century. The new species occurs on forest floors in arid environments on small islands and hills in coastal areas, suggesting that the species likely diverged from S. ferruginea and adapted to the more arid climate that is common in the island areas of the Seto Inland Sea.
