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Thanks for identification to Dr. Juan Lucas Cervera Currado
This species is at Nudi Pixel as Trapania sp. from Koh Rong, Cambodia, identification is verified by Richard Willan, and as Trapania japonica from Anilao, identification not verified.
David W. Behrens labels it also Trapania sp. at Michael D. Miller's The Slug Site discussing the 'Opisthobranch of the Week # 762", with a picture of this species taken by Tet Lara at Anilao.
Trapania sp. 05 is possibly a strange colour variant of Trapania japonica, with large brown patches,
the other extreme variant almost lacking the brown spots is as Trapania sp. 8 at www.umiushi-zukan.com.
There is either one very variable species or a complex of pretty similar species, with a white body colour, yellow pigment on the processes and dark brown pigment on the anterior face of the translucent whitish gills.
Pretty similar are also:
Trapania sp. 08, which lacks the brown spots on the mantle, and differs by translucent white gills.
More information on Trapania japonica are on
Trapania naeva, which differs by dark brown/black lateral processes. Bill Rudman's Sea-Slug Forum!