During my dive on Big Pigeon wall ( Credner Islands ) i run into this little guy called Notodoris Serenae . Took me a little while to get a decent shot...
Size about 5-6 cm for this guy , but they can grow up to 10 cm long
"Notodoris serenae feeds on calcareous sponges belonging to the family Leucettidae. It has been recorded eating Leucetta chagosensis" Wikipedia
There have been sightings of this in Papau New Guinea in different locations
Kavieg for example and many other locations
The most characteristic difference is the development of 3 long extraterrestrial appendages which shield the three distinct gills.
More information on Notodoris Serenae
Papua New Guinea