Wow, this one is great find.
It isn't in my "standard" books, but amazingly is on the dust cover of a Japanese book I have - "Opisthobranchs of Kerama Islands" by Ono Atsushi and is shown on page 75 as a "Trapania sp" which generally means unidentified. It was found in Zamani Island.
It is also on page 71 of another Japanese book I have, "Opisthobranchs of Bali and Indonesia" by Takamasa Tonozuka. He calls it Trapania sp A. His was found in the Karimunjawa Islands.
I looked on the Sea Slug Forum and he is listed as Trapania sp 1 and can be seen here:
http://www.seaslugfo...apwam.htm#m3666Scroll down and you will see photo of a black and white version like yours.
You might want to send in your shot to the forum, as Fiji is a new location to have found it.
Regards, Linda I.
Linda Ianniello - Nikon D200, Sea & Sea housing, Inon ring strobe, dual Inon Z220 strobes, Nikon 60mm and 105mm lenses.