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This species is most possibly Halgerda sp. 3 in IPN page 183:
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp.
Let me quote the description:"The uniform white color and black lines along the ridges immediately distinguishes this species. It has a few black spots and lines on the gill branches and a single black line on the rhinophores." and: "size 25 mm, Distribution: Known only from New Caledonia."
The above picture was taken during a tech dive at a depth of 70 m near Malapascua Island at Monad Shoal seamount .
The photographer Klaus Stiefel writes: "This is the largest nudibranch I ever saw, almost the size of a football."
that is 27 cm, the size of this specimen is 10 times the size of IPN Halgerda sp. 3, which also differs by orange pigment at the gill pocket.