Which nudibranch is this?

[Hintermann]

I took a picture of this critter in Maldives in 2010. I thought that it was a nudibranch but I have never seen anything like it anywhere before or since. Can someone please ID it for me?
Excuse the poor photo quality; there was a lot of current and I had problems to hold the camera steady.

 

[vincent54]

It looks like a Flat worm to me. How big was it?

 

[Hintermann]

It was about the size of a 10-year old child's thumb. It did not look anything like a flatworm - quite bumpy on the exposed side.
I wondered if it was variant of Phyllidiella Rosans. That has similar stripes but AFAIK is not blue and usually found in Hawaii.

Although not obvious in my photo, I recall that the critter did have a couple of little black nudi "horns"

 

[vincent54]

I think you are correct. I looked at the SeaSlug forum and they had a similar looking picture. The Sea Slug Forum - Phyllidiella rosans Wish I could see it in person, the Maldives are on my "bucket" list. Thanks for the picture.

 

[diversteve]

I think it could be: The Sea Slug Forum - Phyllidiopsis striata instead since the center stripes are broken up like the one you shot. Hard to tell without seeing the sides/horns though.

 

[Searcaigh]

I am currently not at home so have no books at my disposal, however it looks similar to Phyllidiopsis annae except for the "broken" lines

 

[Hintermann]

I am currently not at home so have no books at my disposal, however it looks similar to Phyllidiopsis annae except for the "broken" lines

I thought most Phyllidiopsis species are flatter than my critter? One thing I recall well is that this one was rather well bulged out (imagine a longitudinally sliced mini-gherkin), which is why I thought of Phyllidiella rosans. Apparently, they can be blue-black like mine.