This graceful creature is a nudibranch, or "sea slug". There are many varieties of nudibranch, and they are among the most colorful and beautiful creatures in the ocean, ranging in size from a few millimeters to almost two feet long! This one is my own exotic variant, based on the species flabellina cynara, one of the few aeolids that swim. Photoshop over scanned pencil sketch.
While searching for photo reference (yes, I was looking for nudie pictures on the Web), I found that there was actually a nudibranch with my name! Take a gander at the lovely fiona pinnata!

You'll also find this illustration on julian's fabulous Supersnail site, along with lots of other snails.

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