Anna's Nudibranch), but there are plenty of other out there that are much, much more impressive.
Nudibranchs tend to feed on very specific organisms (this one concentrates on Petrosaspongia sp. sponges), and I've lost the source page now but I think I read that some of the colours of the nudibranchs come from their food sources. Kind of like flamingos gaining their pink colouration from shrimps.
While doodling this sea slug I discovered that it's hard (at least for me) to make them look at all realistic because they just plain don't look real. I might have had better luck in graphite, just going for the shape and shading... but then I would have lost the colour, which is pretty much what nudibranchs are known for. Ah well, if it looks like a cartoon it looks like a cartoon.
It's surrounded in scribbles anyway, so why I was even the slightest bit concerned about realism is beyond me.