If you see this little guy at the LFS, please don't buy it impulsively. It is commonly known as a Purple Nudibranch, and its only food is a specific type of sponge. Even if you happen to have the right sponge growing in your tank, it won't be enough to keep this beautiful creature alive more than two weeks, at best. This is one creature that should remain in the ocean, and not be imported, distributed, sold or purchased by hobbyists.
Photo by Bill Lide