Here's an attractive nudibranch from Hawaii. It is commonly known as the Sphinx Nudibranch, or the Blue Sphinx. Its scientific name is Phyllidiopsis sphingis.
This is a small critter -- only about one inch (2.5 cm) in length -- although the macro photo at right might make you think it is larger. It is thought to eat sponges, but not a whole lot is known about its habits.
The species is relatively uncommon. It is found in Hawaii, and also has been seen in other Pacific locations such as Guam and Papua New Guinea. I photographed this specimen off the coast of Hawaii's Big Island, near Puako.
When the species was first discovered, it was thought to belong to the Phyllidia genus. It was later assigned correctly to the Phyllidiopsis genus. The two genera resemble one another externally, but biologists can distinguish them once they are dissected because their innards are quite different -- specifically, the structures of their digestive systems are dissimilar.
Now, about that name. Phyllidiopsis sphingis was named by Dr. David J. Brunckhorst of the University of New England, in Australia. He was the one who identified the species, so he got to name it. He explains his choice of names this way: "Phyllidiids with their lumps, bumps, ridges and bright colours often give the image of tiny monsters; hence, P. sphingis was given the fun name derived from the Sphinx - the mythical monster at Thebes who posed riddles for the people passing by and consumed them if they could not answer!"