The solution to Bit-o-Critter round 16

Miriam from DeepSeaNews correctly identified Glaucus atlanticus, a spectacular pelagic nudibranch.  Like a lot of neustonic animals, they are bright blue, which relates to the intense sunlight exposure they get in the top inches of the ocean.  The same blue coloration is seen in calanoid copepods, jellies and other things that live right at the surface. 

This pic of Glaucus from Wikimedia Commons.