Gaucilla marginata Berg 1860

 

 
Features Dark blue on top and light underneath. Most nudibranchs are bottom dwellers and live foot down. Glaucilla live on the surface with their foot upwards. If turned over they will quick revert to their original position.
Distribution worldwide, temperate and tropical waters
Depth surface dwellers
Max size to at least 3.5cms long. This one 2cms long 3cms across
  They feed on bluebottles and can extract the poisonous nematocysts and store them for their own protection