The Imperial nudibranch is perhaps the most common nudibranch found in shallow waters off of Maui.  It is especially noticeable if it is found trailing another of its kind — a fairly common sight and one which is not really understood.  They have to be thanked for encouraging so many snorkelers to open their eyes to the world of tropical mollusks.  Initially, they can be mistaken for the Trembling or Tom Smith Nudibranch, but they have a distinct deep blue to purple line along their margin that bleeds up onto its yellow-spotted mantle, creating a crown-like effect.  Whether it comes by the name “imperialis” for its regal appearance it’s hard to say — but even its gills stand out in their perfect crown-like definition — trembling white with a marked blue line surrounding each branchia.  Cory Pittman and Pauline Fiene have some excellent photos of the juvenile of this species, noting that the very young have few, if any spots and a much lighter, more lilac coloured margin — the juvenile documented here is also much more purple in colour, but this individual was already 10-12 mm (1/2″) in size.  To about 2.5″ or 70 mm — much larger than the Tom Smith.