My first sighting of these tiny headshield slugs involved literally getting hit in the mask with them.  I encountered a bloom in a back lagoon and these little black specks were everywhere — crawling over rocks, clinging to each other, and spinning in the current and waves. Upon further examination, these black spots appeared to be much more — varied in line structure and colour and decorated in blues and greens, orange and sometimes white — close up, they were stunning.  Some were no bigger than 6 mm, but others were over 30 mm when their long  “swallow-like” tails were accounted for.    For more taxonomic information and a description of the differences in tail formation and colour, please see Cory Pittman and Pauline Fiene’s site, Sea Slugs of Hawaii.