If it weren’t for the expertise of scientists who devote their lives to studying head shield slugs, I’m sure I’d still be baffled regarding an identification of this species.  See the Sea Slug Forum for how involved the discussion of these animals can be.  Apparently, these are quite common and numerous but easily overlooked.  Their dark colouring and minute size (usually no more than 15 mm) makes them hard to detect in the sandy bottoms of eelgrass beds where they live.  I only spotted this individual because it happened to be swimming — an ability that was thought to be very unusual. As one of the experts I consulted put it, “it really looks like it is able to use its parapodial lobes to swim, similar to Gastropteron.”  This individual was no more than 10 mm. When I looked more closely, the eelgrass beds and silty bottom were, indeed, covered in tiny black spots.