The size of 3 sand grains!
http://seaslugsofhawaii.com/species/Thuridilla-multimarginata-a.html
Photographed by Aaron Mickelson... thanks to this photo, an extant Hawaiian population has been rediscovered by Prof. William Mautz!
A juvenile photographed at 100 ft. off South Point, Hawaii. This species is endemic to Johnston Atoll and is only found in Hawaii as an extremely rare waif. I tried to keep the location secret, but it leaked out via FB and the fish was collected and sold to a Japanese aquarist, probably for $5,000 or more! On the bright side, it may will live longer in captivity than it would have on the reef and has made several people very happy.
Nudibranchs are a group of sea slugs characterized by having no shell, thus having their gills exposed. The name in latin, nudibranchia, means precisely that: nudi (nude, exposed) + branchia (gills).
Instead of the protective shell, the nudibranchs use other intricate defense mechanisms and that's one of the reasons they are such an interesting group to study. Not to mention their exquisite appearance.
More info about this species:
www.seaslugforum.net/find/dondbany
hypselodoris.blogspot.com/2008/07/dondice-banyulensis-por...