The Spotted Swallowtail Sea Slug is even smaller than the Blue Swallowtail Slug, but it is easily confused at first.  The bright orange bar of the Blue Swallowtail is replaced with two bright blue spots corralled by orange lines that generally run for a good length of the mantle.  Their undersides are marked in an orange/rust colour and they are generally no bigger than 1/2″ or 13 mm (not including their tail).  This little one (about 10mm)  was found in less than 3 metres (10 ft) of water on Maui’s north shore.  Please see Sea Slugs of Hawaii if you are looking for more images or information.