I’m not sure if the Dendrodoris krusensternii is relatively rare or just hard to see. It tends to blend in very well with its surroundings and does a great job of burying itself in the sponge upon which it feeds.  There is an area in North Kihei, Maui where I would say they are fairly common, but searching the broken rubble and sponge shelves in shallows up to 4 metres (14 ft) deep is a prerequisite for finding them.  Look for their bright blue spots and large tubercles shining out from the edges of accretion platforms covered in sponge. Their large creamy and brown-tinged gills can also stand out against the background and if their reddish brown foot is exposed, you may have even more luck. The animals documented here were all approximately 30-50 mm (up to 2 inches)  Please see Sea Slugs of Hawaii for more details.