These Black Dendrodoris are fairly common, and once you manage to spot one, it’s likely you will spot many.  At first, I thought I was seeing small brown sea cucumbers or drab brown Common Sea Hares. But once I came across an older individual covered in spots it became easy to tell the difference.  Even the juveniles have white spots on their rhinophores, though often very few on their bodies. Juveniles range from blackish/brown to yellow to red.  The yellow one documented below was no more than 3-5 mm and was found out in the open on a patch of algae (unusual diurnal behaviour?)  The reddish juvenile was approximately 12 mm.  Black Dendrodoris usually  have frilly margins that are often coloured blue (but can be red) and a thickish slimy, soft body.  They can be as large as 7 cm (just over three inches) and commonly inhabit rocky, protected intertidal zones under rocks and lumps of old coral, algae beds, etc.  The individuals documented here were all found in less than 5 ft of water from as far south as La Perouse Bay, Maui, to North Kihei. As per usual, check out Hawaii Sea Slugs for more detailed taxonomic information and photos.