The Arminidae are unusual for nudibranchs in that they don’t have visible external gills, but that might be because they spend time buried in the sand, which is not something you want to clog up your gills with. Their diet is pretty specialised too - they only eat sea pens and sea pansies, colonial cnidaria that also live in sandy areas. An Armina eating a sea pen can be quite spectacular, since the sea pens give off a flash of bioluminescence each time the slug takes a bite.

 Location: Beangabang Bay, Pantar Island, Alor, Indonesia. Equipment: Nikon D800, Subal Housing, 105 mm Nikkor lens, Sola lights.