Comment from the photographer: I took this photo in 2004 with a Nikonos V and 1:1 extension tube, so there's no EXIF data. At the time I thought I was looking at a pair of Hypselodoris villafranca mating near a "orange sponge". Later, while looking and the slides I verified this "orange sponge" was indeed a nudibranch. It shows the white rings and caryophyllidia on the mantle characteristic of Rostanga rosi.