Nudibranch (Hypselodoris apolegma) with an emperor shrimp (Periclimenes imperator, centre). Nudibranchs, or sea-slugs, are shell-less marine molluscs that live in seas around the world. Here the external gills (upper centre) can be seen which give the nudibranch its name, meaning naked gills. The emperor shrimp, like other Periclimenes shrimps, lives symbiotically with larger animals, most often anemones and sea-slugs, as seen here. Photographed off Rinca, Indonesia.