GEORGETTE DOUWMA / SCIENCE PHOTO LIBRARY GEORGETTE DOUWMA / SCIENCE PHOTO LIBRARY
Nudibranch (Ardeadoris egretta) on a reef. Nudibranchs, or sea-slugs, are shell-less marine molluscs that live in seas around the world. They have external gills (light blue, centre-left) that give the nudibranch its name, meaning naked gills. Photographed in Raja Ampat, Indonesia.
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