GEORGETTE DOUWMA / SCIENCE PHOTO LIBRARY GEORGETTE DOUWMA / SCIENCE PHOTO LIBRARY
Nudibranch (Hypselodoris obscura) on a reef. Nudibranchs, or sea-slugs, are shell-less gastropod marine molluscs that live in seas around the world. They have external gills that give the nudibranch its name, meaning naked gills. Photographed in the Lembeh Strait, North Sulawesi, Indonesia.
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