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Thorny oyster mantle. Thorny oysters (Spondylus varius) are filter feeders and are common on steep reefs and walls. Along the edge of their mantle are simple eyes (small dots). When these detect an approaching threat, the oyster quickly closes its shell. Photographed off Komodo National Park, Komodo, Indonesia. (KEYSTONE/SCIENCE PHOTO LIBRARY/MATTHEW OLDFIELD)