ALEXIS ROSENFELD / SCIENCE PHOTO LIBRARY ALEXIS ROSENFELD / SCIENCE PHOTO LIBRARY
Sea urchins (Diadema setosum). These sea urchins are social, often forming dense groups like that seen here. Sea urchins feed mainly on algae. The spines serve to protect them from predators. Diadema urchins are found on reefs in the tropical Indo-Pacific region. Their thin spines can grow up to 30 centimetres long and their shell up to 9 centimetres across. An urchin uses tube feet on its underside to crawl over rocks. The mouth is also on the underside, and has five jaws that scrape up algae growing on the rock surface. Photographed in the Indian Ocean, off the coast of Madagascar.
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