Sea urchins (Astropyga radiata) with a school of cardinalfish (family Apogonidae). Sea urchins are echinoderms, relatives of the starfish. They possess a rigid body covered in numerous movable venomous spines, which protect them from predators. They also have five-fold body symmetry. Urchins feed on algae and other organic material which they scrape from the surface of rocks. Photographed off Sulawesi, Indonesia.
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