Slate pencil sea urchin (Heterocentrotus sp.) on a reef. The sea urchin is a marine invertebrate of the phylum Echinodermata that includes starfish. Like a starfish, a sea urchin has five-fold radial symmetry. The arms have fused to form a spherical shell from which protective spines protrude. Here, the spines are thick to withstand the turbulent water of its shallow water habitat. This urchin is named for the fact that its spines were used to write on blackboards. A sea urchin uses tube feet on its underside to crawl over rocks. Its mouth is also on its underside, and its five jaws scrape up algae (microscopic plants) growing on the rock. Photographed in the Red Sea.

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