Common Sea Urchin Spines, coloured scanning electron micrograph (SEM). Common Sea urchins (Echinus esculentus) are marine echinoderms, relatives of the starfish. They possess a shell, or test, covered in numerous movable spines which protect them from predators. The sea urchin test has five-fold symmetry. They feed on algae and other organic material that they scrape from the surface of rocks. Magnification unknown.