The movable spines are purple and the most conspicuous of the three. All of the appendages are sensitive to touch and may also respond to chemicals released into the water by predators. Touching the urchin's body with a sharp object causes all the spines to point to the region touched. The spines turn away if a blunt object is used, allowing the pedicellariae to be the primary mode of defense. They also show a shadow response, making rapid movements with the spines when a shadow suddenly appears.
Sea urchins exhibit a generalized light sensitivity all over the body.
Some urchins cover themselves daily with stones and shells held in place by the tube feet. This occurs a few hours after dawn and may protect light sensitive pigments in the body. It has adapted well to the pounding surf by burrowing at least half of its body into the rock on which it is found. The spines of the urchin, aided by the motions produced by waves and tides, account for the excavations. If a spine is damaged in the process, it will be regrown.