While they possess a number of adaptations that allow them to survive in their marine habitats, the most important adaptation of sea urchins is their protective coat of spines. Additionally, sea urchins have highly modified mouths, composed of five bony plates, which enable them to scrape algae off rocks and coral. While often solitary, sea urchins sometimes form large aggregations, which help to protect the invertebrates from predators.
The spines of urchins are crucial for their survival. Unable to swim or crawl quickly, sea urchins would be easy prey for crabs, large fish and sharks. However, the spines provide very effective protection for these peaceful herbivores. When a shadow passes over a sea urchin, the urchin wiggles its spines, which is presumably an adaptation designed to dissuade a lurking predator.
The spines of sea urchins are very sensitive to touch and chemicals in the water. If the spines touch something sharp, they will all turn and point in that direction. Sea urchins use their spines to wear away depressions in rocks, which gives them a protected place to hide. This often erodes the spikes, but since they grow throughout the urchin’s life, this does not cause many problems.
Scientists suspect that sea urchins grow throughout their lives, but this is hard to determine with free-living animals.