The sea urchin is a common site in aquariums and reefs around the world, and the Great Barrier Reef is no different. They abound on the shallows of the reef as well as the deeper areas and walls. Sea urchins are related to sea cucumbers and starfish. Like their cousins they have tubular feet they use to crawl across the reef. They use both water and blood as circulation systems in their bodies and eat mostly algae as part of their diet. When food conditions abound they can reproduce very quickly and therefore reach large numbers in a very short period of time. Likewise when the food sources dwindle they die of in masses just as quickly.
The spines are method of protection against predators but also offers a home to juveniles of smaller fish, offering them protection also. These spines are not dangerous to humans but as always it is best not not play with the wildlife.