After being stung by a sea urchin, a person will feel a sharp pain in the area of the sting. The sea urchin uses its bony spines to puncture the skin, which is what causes the pain.
The bony spines of the sea urchin are able to penetrate through skin, and pieces of the spines often break off during the process of stinging. If the spine pieces are not removed, they can cause inflammation and infection. It is not clear whether or not sea urchins release venom when stinging.
Medical complications from a sea urchin sting can vary. Oxford University reports that people can suffer from arthritis, granulomas or tenosynovitis, which is inflammation around the tendons. Early intervention with steroids can help, but other common courses of treatment, such as antibiotics and anti-inflammatory medications, do not achieve significant results.
Sea urchin spines can be difficult to remove, but doctors can remove them surgically. The New England Journal of Medicine has reported some success with freezing the affected area with liquid nitrogen. If they can get the skin to blister, doctors can remove the fragments without surgery.
While sea urchins are not deadly, their cousin, the flower urchin, can actually be lethal. The flower urchin is covered with pincers that contain a toxic venom. This venom can cause paralysis or death.