Some sea urchins are poisonous and those with venom are deadly to humans. All sea urchins are hazardous to humans, however, because of the sharp spines that can dig deep into the skin.
These spines will not only inflame the skin, but they can penetrate internal organs and often require surgical removal. People who are swimming, surfing, snorkeling or diving in the ocean should be aware of sea urchins and avoid them. Sea urchins tend to reside near the ocean floor. The most dangerous and deadly sea urchin is known as the "flower urchin." The flower urchin looks as though it has many little flowers on it, but these flowers are actually pinchers and are full of toxic poison.