Sea urchins, are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres. Their tests are round and spiny, typically from 3 to 10 cm across. A few species have venomous spines with potent and potentially deadly effects. ... Some sea urchins “bite,” and a few have venomous bites. Unlike a sea urchin sting, a bite does not leave spines behind. Sea urchins may also trigger allergic reactions that can range from mild to potentially deadly.