A sea urchin is a small round sea creature that has a hard shell covered with sharp points.
sea urchin in British English
noun
any echinoderm of the class Echinoidea, such as Echinus esculentus (edible sea urchin), typically having a globular body enclosed in a rigid spiny test and occurring in shallow marine waters
sea urchin in American English
any of various orders of echinoid echinoderms having a somewhat globular body of fused skeletal plates studded with long, calcareous, movable spines