Diadema antillarum is a regular (round) urchin, and displays the pentamerism of echinoderms. Mature individuals of D. antillarum can reach up to 500 mm in diameter. Diadema antillarum has thin spines that range from 300-400 mm in length and can be up to four times the diameter of the test (skeleton formed inside the body). The spines are thin, hollow, and break easily. The test is rigid and there is a reduced amount of soft tissue in the body wall as compared to other species in the family Diadematidae.
The test and spines of a mature adult are typically black, but lighter colored spines may be intermixed, and in rare cases the urchin will be almost entirely white. The spines of juveniles are always banded with black and white. When the urchin dies, the spines falls off and the test remains.
At the base of the urchin are branched tentacles called tube feet, which help in gathering food, respiration, locomotion, and mucous production.
Other Physical Features: ectothermic ; heterothermic ; radial symmetry