Echinothrix diadema is a long spined urchin measuring between 10 to 20 centimeters. It belongs to the Diadematidae family of sea urchins, which are known for their long and venomous spines. Like other sea urchins they have a spherical shell, known as a test, with the mouth found on the underside and the anus at the top. E. diadema is dark in colour, as reflected in its common name. It can measure between 10 and 20 centimeters. Although juveniles have some banded spines, adults do not.
 Sealife Base. n.d. Echinothrix diadema. Accessed April 2018. https://www.sealifebase.ca/summary/Echinothrix-diadema.html.
E. diadema is found throughout the Indo Pacific. It tends to dwell under rocks during the day and to be active at night. It typically lives at a shallow depth and down to a maximum of 10 meters. It feeds by grazing on organic material and occasionally adults feed on hard corals. Sea urchins are gonochoric and fertilization occurs externally. Planktonic larvae live in the open waters for several months before settling on the substrate. They use their bottom tube feet to adhere to the ground and eventually metamorphose into young urchins. It is sometimes associated with the shrimp Stegopontonia commensalis.
This sea urchin is currently not assessed by the IUCN Red List of Threatened Species. It appears to be common throughout its range.