About Echinothrix diadema (Linnaeus, 1758)
Echinothrix diadema is a long-spined urchin. When including its spines, its typical diameter ranges from 10 to 20 cm (3.9 to 7.9 inches). Its internal organs are enclosed within a hard test, which is covered by a thin dermis and epidermis. This species is generally black or blue-black, and is always dark-colored; its spines display a blue sheen when exposed to light. The spines are closed at the tip, and its anal sac is small and dark. It can be distinguished from the related species Echinothrix calamaris by several features: E. diadema does not have banded spines except when it is a juvenile, its anal sac is small and barely visible, and its spines have a blue sheen, while dark morphs of E. calamaris have a greenish tinge to their coloration. Another similar species is Diadema setosum, which has longer spines and a smaller test; the key distinguishing feature of D. setosum is an orange ring surrounding its anal sac. This species occurs across all Indo-Pacific coral reefs, ranging from the Red Sea to Hawaii. It is a shallow-water species that most commonly lives between the sea surface and 10 meters deep, though it can occasionally be found as deep as 70 meters.
