Diadema Urchin

The diadema urchin, scientifically known as Echinothrix diadema, belongs to the Diadematidae family and is a species of tropical sea urchin.

Characterized by its long spines, the typical diameter of Echinothrix diadema ranges from 10 to 20 cm (3.9 to 7.9 in). Its internal organs are enclosed in a test, which is covered by a thin dermis and epidermis. The color of this species is generally black or blue-black, with the spines displaying a blue sheen when exposed to light. The spines are closed at the tip, and the anal sac is small and dark.

Echinothrix diadema can be distinguished from Echinothrix calamaris by its non-banded spines, except in juveniles, and its small and inconspicuous anal sac. Dark morphs of E. calamaris show a greenish tinge, unlike the blue sheen observed in the spines of E. diadema. Diadema setosum is another similar species, but it has longer spines and a smaller test. The distinguishing feature of D. setosum is an orange ring around the anal sac.

In 🇫🇯 Fiji, the grazing preferences of Echinothrix diadema have been found to correlate with its favored algae and seagrass species, specifically Codium geppiorum. The grazing activity of this species contributes to echinoid bioerosion.

This particular urchin can be found inhabiting the Indo-Pacific coral reefs, spanning from the 🌊 Red Sea to Hawaii. While it primarily resides in shallow waters between the surface and depths of 10 meters, it can also be found at depths of up to 70 meters.

Echinothrix diadema is predominantly active at night, seeking shelter in crevices, burrows, and under rocks during the day. Females often choose low-lying locations, presumably to offer better protection to their tiny larvae against predators. This species also hosts commensal organisms such as the shrimp Stegopontonia commensalis, while fish from the families Apogonidae (cardinalfish) and Centriscidae (razorfish and relatives), like Saron marmoratus, stay in close proximity for protection.