Summary

Sea urchins of the genus Diadema are one of the most widespread genera of tropical urchins. While there are many similar–looking species of Diadema on the Great Barrier Reef, Diadema savignyi can be identified mainly by the iridescent blue patterns on its test and the blue or black rimmed anal pore. Inhabiting shallow, warm waters on the rocky fore-reef or reef crest, this urchin is usually found wedged in between crevices or in holes to protect itself from predation and excessive wave action. D. savignyi is very sensitive to light and touch, and will point and wiggle its long spines in the direction of any disturbance in an attempt to ward off intruders. Previous research has been done into the light-sensing ability of Diadema species, and has concluded that light sensing pigment cells are present on the test as well as on the spines, pedicellariae and tube feet. A short experimental investigation was performed on D. savignyi to test the ability of the species to respond to light when the test was covered with sediment. Results suggested that the light-sensing cells on the test play a major role in the ability of the species to detect light, as opposed to those on the spines, tube feet or pedicellariae.  As an algae grazer, D. savignyi plays an ecologically important role in maintaining the health of coral reefs, as it, along with other urchin species, prevents the coral reef ecosystem from shifting into an algal-dominated state. The adverse effects of increased sedimentation as a result of human activity should therefore be considered with respect to the health of coral reefs.

A Diadema savignyi found on the reef flat in a crevice between hard corals. The 
iridescent blue patterns on the test and surrounding the anal pore are highly visible.