A tropical species of sea urchin often found in shallow fringing reefs.

Blue-black urchin - Echinothrix diadema in shallow fringing reef near Port Douglas - Photo by Alex Pawlow

As can be see in the photos, this species lives in shallow waters and can occasionally be exposed on a very low tide.

These urchins are primarily herbivorous and eat alagae and occassionally sea grass. They play an important roll in controlling algae growth and allowing corals to form. They do, however, erode rock surfaces and over time, make themselves "base stations" through this erosion.

They are active feeders at night, however during the day they prefer to anchor themselves in a crevice, or hole.

Its protective spines are often used by small fish and prawns as safe hiding place from larger predators.

When exposed, they may be preyed upon by birds, but underwater, their main predators are starfish, crabs, lobsters and some fish.