### Identification The Banded Sea Urchin (*Echinothrix calamaris*) is a gothic masterpiece of the reef. Reaching a diameter of about 20 cm (including spines), it is easily recognized by its dual-layered defenses. It possesses two distinct types of spines: long, hollow primary spines that are often strikingly banded in black and white (though they can be solid bronze or charcoal), and shorter, needle-like secondary spines that fill the gaps.
To distinguish it from the similar *Diadema* genus, look for the "fuzzy" appearance created by those dense secondary spines. The most reliable field mark, however, is the prominent, bulbous anal sac at the center of the dorsal surface. In *E. calamaris*, this sac is often beautifully patterned with iridescent blue or green spots, looking like a tiny, glowing disco ball.
### Habitat & Range This species is a staple of the tropical Indo-Pacific, ranging from the Red Sea and East Africa to the shores of Hawaii. It thrives in shallow coral reefs, lagoons, and rocky subtidal zones, typically at depths of 1 to 70 meters. You’ll find them tucked into crevices or sheltered under plate corals where the surge is minimal.
### Behaviour & Diet By day, the Banded Sea Urchin is a shy recluse, wedging itself into rockwork so only its spines are visible. At night, it becomes an industrious grazer. It "walks" across the substrate using its tube feet and spines to forage for its primary diet: macroalgae and biofilm. Using a complex five-toothed apparatus known as "Aristotle’s Lantern," it scrapes algae off rocks, acting as a vital reef janitor that prevents coral from being smothered.
Be wary: those spines are brittle and venomous. If brushed, they break off in the skin, delivering a painful sting. Despite this, they often host "hitchhikers" like the Coleman shrimp or tiny cardinalfish, which seek refuge within the spine thicket.
### Fascinating Fact That shimmering, iridescent "eye" in the center of the urchin isn't an eye at all—it’s a highly decorative rectum. The anal sac is inflated with water and adorned with light-reflecting cells (iridophores) to mimic a large, intimidating eye, successfully tricking predators into thinking the urchin is watching them!