### Identification The Long-spined Sea Urchin (*Diadema setosum*) is the "pincushion" of the Indo-Pacific. It is defined by its incredibly long, needle-thin black spines—sometimes reaching 30cm—which radiate from a small, dark spherical body (the test). While it looks similar to its cousin *D. savignyi*, the key field marks for *D. setosum* are five brilliant white spots on its back and a vivid, unmistakable orange-to-red ring surrounding its central anal cone. If you see that "bullseye," you’ve found *setosum*.
### Habitat & Range This species is a staple of the Indo-Pacific, spanning from the Red Sea and East Africa to the shores of Australia and Japan. They are shallow-water specialists, typically found from the intertidal zone down to 30 meters. Look for them wedged into the nooks of coral reefs, sheltering under rocky ledges, or congregating in large "herds" on sandy lagoon floors where seagrass is abundant.
### Behaviour & Diet By day, *D. setosum* is a shy recluse, huddling in crevices to avoid triggerfish. At night, the reef comes alive as they emerge to forage. They are the reef’s essential "mowers," using a complex five-toothed mouth called Aristotle’s lantern to scrape algae and detritus from rocks. This grazing is vital; without them, algae would quickly smother the coral. If you pass your hand over one, you’ll notice the spines pivot instantly toward your shadow—a defensive "shadow reflex" triggered by light-sensitive cells across their body.
### Fascinating Fact While that striking orange ring looks like a glowing eye watching you, it is actually the urchin's anus! Even more incredible: despite having no actual eyes, *D. setosum* "sees" with its entire body. Its skin is so packed with photoreceptors that the animal functions as one giant, walking compound eye, allowing it to detect threats from any direction.