How a sea urchin’s iridescent shimmer turns physics into reef art
Every now and then, when I’m drifting over the reef, I catch a flash of blue light from a dark crevice and stop to look closer. It’s not a trick of my mask – it’s one of those long-spined sea urchins, Diadema savignyi, quietly minding its own business but looking rather fabulous while doing it.
They’re easy to spot once you know what to look for: long, fine black spines and five shimmering blue V-shapes radiating across the body. In the right light, those blue lines seem to glow, almost like a neon sign. And sometimes it’s as if the whole animal is incandescent. But it’s not true light production – no chemistry involved – just iridescence. That’s different from bioluminescence, where an animal actually produces light through a chemical reaction. With irridescence, the effect comes from cells called iridophores that bend and scatter light in clever ways. Inside, they hold stacks of reflective plates that can throw back iridescent greens, blues, silvers, and golds – a bit of optical magic that the reef does so well. Diadema savignyi tends to favour the blues, sometimes with a hint of green.
You quickly learn not to get too close. Those spines are sharp and brittle; if you block their light, they’ll swivel and aim them straight at you. It’s a little unnerving, as if they’re saying, ‘I see you – and I don’t like where you’re standing’.
There’s something mesmerising about them – beautiful yet still slightly menacing.
A close up of the blue lines and the anus of the Diadema savignyi, Norfolk Island
A Diadema savignyi mid poop!
And, yes, I do have a photo of one mid-poop. Urchins are methodical about it, sending out a neat line of pellets from the top – technically the anus, which is right in the centre. Everything on the reef has its place and purpose, and even that small act becomes part of the grand recycling show that keeps the lagoon alive.