A lot of animals in a coral reef are exquisitely specialized for a very particular ecological niche. So often, an animal has evolved especially to live on another animal, usually a sessile or slow-moving animal like a sponge, coral or, as in this case, urchin.
Look at how the tiny shrimp – about 2 cm long – fits in with the spines of the sea urchin I saw in Tulamben, Bali. Its color matches the color of the thinner spines, and the white stripe in the center of the shrimp’s body visually splits the animal in half, with each half about as thick as a bundle of spines. It was quite hard to take the picture – the shrimp moved around quite hectically – and it was quite hard to spot this animal in first place. Realistically, one does not simply come across a well camouflaged crustacean like that by chance. I knew what I was looking for and scanned every urchin I saw carefully. I’m happy that I eventually found this little guy.