I got a cheap white pincushion? type urchin today, and I can say without a shadow of a doubt that Zebrida adamsii eat urchins and as far as I can tell, nothing else. I never saw it eat anything I target fed it with...mysis, cyclops, cyclop eeze, prawn eggs, rotifers, etc. But as soon as I put that urchin in the tank, it was love at first sight. The crab is now picking at the urchin's feet I think and putting little bits into his mouth. Guess I'll have to get a couple more urchins. This one's pretty big, about 3 1/2 Inches in diameter.