Blue Tuxedo

[Delphinus]

I'm pretty certain its done on purpose. They're sometimes called "decorator" urchins for this behaviour. I suspect it's for camoflage reasons. There have been times I've been looking directly at my urchin and not realizing he's right there thanks to all the rubble they're carrying.

If you have small frags, particularly zoo frags, be sure to glue or tie them down to something heavy otherwise they'll go walkabout on you. Many a reefer gets frustrated realizing their zoos are now buried in the rock somewhere ...