This is the shape of the holes in the walls of Reef Balls (tm), note that they are designed to create a vortex (the water swirls as it moves through the hole) which brings nutrients to the hole's inhabitants. This is just five months after deployment and you can see (if you look past the arrow crab) a camel cleaning shrimp, a black sea squirt, a Grey sea squirt, a scallop and several species of sponges in just this one hole.
