'Purple Forest' from MORPHOLOGIC on Vimeo.
In this video titled Purple Forest, you can see how the decorator crab makes its way in the world. By adding predictable detritus to its wardrobe, and after waiting for the currents to land an appropriate morsel of tasty goodness onto its shell, the unseen crab hooks his prey.
The unsuspecting isopod has no idea that it has landed upon an algae covered beast. Furthermore, it appears that the crab is not aware of the unexpected visitor until the isopod begins to explore its decorated exoskeleton. 50 seconds into the clip the isopod meets its fate with a few swift snatches of the crab's claws. Without missing a beat, the crab continues scavenging amongst the rocks and algae. And life on the reef goes on...
Decorator Crab {Dive Gallery}
The algae that Johnny Cat refers to actually lives within the skeleton of the coral itself. Those are golden-brownish in color.