The first time I bumped into a Decorator crab I didn't realise what I was looking at. We were diving the Lembeh Strait in North Sulawesi and I saw a sea urchin moving impossibly quickly on the sand. It was only when I swam closer that I realised the urchin was perched on a crab.
These crabs have spikes on their eco-skeletons which enable them to attach sponges, seaweeds, algae - and in the case of the crabs I saw in Lembeh - sea urchins, onto their backs as a form of disguise. I've even seen a decorator crab with an unwieldy mess of dead coral on its back, but alas I haven't been able to unearth that particular photo from my files.
I think there's something rather endearing about these little critters and I'm looking forward to a wealth of diving surprises in the Maldives next week.
As a result of a question put to me (see the comments) I did further research and discovered that my 'decorator crabs' are in fact 'urchin crabs' which resemble decorator crabs. The big difference being: urchin crabs carry sea urchins and decorator crabs carry sponges, hydroids, bits of algae and other inert objects on their shells.
This is a classic case of believing what a dive leader told me without thoroughly checking the facts - so slapped wrist to me - and big thanks to AW for asking the questions that prompted my research.
In answer to AW's question about the advantages: the sea urchins benefit from being carted around by the crabs because they are transported to new feeding grounds.