On the seventh day of Fishmas, my dear love gave to me… a decorator crab!
Decorator (or dresser) crabs (superfamily Majoidea) consist of a group of several species of crabs known for their ability to use living and non-living materials found on the sea floor as camouflage and protective shields. They will remove anemones, sponges, algae etc. from the sea floor and after chewing them to get them to size they set them on their own shells.
Fun fact: Decorator crabs recycle their living decorations during the molting process! They always remove the anemones, sponges and other decorations from their old shell and use them to decorate their new shell. Waste not, want not!
Check out this great video by the BBC about dresser crabs.