Last year, Harry the Hermit Crab was in need of a new shell. Since he lives at the Atlantis Discovery Area in Legoland Windsor, the caretakers decided to offer him a custom-built Lego shell along side the more traditional assortment. Much to their surprise and delight, he picked the bricks.
Hermit crabs have no shells of their own, but must scavenge discarded ones to protect their soft little bodies. They'll stay in a shell until it becomes too small or too dirty for them, and then they'll move on - but in the case of Harry, his new shell is completely modular, so it can always be expanded as he grows.