Hermit crabs borrow empty shells for protection and swap them as they grow larger.

The longer version

Soft abdomens leave these crabs exposed, so a borrowed shell serves as portable armour that costs nothing to maintain. A curved tail and modified limbs lock the body deep inside, and the larger claw seals the opening when danger approaches.

Growth, however, means the house must be replaced, and candidates are tapped, turned and measured against the old one before a move is made. Shortages produce a chain of swaps, with several animals waiting in size order until each improves. Where shells run short, alternatives such as tubes, cavities and debris are pressed into service.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.