Water bears (or tardigrades) are tiny invertebrates that live in aquatic and semi-aquatic habitats such as lichen and damp moss. They require water to obtain oxygen by gas exchange.
In dry conditions, they can enter a cryptobiotic state of desiccation, known as a tun, to survive. In this state, water bears can survive for up to a decade.
Water bears are found throughout the world, including regions of extreme temperature and pressure, like hot springs and deep underwater. They can even survive high levels of radiation and the vacuum of space.