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, such as hot springs, and extreme pressure, such as deep underwater. They can also survive the high levels of radiation and the vacuum of space.
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