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If you have found a strange balloon-like object washed up on the beach, you may be looking at a fish's gas bladder.
The gas bladder (also called a swim bladder) is a flexible-walled, gas-filled sac located in the dorsal portion of body cavity. It controls the fish's buoyancy and in some species is important for hearing. Most of the gas bladder is not permeable to gases, because it is poorly vascularised (has few blood vessels) and is lined with sheets of guanine crystals.
Spiny Pufferfish gas bladders are not commonly encountered. Only a few examples of Porcupinefish gas bladders have been brought to the Australian Museum for identification during the last twenty years.