What is a gas bladder?

This image shows a gas bladder found by P. Merrick on Avoca Beach in January 2005. It is the gas bladder from a  Porcupinefish (often called Spiny Pufferfish or Burrfishes).

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