A long while back I bumped into neighbours on the beach and they showed me an odd piece of plastic rubbish shaped like a molar that they'd picked up. Except it wasn't plastic — I'd come across one of these before and had spent a while researching, to discover it was the gas bladder from a porcupine fish.
What is a gas bladder? Australian Museum says:
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.
Porcupine fish turn up on Waikawa Beach from time to time, so it’s not surprising to sometimes find their swim bladders, I guess. The bladders are curious, firm things, that definitely appear as a form of plastic. They’re not tiny, either. I didn’t measure this unfortunately, but it’s about the size of a deck of cards.