Porcupinefishes are slow-swimming, medium-sized fishes that are found mostly in shallow temperate and tropical marine waters. The teeth in both jaws are fused into a parrot-like "beak". The family name Diodontidae literally means 'two teeth'. Diodontids use these fused teeth and plates on the roof of the mouth to crush hard-bodied prey such as molluscs and sea urchins.
Porcupinefishes are covered in spines. They can inflate their bodies into a ball-shape by swallowing water. The inflation of Porcupinefishes is a defence mechanism, and as such there is no limit to the number of times an individual can inflate (and deflate). If the fish inflates at the surface it is likely to ingest air, which can be difficult to expel. This can lead to death of the fish because it floats and cannot leave the surface. However, if the fish inflates under water, it ingests only water and has no problem deflating once the danger has passed.