Porcupinefish are fish belonging to the family Diodontidae (order Tetraodontiformes), also commonly called blowfish and, sometimes, balloonfish and globefish. They are sometimes collectively called pufferfish, not to be confused with the morphologically similar and closely related Tetraodontidae, which are more commonly given this name.
Porcupinefish are medium- to large-sized fish, and are found in shallow temperate and tropical seas worldwide. A few species are found much further out from shore, wherein large schools of thousands of individuals can occur. They are generally slow.
Porcupinefish have the ability to inflate their bodies by swallowing water or air, thereby becoming rounder. This increase in size (almost double vertically) reduces the range of potential predators to those with much bigger mouths. A second defense mechanism is provided by the sharp spines, which radiate outwards when the fish is inflated.
Some species are poisonous, having a tetrodotoxin in their internal organs, such as the ovaries and liver. This neurotoxin is at least 1200 times more potent than cyanide. The poison is produced by several types of bacteria obtained from the fish's diet. As a result of these three defenses, porcupinefish have few predators, although adults are sometimes preyed upon by sharks and killer whales. Juveniles are also preyed on by Lysiosquillina maculata, tuna, and dolphins.
The porcupine fish (as Diodon antennatus) is mentioned in Charles Darwin's famous account of his trip around the world The Voyage of the Beagle. He noted how the fish can swim quite well when inflated, though the altered buoyancy requires them to do so upside down. Darwin also mentioned hearing that a fellow naturalist, Dr. Allen of Forres, had "frequently found a Diodon, floating alive and distended, in the stomach of the shark; and that on several occasions he has known it eat its way, not only through the coats of the stomach, but through the sides of the monster".