The Cost Of Inflation

The inflation of the Porcupine Pufferfish is possibly the most obvious and well-known characteristic of this species. Beautifully depicted in the Disney film 'Finding Nemo', 'Bloat', the Porcupine puffer, was often seen instantly inflating when he felt threatened. 

In truth, this ability to inflate is far less rapid and used solely as a defence measure. Upon encountering a hungry predator, a Porcupine puffer will rapidly swallow mouthfuls of water (or air when removed from the water) that are pumped into an extremely expandable stomach. The resulting increase in size (almost double in vertical measurement) makes them harder to swallow.

Porcupine Pufferfish belong in the family Diodontidae, (order Tetraodontiformes) and are also commonly called 'blowfish' (as well as 'balloonfish' and 'globefish'). Porcupine puffers are medium to large-sized fish that are found in shallow temperate and tropical seas worldwide. Species found much further out from shore, form large shoals of thousands of individuals.

Not surprisingly, to achieve this drastic change in shape there has been a radical re-arrangement of the internal skeleton, and their mouths (buccal cavity) have specifically evolved to function in the pumping mechanism. However, the evolutionary origin may not be particularly that mysterious, because two functionally similar behaviours that involve buccal compression are common in puffers. All exhibit a 'coughing' behaviour to spit out unwanted objects and most are good at 'blowing' strong jets of water out of their mouths (e.g. to uncover or manipulate food). Coughing, blowing and inflation are all based on a similar cycle of expanding and compressing the buccal cavity in order to draw water into the mouth and force it out again. The main difference is that inflation involves the pumping of water into the stomach and not out of the mouth.

As such, there is no limit to the number of times an individual can inflate. The only real issue comes if the fish inflates at the water's surface and ingests air. Air can be difficult to expel and can lead to the death of the fish, as the fish floats and cannot leave the surface. However, if the fish inflates underwater, it only ingests water and has no problem deflating once the danger has passed. In either case, it is never a good practice to encourage the inflation of a pufferfish under any circumstances! 

The Porcupine puffer does posses a second defence mechanism and this is provided by their sharp spines, which radiate outwards when the fish is inflated. They are also poisonous, having a tetrodotoxin (a potent neurotoxin with no known antidote) in their internal organs (such as the ovaries and liver). This neurotoxin is at least 1200 times more potent than cyanide and is produced by several types of bacteria that are obtained via the fish's diet. As a result of these three defences, Porcupine puffers have very few predators, although they are sometimes victim to sharks, tuna and killer whales. 

And finally, a limited degree of inflation can also found in a few other groups of fish. For example, one species of Catshark, commonly known as the 'Swellshark' (Cephaloscyllium ventriosum), swallow water or air to inflate themselves and defend against predators. When the Swellshark feels threatened, it can bend its body, seize its tail with its teeth and then suck in water, resulting in almost doubling in size! Impressive stuff!