Pufferfish

Also known as the globefish, blowfish or swellfish, the pufferfish belongs to the order Tetraodontiformes, and over 100 species are known, divided into two families: the short-nosed puffers (tetradontidae) and the long-nosed puffers (Canthigasterinae). They tend to inhabit warm coastal waters, though several species can live in fresh water. Their diet differs from species to species - some species have teeth that are fused together into a beak, and these tend to feed on small crustaceans, snails and sea urchins, whereas other species with normal teeth can feed on smaller fish. Their most well-known characteristic is a bizarre defense mechanism in which, when threatened, the pufferfish will gulp large quantities of water and swell its body up to twice its normal size. This can also be done as a territorial display, to ward off aggressive males. Pufferfish are highly territorial creatures, reacting very aggressively to incursions on their little patch of the ocean.

Instead of scales, pufferfish have a thick, leathery skin studded with small spines which remain invisible while the fish goes about its normal daily business, but when it expands the spines are forced to stand up from the skin, making the pufferfish look like a football covered with pins. Most pufferfish are only a few inches long, though a couple of species can grow over three foot in length. When they inflate, their swimming speed and manoeverability are drastically reduced, but apparently this is made up for by the intimidation effect, as any predator will think twice about closing its jaws on a big, spherical, spiky thing that doesn't look much like any kind of fish it's familiar with. A three foot long pufferfish inflating its body would probably scare off a medium sized shark.

The pufferfish is actually not swallowing the water, but sucking it into a specially formed sac inside its body, after first sealing off its intestines and stomach with a muscular valve. Their lack of ribs or pelvis means that they can expand enormously, but they have a regulatory system in the muscles of their skin that prevents them from overstretching. Their skin itself is very tough, and can withstand great pressure from the jaws of predators.

As if this wasn't enough, many species of pufferfish have toxic flesh, impregnated with a poison known as tetraodontoxin which is possessed by many sea urchins and crustaceans. In Japan, pufferfish are known as Fugu, and they are eaten as a delicacy in spite of their poison - in fact, the poison itself is one of the attractions. Fugu must be prepared in exactly the right way by a skilled chef in order to remove the poison, after which the fish is said to taste delicious and even to have a slight narcotic effect. However, many deaths are caused every year by incorrectly prepared Fugu. The toxin, which is 1250 times more powerful than cyanide, acts on the nervous system, blocking nerve transmissions and causing muscle paralysis. Death comes by respiratory arrest. In highly diluted form, pufferfish toxin is used as a treatment for arthritis and rheumatism. A common Japanese saying goes:

Those who eat fugu soup are stupid.
But those who don't eat fugu soup are also stupid.

Finally, the pufferfish species Fugu rubripes played an important role in the mapping of the human genome. It has the smallest genome of any vertebrate, one eighth the size of that of a human, and yet there are significant structural similarities. In fact, the pufferfish has roughly the same number of genes as a human, and only lacks what is known as 'junk DNA', the filler of the human genome that according to current scientific thought, seems to play no important role in determining our makeup. Scientists have been able to make advances in figuring out the different functions of the human genome by comparing it to the much simpler pufferfish genome.