Sunday, September 5, 2010

How and why pufferfish inflate?

Pufferfish belong to an order of fishes known as Tetraodontidae. These tropical fish include the pufferfish’s closest relatives, species like triggerfish and tripodfish. There are more than 120 species of pufferfish worldwide. Most are found in tropical and subtropical ocean waters, but some species live in brackish and even fresh water. They have long, tapered bodies with bulbous heads. 

The pufferfish we see daily in Tayrona Park is the species Diodon Holocanthus.

The spiny puffer seems relatively normal when relaxed. But when threatened, the puffer undergoes an amazing transformation. Biologists think pufferfish, developed their famous “inflatability” because their slow, somewhat clumsy swimming style makes them vulnerable to predators. In lieu of escape, pufferfish use their highly elastic stomachs and the ability to quickly ingest huge amounts of water (and even air when necessary) in order to increase three times its usual size and become a rigid, near-perfect sphere covered in spiky armor-not a good design for swimming but decidedly discouraging to attackers.

But how they do it?

Pufferfish typically pump themselves up by taking 35 gulps or so in the course of 14 seconds. Each gulp draws in a big load of water thanks to some peculiar anatomic changes in their muscles and bones. Most fish, for example, have shoulder bones that anchor firmly to the back of their head, but in pufferfish the connection is hinged. When a pufferfish opens its mouth, it can therefore rotate its shoulders back and increase the size of its mouth cavity, pulling in even more water. Once a pufferfish has taken in water, its gill slits clamp shut and a powerful valve flips up over the inside of its mouth, acting as a seal. Now when the pufferfish compresses its mouth cavity, the water flows down its esophagus rather than out its gills or mouth.

Scientists of the University of Massachusetts, studied the biomechanical tricks the puffer uses to accomplish its swell feat. 

The stomach expands to nearly a hundred times its original volume, an astonishing increase made possible by the stomach's being pleated, like a skirt. As the puffer fills with water, the fish's spine, already slightly curved, bends into an upside-down U shape, and the intestines, liver and other internal organs become squeezed between the fish's backbone and its rapidly expanding stomach.  Meanwhile, the fish's skin is pushed out, obscuring most of the puffer's features. Only the mouth-a cartoonish orifice containing heavy, crushing plates capable of pinching a human finger to the bone-remains unaffected.

Ilustration by Sally J. Bensusen. American Museum  of Natural History.

The skin of a fully inflated puffer is stretched to one and a half times its resting length. But while the skin's elasticity accounts for the change in the volume of the fish, stiffness and strength are required to turn the creature into a rigid ball. For this the puffer again turns to pleats. The spiny puffer's skin consists of two layers-a thin, elastic outer layer and a fibrous inner layer. This inner layer of skin is pleated and, when extended, quite stiff.

The skin also helps deploy the puffer's armor. The scales of this fish have been modified into slender spikes, each on a tripod-shaped, bony base embedded in the skin. Normally the tripod lies on its side, with the spike flat against the skin, pointing backward. But when the fish puffs up, the stretched skin pulls two of the tripod's legs backward and one leg forward, snapping the spike upright. The three legs provide a secure base that blunts the force of anything pushing against the spike's sharp tip.

As you can see, this transformation is a high-cost process for the fish, so next time you see this fish underwater.. please don`t chase it or try to catch it just to see how it blows up.

Enjoy the marine life with respect and have great dives.

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