Fish's 'spitting mystery' solved

25 October 2012 Last updated at 07:50 By Michelle Warwicker BBC Nature

Italian scientists say they have solved the mystery of how archer fish can spit powerful jets of water.

Archer fish shoot water jets with enough force to knock prey into the water from overhanging vegetation.

Research by a team from the University of Milan shows that the fish's forceful strike is formed externally using water dynamics, rather than using the body's internal muscles.

This technique allows the animals to take accurate shots from up to 2m away.

For years scientists studied archer fish (Toxotes jaculatrix), searching for evidence of specialised internal organs that are adapted to this water-pistol-like hunting technique.

But previous studies have ruled out this idea.

The new research, published in the PLoS ONE journal, demonstrated that archer fish "modulate" the water they spit out so that the jet increases in velocity (speed in a given direction) as it travels from the fish's mouth through air.

The team used high-speed video recordings to make their discovery.

Professor Alberto Vailati, a member of the research team from the University of Milan, Italy, described the action as being similar to "when you shoot water... using a water pistol or by squeezing a rubber toy".

Underwater Olympians

It is the accuracy with which archer fish shoot deadly darts of water at prey that earned the species its name.

The jets of water are produced when archer fish press their tongues against a groove in their mouths to form a gun-barrel-like shape, and close their gills to force out a spurt of water.

The water jet travels through the air, increasing in velocity, and forms into a single large drop of water, Prof Vailati explained. The energy gathers at the head of the water jet, for maximum impact when it hits the prey.

The water spurts are up to six times stronger than the fish's muscular power - strong enough to abruptly knock even firmly anchored prey into the water so that the archer fish can devour them.

Other animals, such as chameleons, use a "catapult" technique by storing energy in collagen fibres within their bodies.

These internal structures "act as a kind of spring", allowing chameleons to suddenly shoot their tongues out to capture prey, said Prof Vailati.

The findings by Prof Vailati and his colleagues are the first to suggest that it is external powers of physics rather than biological causes that afford archer fish with their deadly darts.

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Source: http://www.bbc.co.uk/nature/20068607

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