Sunday, April 18, 2004

Fishy shooters
Nutan Shukla

The  archer fish uses water droplets as weapons to shoot at  prey
The archer fish uses water droplets as weapons to shoot at prey

THE archer fishes, found in the mangrove swamps of the India-Pacific oceans, are a real sharp shooters, probably the most prominent example of tool-users among fish. They usually targets unwary insects on overhanging vegetation. Using water droplets as weapons, the archers bombard their unwary prey and knock them down into the water. They are capable of hitting the target with great precision, usually from a distance of 2 feet to 3 feet, sometimes even 10 feet. As tropical insect-controllers, they are clearly in a class of their own.

Found both in brackish and fresh water, these fish have snouts that are tilted slightly upwards to act like a gun barrel for shooting droplets. The fish makes a tube by pressing its tongue against a central groove in the roof of the mouth. Once the barrel is ready, it forces water along the tube by contracting its gill covers suddenly. Their forward-facing eyes, like hawks, help them to accurately judge the distance of the insect.

The most amazing aspect of this performance is that it compensates for surface refraction of water (the bending of light as it enters from one medium to another, in this case from air to water). The fish needs a corrective mechanism for this distortion, for when it fires its watery missiles, its lips are just out of water and eyes below the surface.

Scientists are of the opinion that this technique has to be learnt by each fish, individually. Youngsters learn the art of precision shooting by trial and error. They can be seen spitting at almost anything with little directional sense, but as they grow older, their aim gradually improves. By the time they’re adults, they have become expert ‘marksmen’. This shows that the habit of spitting is inborn in the archers but they master the art through learning.

To demonstrate their firepower, some archers were kept in an aquarium and fed on finely ground raw meat thrown against the glass wall of their tank, above the water-level. The meat, stuck to the vertical surface barely out of reach, soon attracted the shoal’s attention. A few fishes jumped up to snatch the food, leaping as much as 12 inches out of the water but when they failed, the shoal jointly aimed the water droplets. Some used single shots with pauses; others employed machine-gun bursts. In just 15 minutes, one-third of the raw meat had been washed off the glass into the water below.

Sometimes, if the prey is within easy reach, the archers do not even waste time to shoot it down; they just leap out of water and grab it. This way, it doesn’t become somebody else’s prize catch.

This feature was published on April 11, 2004