Tuesday, January 25, 2011
January 25, 2011 : Archerfish
The archerfish (Spinner Fish or Archer Fish) are a family (Toxotidae) of fish known for their habit of preying on land based insects and other small animals by literally shooting them down with water droplets from their specialized mouths. The family is small, consisting of seven species in the genus Toxotes; which typically inhabit brackish waters of estuaries and mangroves, but can also be found in the open ocean as well as far upstream in fresh water in India to the Philippines, Australia, and Polynesia.
Archerfish or Spinnerfish bodies are deep and laterally compressed, with the dorsal fin set far back, and the profile a straight line from dorsal fin to mouth. The mouth is protractile, and the lower jaw juts out. Sizes are generally small,about 5–10 cm but T. chatareus can reach 40 centimetres (16 in).
Archerfish are remarkably accurate in their shooting; adult fish almost always hit the target on the first shot. They can bring down an insect that includes grasshoppers, spiders and butterflies on a branch overhanging the water, 3 m above the water's surface. This is partially due to their good eyesight, but also their ability to compensate for the refraction of light as it passes through the air water interface when aiming for their prey. They typically spit at prey at a mean angle of about 74 degrees from the horizontal, but can still aim accurately when spitting at angles between 45 and 110 degrees.
When an archerfish selects its prey, it rotates its eye so that the image of the prey falls on a particular portion of the eye in the ventral temporal periphery of the retina and its lips just break the surface, squirting a jet of water at its victim. It does this using the narrow groove in the roof of its mouth. It presses its tongue against this groove to form a narrow channel, then contracts its gill covers to force a powerful jet of water through the channel. The resulting jet of water can be up to 2–5 m long, but their accuracy only allows them to shoot insects 1–2 m away depending on body size. The fish can alter the power of the shot for prey of different sizes. If the first shot does not knock the victim into the water, the archerfish will keep trying.
Archerfish will often leap out of the water and grab an insect in their mouth if it happens to be within reach. Individuals typically prefer to remain close to the surface of the water.