Virtually all species of fish live their lives underwater. Those that do emerge, usually do so only for short spells and in these cases it is often to escape unforgiving conditions. For example, Clarias catfish escape from pools where the water level is dropping and both oxygen and food are scarce. Despite this, a large number of fish feed on terrestrial animals, especially flying insects, either picking them from the surface or leaping out to catch them -- even shooting them down as they rest on overhanging leaves and branches.
Do fish benefit from the windfall of insects on the water surface?
Billions of insects every year are fatally attracted to rivers and lakes. Often, they may breed there and spend the early part of their lives underwater, but while water provides a nursery, it also claims huge numbers of adults. Should an insect blunder onto the water, the surface tension is often sufficient to overcome their feeble struggles. Huge numbers of flying insects die in this way, but bad news for the flies is good news for the fish. They represent a rich food resource for fish and a number of species specialize in feeding at the water surface. Typically, these species have upturned -- so-called "superior" -- mouths sited towards the top of their heads. In addition, they tend to have fairly flat backs -- ideal for cruising just below the water surface. The African butterflyfish (Pantodon buchholzi) is one such fish, lurking among floating vegetation, waiting to pounce on any insect unfortunate enough to drop onto the water surface.
Do fish ever leave the water to feed?
The incredible four-eyed fish (Anableps onableps) is even more adept at this. It too lives at the water surface, but its uniquely positioned eyes allow it to see both above and below the water line simultaneously. In this way it can even ambush low-flying insects, leaping out of the water to snatch them on the wing. In fact, most surface-feeders are capable of jumping from the water to take prey, but the silver arowana (Osteoglossum bicirrhosum) has developed these skills to a remarkable extent. Large individuals, measuring up to 3 feet (1 m) long can jump considerable distances out of the water to snatch unwary insects and even small birds from overhanging vegetation in their Amazonian home.They accelerate from deeper water, only opening their mouths fully after clearing the surface, thereby reducing drag. The comparatively large mouths of arowanas allow them to take a variety of prey in this way -- there are even claims of small monkeys being on the menu.
How do archerfish capture their prey?
When it comes to solving the problem of feeding on flying insects, the prize for innovation must go to the archerfish, which is found in estuaries and brackish water regions across Asia. Archerfish capture their prey by shooting them down with rapid and precise drops of water. They are able to spit the water to heights of up 4 feet (1.2 m) above the surface, giving them a huge range. The pressure needed to achieve this is generated by the tongue, while grooves in the mouth serve to direct the shot The fish are capable of amazing accuracy, especially over shorter distances.There are stories of some individuals who would shoot and put out their owners' cigarettes if they approached the tank although whether this was for health reasons or because they mistook them for glow-worms is not known!