Sunday, August 18, 2002		Nature

Catfish uses whiskers to get information
Nutan Shukla

THE ability of animals to detect and react to objects with which they come into physical contact is so widespread that it may almost be classed as a fundamental property of living matter. Every animal shows this property in some way, even the simple one-celled amoeba; while among plants it is also found in many species.

In most lower forms of life, response does not rise above the level of a simple reaction. A typical example is the one-celled animal ‘paramecium’, a tiny organism found in ponds. It swims by means of a cilia or tail which it moves convulsively to force its way through the water. When it encounters an obstacle, the animal ‘backtracks’ and approaches again from a slightly different angle. The organism can clearly notice in some way that its path is blocked and take some form of avoidance action.

A marine animal, the sea anemone, not only pulls in its tentacles when they protrude above the level of the tide — a necessary reaction if it is to avoid becoming dehydrated by being left high and dry — but they also seem to be able to distinguish food from other underwater objects. The anemones, who ‘ride’ on the back of hermit crabs, are frequently bumped against rocks by the movement of their ‘chargers’, but in these cases they scarcely bother to contract. They pull in their tentacles actively only when they feel the motion of a small fish or shrimp that brushes against their fronds. Thus they can tell ‘touching’ from ‘being touched’.

This distinguishing ability is even better developed in one particular species of anemone in the Red Sea that works in close conjunction with the common small fish ‘Amphiprion bicinctus’. The fishes drive away the anemones’ enemies, while the anemones will not open out if they do not feel the frequent touches of the fishes passing and repassing over them. In normal circumstances, the anemones make no attempt to sting the fish, which pass through the fronds unharmed. However, if the fish is passively pushed on to the anemone’s tentacles, the anemone can no longer tell it from food and will sting and eat it if it is small enough.

The catfish of Mississippi, like the cat itself, relies heavily for information on the whiskers fringing its mouth. In the case of the catfish, the whiskers droop on to the bed of a muddy river, trailing over it and warning the catfish when it comes into contact with anything unusual. Unlike those of the cat, however, the catfish ‘whiskers’ are not hair. Instead, they are fleshy outgrowths of the face, but they serve the same function.

Shrimps, too, make use of their antennae as probes. The tropical barbershop shrimp keeps its extensions in constant motion, probing the surrounding water with them. Each of its antennae is two or three times its own length, and they clearly provide it with a great deal of information.

Touch is particularly developed where the senses of sight and sound are unable to operate effectively. In the cat, a nocturnal hunter, the face is fringed with sensitive whiskers which provide instant information about the whereabouts of small prey that may have been tracked down into a dark corner. If a mouse touches its whiskers, a cat will react with hair-trigger speed, and is instantly aware of the mouse’s position.