The Tribune - Spectrum

Sunday, October 15, 2000

The world of sea-urchins
By Nutan Shukla

THERE are many animals who use sense of touch to find food. Under the sea, starfish and sea-urchins creep about on the sea floor and use touch, together with a sense of smell, to find and to examine a prospective meal. At the bottom of the deep sea, bright red prawns with exceptionally long feelers and deep sea fish with long, under-chin barbels, many times the length of the body, are thought to use these structures, not only to find their way about like a blind man with a stick, but also to make contact with food.

Sea-urchins use touch to find their mealAlso on the calm water-surface might be water-striders (pond skaters), back swimmers, whirligig beetles and clawed toads. All these creatures are in close contact with the surface film and are able to locate prey, trapped in the surface tension, by the tell-tale ripples that emanate from their struggling bodies.

Sense of sight has no meaning for the animals living in dark caves. It is inappropriate to the extent that many of these creatures have lost their eyes altogether, and can range from blind crickets and cave-fish to blind salamanders. Some of the cave-dwelling creatures depend on smell, whereas many of them have adaptations of parts of the body to afford a better sense of touch. Cave crickets, who are voracious eaters, and earwigs have long antennae, as have poisonous, spindly-legged centipedes and giant, six inches-across, red-eyed spiders.

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Surface-living cousins of these cave-dwelling spiders are also touch experts. Waiting for the prey, garden spider sits at the side of its circular web with its one leg on a tension line. If the vibrations are too great, the spider will ignore the signal because they could be either created by large animal or wind. They could also be due to a large insect which will work itself free and fall out The spider knows if the insect caught in the web is small its struggling legs will create the just right kind of vibrations and the spider will dash out and immobilise the prey with a single, poisonous bite.

Many members of the bird world too rely on sense of touch to find food. For instance, debbling ducks and probing plovers use touch to find prey. The number of touch receptors or corpuscles in the end of the bill of these birds exceeds even the nerve endings in the human finger-tip. Using this sense, most waders can blindly explore sand and mud, and avocets are able to sweep ahead of them, snapping up any tiny aquatic creature it might touch. Spoonbills work with curious-shaped bills in the same way.

Woodpeckers have touch-sensitive cells at the tip of the tongue. With these, they can explore the tunnels of wood-boring insects and insect larvae. Once located, the juicy morsel is hooked out by the bird’s barbed and sticky tongue.

The most extraordinary reliance on touch is, perhaps, shown by the skimmer. This remarkable, tern-like bird seems to have its bill built upside down: the long mandible is at the bottom and the shorter one at the top. Seen head-on, it is very thin and is built to offer minimum resistance. The bird feeds by flying up and down with its head very close to the water. The bottom mandible cuts through the surface, hence the name ‘skimmer’, until it makes contact with a fish. Automatically, the bill snaps shut and the head drops down, trapping the fish sideways between the scissors of the bill; it is then lifted from the water.

If an object, like a rock, offers slightly more resistance, then the head bends round and special muscles in the neck lessen the shock. Using this method of fishing, the skimmer is only able to feed on lagoons, lakes and at the sides of wide, slow-moving rivers. It is restricted to times when the water is mirror-clam, often at dawn, dusk and through the night.

It takes advantage of the fact that it is about when there is no competition from other birds.

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