Sunday, July 8, 2001		Nature

 A colour for every occasion
Nutan Shukla

CUTTLEFISH and squids are known for the fastest changes of colour in the entire animal kingdom. Now the question is why do they do this? Their capability to change colour of their body serves both as a defensive as well as hunting strategy.

Cuttlefish, which are very fond of crabs and shrimps, approach their prey very slowly, meanwhile blending themselves to their surroundings, but when they are almost near they demonstrate a remarkable play of colours which mesmerises the crustacean. As the predator nears the prey it strikes a pose, with the two of its eight short arms raised. A fantastic, changing kaleidoscope of colours passes down its body. Moving patterns of zebra stripes, longitudinal and transverse bars, spots and blotches confuses the party and this is the moment when cuttlefish strikes with its two tentacles that shoot out from the circle of arms. These tentacles, after catching the prey, drag it towards the bird-like beak at the centre of the arms. This tears into the shrimp’s exoskeleton and extracts flesh.

How does a cuttlefish change its colour? In its skin there are thousands of elastic-walled colour cells that can be filled or drained of pigments (coloured substances occurring naturally in plant or animal tissues) in an instant. Cuttlefish, squid and octopuses belong to the group of cephalopod (literally meaning ‘head-foot’ in other words they are a class of molluscs that have tentacles on their head) who have large brain and eyes resembling that of humans.

The body of the common cuttlefish is about 18 inches long and the tentacles are about half- the length of its body. Imagine the snatching power of a giant squid with a body 20ft long and tentacles extending up to 35 ft. This specimen had washed ashore alive at Thimble Tickle in 1878. It was the largest, authenticated giant squid known to science. Its eyes were 18 inches across and the largest suckers on the ends of the tentacles were 4 inches in diameter. It was truly a giant amongst giants.

Cephalopod eyes are the most developed ones among the invertebrates. However, they have evolved independently, but they are very similar to human eyes, although there is one major difference. Unlike our eyes that change the shape of the lens to achieve fine focusing, eyes of these creatures do this by moving the lens backwards and forwards, more like the focussing mechanism of a slide projector.

Among most-living animals, the cornea, a curved, transparent outer covering at the front of the eye, assists in bending incoming rays of light so that they converge on the lens. The greater the curvature of the cornea, the more the ray of light is bent. The cornea developed to compensate for the bending of light as it passes from the air outside the eye to the liquid on the inside. Once the cornea has more or less focused the light entering the eye, the lens then fine-tunes the image projected onto the retina, a screen of light-sensitive cells at the back of the eye.

Marine creatures, however, have no need to compensate for the bending of light as it passes from air into liquid. In octopus and the squid, the cornea contributes little to focusing, which is done almost entirely by the lens.

Experiments have proved that an octopus is capable of recognising and remembering different sizes and shapes. For example, it can tell the difference between vertical and horizontal rectangles. A sedentary predator, the octopus ambushes crabs and other marine animals from the safety of a crevice in a rock.

Cephalopods are also among the very few marine creatures who perform distraction display to save themselves from predators. The cuttlefish, when attacked, releases a large blob of ink into the water. Originally it was believed that this was a simple ‘smoke screen’, a kind of instant fog in which the cuttlefish became hidden and could escape. But careful observation suggests that this is not the case.