Answers Activity 148


If you're snorkelling around a coral reef, you'll see the local marine life in all its carnival colours. But the show clearly isn't just a tourist attraction. For the fish that live on the reef, it's more a matter of life and death. As with any other creature, the survival of a fish species depends on two things
food supplies and breeding success.

Seeing a coral reef in all its glory, you can't help feeling that fish have completely failed to solve this dilemma. The picture, however, only comes into focus when you take the fish's-eye view. For fish, according to Justin Marshall from the Vision, Touch and Hearing Research Centre at the University of Queensland in Brisbane, see things differently.

This means that the carnival looks quite different to the marine life itself. To help him discover exactly how different it looks, Marshall has designed a unique underwater 'spectrophotometer', which analyses the colours of things objectively in terms of their physical reflection. He is also measuring the light available in different micro-habitats.

The general shift towards the blue end of the spectrum in underwater light explains why most nocturnal reef fish, such as the soldierfish, squirrelfish and big-eyes, are mainly red in colour. According to Marshall, some reef fish might see red, in which case they could capitalise on the colour blindness of others and use red markings for private communication. But in most cases, red species are surprisingly inconspicuous.

As any snorkeller will know, lots of reef fish display the sort of colour combinations that suggest camouflage is the last thing on the fish's mind. The bright blues and yellows that are most common, however, are only conspicuous at a certain range. They fade to grey at a distance, because the colours are so close together that they merge.

Wider colour bands will be visible much farther away, of course, but still the fish's-eye view is different from ours. Most recently, Marshall has discovered that fish may see hardly any contrast between the blue of many species, such as tropical angelfish, and the colour of the water around a tranquil reef. More surprisingly, says Marshall, a fish with blue and yellow stripes can be just as well camouflaged, as even this distinct pattern will merge into some backgrounds. When the fish are all together in a shoal, it's hard for a predator to spot where one individual starts and another ends. It's what Marshall calls 'the zebra effect'. If Marshall is correct, then a fish with bold blue and yellow markings can either advertise or hide itself by simply adjusting its behaviour.

In other words, one set of colours can send out very different signals depending on the setting. To complicate things further, most reef fish can vary their colours, whilst it is common for species to change colour from night to day or as they grow older. Colours may even change with a fish's mood -whether it's fighting or fleeing from predators.


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