How fish colours work in the battle for survival

Today the Old Wife is only found in southern Australia. The markings on the single living species are almost identical to those on its predecessor 50 million years ago. (Photo Joao Paulo Krajewski)

The spectacular colours of coral reef fishes represent a ‘language’ that has been around for at least 50 million years – to which humans have lost the key.

Colour in fish is most probably a highly developed form of communication, some of which we can interpret, but much of which remains cryptic says Professor David Bellwood of the ARC Centre of Excellence for Coral Reef Studies and James Cook University.

“Colour is one of the languages fish use to communicate. By brightening or darkening their hues they can display aggression or fear, identify mates, advertise status, hide from danger and probably many other things we can only guess at,” he says.

Sometimes colour is camouflage, so the fish can hide from or deceive predators, and this is more easily interpreted and tested by science. But there is no easy explanation for the rainbow displays of the most spectacularly brilliant reef fish, he says.

Superbly preserved fossil specimens of reef fish from Monte Bolca, Italy, studied by Prof. Bellwood reveal that even as far back as 50 million years ago fish were spotted, striped and probably highly coloured also. “If it has lasted this long, bright colouring is clearly a highly successful trait. The odd thing is that mammals, including humans, appear to have lost it.'

Faint but still visible. This 50 million year old damselfish (Palaeopomacentrus orphae) from the exceptional fossil deposits of Monte Bolca in Northern Italy still bears the faint mark of an ocellus on the dorsal fin. (Photo David R Bellwood)
“We are ‘fish’. We are descended from them and we have inherited many of their biological attributes like a brain, eyes, a backbone, paired limbs and so on. But one thing we and the other mammals seem to have lost over evolutionary time is the fishes’ spectacular array of skin colours - and the ability to modify them or to communicate with them.”

Science has been able to figure out the reasons for some patterns observed on coral reef fish. A black stripe across the eye helps to conceal the position of the organ from predators who may then find it hard to tell which end of the fish to attack. Many fish further confuse them by adding a large dark eye-spot towards the end of the tail, presumably so the predator heads in one direction while the fish escapes in the other. These eye spots (ocelli) and other patterns also appear on fish fossils millions of years old and are clearly a long-standing evasion device.

“One parrot fish can display a large black and white eye to appear on its tail when a predator is nearby,” Dave adds “The function of these sorts of patterns is easy to guess but can be challenging to test scientifically.”

A juvenile damselfish (Pomacentrus amboinensis) displays a prominent ocellus on its dorsal fin. This ‘false eye’ may confuse predators and draw attention away from the real eye. (Photo David R Bellwood)
“We know the colours that contrast most strongly under water are blue and yellow, and that blue and yellow fish tend to have more yellow stripes towards the back end. This may be a way of drawing the predator’s eye in the wrong direction. It says “Look at me – but look at the wrong bit!”

Remarkably, too, many fish also ‘glow’ in the ultra-violet. In some cases this colour, invisible to humans but seen by other fish, may signal aggression and experiments have shown that when the UV is muted artificially, other fish react less strongly.

In such ways, he says, fish colour appears to communicate both information – useful for finding mates and advertising status in the pecking order - and disinformation, intended to mislead predators and competitors, and this ambiguity makes it hard to interpret.

In the sad case of gobies, who lose 8 per cent of their brothers and sisters to predators every day, sitting on the bottom and looking inconspicuous is the safest survival strategy – and their muted colours reflect it.

But why many reef fish display brilliant blues, crimsons, purples and oranges, in striped, spotted, variegated and harlequin patterns - and are apparently doing everything possible to catch the eye remains a scientific puzzle. The language of fish colour remains to be interpreted.

“We do know that the most brightly coloured fish are associated with very clear waters – round coral reefs and in the Great Lakes of Africa, for example. But the meaning their colours convey is still not entirely clear, despite many decades of scientific study.

Mammals probably lost the ability to grow and change bright skin colour when they became furred, Prof. Bellwood speculates. “However it is pleasant to think that that humans have rediscovered the ancient art of pattern and hue through our fashion and jewellery industries – and that in using these things to attract mates, advertise status, communicate aggression or evade predators we may still be responding to something we learned from our ancient fishy lineage - the language of colour.”

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The copperband butterflyfish (Chelmon rostratus). Fishes use colour to both inform and mislead. With bands and an ocellus this fish uses two of the commonest ‘tricks’ to confuse predators. (Photo Joao Paulo Krajewski).

Windfall, Southern Wind 94 - next stop Grenada - RORC Transatlantic Race 2014.
Puerto Calero/James Mitchell