‘…the clearness of the water afforded me one of the most astonishing and beautiful sights I have ever beheld. 
The bottom was absolutely hidden by a continuous series of corals, sponges, actinic, and other marine productions 
of magnificent dimensions, varied forms, and brilliant colours. The depth varied from about twenty to fifty feet, and 
the bottom was very uneven, rocks and chasms and little hills and valleys, offering a variety of stations for the 
growth of these animal forests. In and out among them, moved numbers of blue and red and yellow fishes, spotted 
and banded and striped in the most striking manner, while great orange or rosy transparent medusa floated along 
near the surface. It was a sight to gaze at for hours, and no description can do justice to its surpassing beauty and 
interest. For once, the reality exceeded the most glowing accounts I had ever read of the wonders of a coral sea…’

 Alfred Russel Wallace, The Malay Archipelago, 1869

In the 150 years since naturalist-explorer Alfred Russel Wallace made his observations, successive generations of scientists, explorers, adventurers, scuba divers and tourists have been entranced and captivated by the glory of the oceans’ coral reefs. Exploring a tropical coral reef, with mask and snorkel or, even better, with full dive kit, you discover a complex, dynamic – perhaps even chaotic – universe and one of the most astonishing revelations is the amount of colour you find.

Stony corals covered in a velvety sheen of muted pastel tones. Orange or purple gorgonian sea fans and crimson sea whips sprout and branch upwards. Blue or yellow or brown sponges compete with the coral colonies for reef real estate. Soft corals in an outrageous colour spectrum, between pastel and fluorescent, feed in the current. Emerald-green marine plants thrive in the dappled sunlight. And algae in pinks and greens and reds and blues act as a living cement to amalgamate and unify the diverse multicoloured reef.

Upon closer investigation, you see the tell-tale signs of life and diversity in every conceivable size, shape and form. Clusters of brightly hued Christmas tree worms fan out into the current. Colourful antennae, claws or appendages of shrimps, crabs, and squat lobsters seek food in the water column. Vividly patterned and varicoloured nudibranchs and flatworms ooze across the substrate, their colouration presumed to send a warning message of their unpalatability.

Cephalopods hunt prey while shapeshifting and alternating tones and patterns within their transformative skin. Starfish in red or blue or green amble along the substrate, passing sea cucumbers in variegated patterns of vibrant or mute colouration. But even more mesmerising than this seascape, seemingly sculpted by the Surrealists, is the teeming multitude of hyperactive reef denizens in constant motion: the fish.

There are so many fish of so many species in so many sizes and so many diverse shapes, that they at first overwhelm the observer’s senses. There are roughly 100 reef-fish families and each family contains a range of different species. As they swim, dart, freeze, flit, float, dive, dash, stall, or hide as they go about their business – which is mostly to do with reproduction, feeding, and avoiding being eaten – the viewer is transfixed by the endless variety. It is a mind-boggling explosion of behaviours and forms and, especially, colour.

COMMUNICATION AND CONCEALMENT

Confronted with a mosaic of colours – dazzling in every conceivable shade of the spectrum – the burning question is: ‘Why are coral reef fish so colourful?’

Colouration in animals has been a topic of speculation since initial hypotheses proposed by Charles Darwin and Alfred Russel Wallace in the late 1800s. The consensus for the reason for exuberant animal colouration on land is that it fulfils the functions of communication and/or concealment. According to sexual selection theory, investment in colourful plumage in male birds is a signal of reproductive fitness and serves the role of signalling to attract potential mates, as well as a declaration of dominance to forewarn competitors.

Colour and pattern evolved in some species, such as the poison dart frogs of the South and Central American rainforests, to advertise their unpalatability or toxicity, a strategy known as aposematism. Skunks, wasps and bees also have well-known colour patterns that serve as an effective deterrent most of the time. Mimicry is the strategy of resembling a wholly different species that is not desirable to predators. And camouflage is a form of disguise, enabling an animal to hide in plain sight, utilizing colour and pattern, and in some species protuberances or appendages, to seamlessly blend with the surroundings.

Camouflage and mimicry are also attributes successfully exploited by ambush predators, such as crocodiles, praying mantis, snapping turtles, and the green and the black mamba snakes. Yet, in spite of these noteworthy examples, compared to the brilliantly-hued marine denizens of a tropical coral reef, the terrestrial world comes off as relatively lacklustre, almost muted. And, again, the question of why?

Why should so many coral-reef fish be the brightest possible shades of yellow, orange, red, with some colourations so off-the-charts intense that they are difficult to synthesise organically as paint or ink? Why should so many contrasting colour patterns of black and white, black and yellow, yellow and blue, orange and blue, blue and green, dominate the complexion of so many species of reef fish?

These questions have only recently achieved a degree of understanding, through multi-disciplinary research and the development of new tools for scientific investigation in the past fifty years. To understand why the shallow tropical sea is filled with colour requires an abandonment of both a terrestrial and an anthropocentric view of vision, and to contemplate the world as it is perceived by life forms other than human.

One also has to take into account the properties of light travelling through water. Light is electromagnetic radiation produced by the sun. Human vision perceives a narrow part of the electromagnetic spectrum, called visible light. This is the band of wavelengths that separate into the constituent colours of the rainbow as white light passes through a prism. These wavelengths refract (bend) at slightly different angles, according to their respective frequencies. Human vision perceives the spectrum lying from 390 nanometres (violet) to 700 nanometres (red) – a very small part of the electromagnetic spectrum between infrared and ultraviolet.

Not being visible to humans does not mean that other life forms can’t see other wave lengths; it is now established that ultraviolet is within the visual system of some invertebrates, birds and fish. Frogs, mosquitoes, some snakes and some fish can also perceive the world in infrared light. Contemporary research has discovered that very few animals, land or sea, have colour vision. And underwater, very few creatures have eyesight comparable to that of humans in terms of visual acuity, most being unable to resolve a sharp focus at distance.

The majority of fish obtain sharp focus only over a relatively short range, with the majority of fish unable to achieve it beyond a one-to-three-metre range; and very few species, mostly larger pelagic predators, can see clearly beyond five metres. Colour vision developed in our primate ancestors as a genetic mutation that gave an advantage to those that possessed it. The theory is that being able to differentiate yellow, red and orange fruits and berries at distance from the monochromatic green jungle had a great benefit to our ancestors.

The world underwater does not offer such corollaries of ripening fruit, nor does the transmission of light through the medium of water allow for such refined visual acuity across a broad expanse. On the brightest day imaginable in the clearest visibility that can be found on a reef, light can only travel so far horizontally, perhaps 40 metres. In reality, horizontal visibility is normally far less. This is because light transmission through water is immediately subjected to the twin processes of absorption and scattering. Light intensity declines quickly with depth.

Sunlight enters the water, and immediately its intensity meets interference, so when it reaches three metres’ depth, 60 per cent of that light is absorbed, including most of the red spectrum. As depth further increases, the remaining red, followed by orange and finally yellow are subsumed, so that by ten metres depth, 85 per cent of the light will have been absorbed and only shades of green, blue, indigo (cyan) and violet persist. It becomes a blue-green dominated visual world.

Phytoplankton and dissolved organic matter are suspended in water and, as light collides with the particles and the water molecules themselves, light is absorbed and scatters, which lowers contrast, similar to a terrestrial fog. The twin impacts of absorption and scattering make it difficult to focus on fine detail over any distance underwater.

DIFFERENT EYES

Human eyes have diverged from fish eyes because light is transmitted more easily through molecules of air than through the more dense molecules of water. In human and other terrestrial animals’ eyes, light enters through the cornea, passes through the iris (the diaphragm), which dilates and contracts the pupil (the aperture) to control how much of that light reaches the retina. Through contractions or relaxations of the ciliary musculature surrounding the eye, the shape of the lens is altered and focus is achieved.

Fish eyes have to work with less light intensity and have evolved thick spherical lenses to achieve increased refraction. The fish iris is fixed, and the lens is inflexible, so focus is achieved by muscles moving the lens closer to or further from the retina, giving a sharp-focus telephoto effect without much depth of field and blurred vision at other distances. The eyes of most fish are situated on the sides of their heads, giving fish a wide angle of monovision, with a small overlap of stereoscopic vision where the right and left fields of view overlap in front of the head.

When it comes to colour perception in fish, there is no typical fish vision. Colour vision requires different types of cone cells to detect each of the three primary colours. Most fish, like most mammals, are dichromatic, possessing at least two receptors. Some fish have three types of cone cells, like humans, giving them trichromatic vision, though in some species this may be in conjunction with abilities to see into the ultraviolet spectrum. What is ‘colour blindness’ to humans is likely normal to the majority of fish species.

Many species have the equivalent of our ‘night vision’, where the world is monochromatic in shades of yellow or green. In general, fish eyes are adapted to see contrast and movement more than detail. Any colour vision they may possess is used more to help them find prey in near distances, or to detect predators, potential rivals and potential mates at greater distances against backgrounds of open water or those blending with the illumination or dynamic structure of the reef.

The reef structure has an average reflection, which may be bright or diffuse, depending upon the eye of the beholder and upon the light playing upon the reef due to wave action, the time of day or cloud cover. Scientists investigating Hawaiian reefs using spectrographs found the average coral reef in shallow water to have the same reflectance as the hues of bright yellow found in 30 per cent of reef fish. This indicates that the fish colouration is, when the fish is close to the structure of the reef, a highly effective camouflage rendering the fish difficult to detect for a passing predator.

The reef ’s reflectance originates primarily from the chlorophyll found in the coral and the zooxanthellae living in the coral polyps. It was further observed that, when that same fish ascended into the water column and the background was the blue of the open water, yellow was a contrasting colour useful for advertising its presence and communicating to potential mates or rivals across an expanse of a reef. The function of bright colour on the reef appears to have a dual purpose: it can conceal or reveal. A contrasting colour seen against an empty backdrop is a bold act of communication plus, the  way colour is seen and experienced is wholly dependent upon sensory ability and context.

Vision is relative to the observer… researchers have now established that the bright colour can also be a form of camouflage against the dynamic background of a reef structure. The hues of yellow, red and orange turn out to be colourful only to human eyes and are to most fish likely to appear drab. Indeed, to blend into the reef that is itself so bold and highly contrasting, it is only by standing out – to human eyes – that an animal disappears within its own world, one populated by animals sharing the same visual system and set of optical perceptions.

The way colour is seen and experienced is wholly dependent upon sensory ability and context. Vision is relative to the observer and a non-human, non-primate life forms such as the majority of the creatures on this Earth – and within the sea – have a very different sensory experience of vision and colour. But for a human visitor into the ocean, beauty – and colour – are truly in the eye of the beholder, nowhere more evident than on a very fishy, healthy, coral reef.

Ocean Colour Scenes…

OUT OF THE BLUE

In the water column between the reef and the surface, and just off into the blue open ocean adjacent to the reef, are the varieties of silvery and/or bluish fish, as innumerable as the stars in a bright night sky. Mackerel, mullet, herring, shad, sardines, barracuda… Blue-hued fish are an excellent match to the background light radiance of open water over drop-offs and when viewed from above. A large percentage of these fish display a camouflage defined as countershading, which consists of a darker pigmented dorsal surface and light or white ventral surface; it is believed the purpose is to make the animal blend in with the background light and disappear from view for a predator looking from the top down or bottom up.

YELLOW SUBMARINE

The colour yellow in all its tonal variants is the most common found on the reef, with up to 30 per cent of fish adopting the hue. Species include rabbitfish, snapper, surgeonfish, gobies, blennies, wrasses, trumpetfish, pufferfish, filefish and boxfish. It is often combined with blue patterning. Yellow-hued fish are well-matched to the average reef colour and with increased depth and further filtering out of the yellow spectrum wavelengths, the differences between average reef colour and the yellow-hued fish become increasingly difficult to detect. There is the added benefit for when you want to be noticed, as yellow stands out strongly against the backdrop of the blue of open water.

GRAZING GREENS & BLUES

In the reef shallows, meandering just above the substrate, slender wrasses in colouration of greens and blues zig-zag hither and yon as do larger and stockier parrotfish in shaded and patterned greens, blues and purples; the wrasses preoccupied with foraging for tiny invertebrates; the parrotfish grazing upon algae and coral polyps. Green-hued fish match the interplay of light in the shallow reef substrates and algae-prevalent environs of the reef-tops they occupy. The multi-patterned green, blue/ red, and blue/deep-red hue combinations found in parrotfish and wrasses work in concert to reflect the same luminance wavelengths of the average coral providing a similar camouflage to that enjoyed by yellow-coloured fish.

TANGERINE DREAMS & PURPLE HAZE

Atop the coral bommies, a veritable treasure trove of living jewels bedazzles: shoals of tangerine, orange, fuschia, pink and purple anthias swarm, their numbers expanding outwards as they deftly pluck zooplankton from the water column, and as quickly contracting en masse as they retreat from perceived potential predatory threats.

FIGHTING COLOURS

Red-patterned hawkfish flit between the coral branches or sponge roosts of their chosen territories. Small, pugnacious damselfish in blue, ultramarine, sapphire, yellow or black frantically patrol their reef fiefdoms, audaciously chasing off intruders. Their cousins, the anemonefish, resplendent in finery of reds, oranges and yellows, rely upon the symbiotic relationship they have with sea anemones for protection.

BUTTERFLY BALLS & ANGEL DELIGHTS

Butterflyfish in pairs or small groups wind through coral structures, picking at detritus, coral polyps and small prey. They are pancake-shaped and dramatically coloured with a base white or yellow, many displaying an eyespot splotch near the tail, and intricately patterned with stripes and bands in black or orange. They share the reef with angelfish, superficially similar in appearance sharing the laterally compressed shape and elongated body, dramatic in their colouration of blues and yellows, with green and purple patterns. They are frequently larger in size than butterflyfish.

MATCHING COLOURS & JUVENILE DELINQUENTS

Ambush predators such as lizardfish, stonefish and crocodilefish are patterned to blend with the substrate, where they lie motionless. Fellow ambush hunters, the frogfish, are often brilliantly coloured as juveniles but as adults adapt their colour to match nearby reef structures, most often sponges.

ABSTRACT EXPRESSIONS

Some scorpionfish species, particularly the leaf scorpionfish, rhinopias and stonefish, frequently exhibit fantastical, psychedelic colours. Many eel species take on patterns and colourations that defy understanding when you consider they are animals with poor eyesight that spend their lives by day hidden in rocky recesses of the reef and hunt under cover of darkness by scent. Since many predatory fish have poor resolving power at distance, the seemingly psychedelic patterns, pointillist dots and swirls, and multicoloured hues of many reef fish have the effect of perceptual blending at distance, abetting their hide-in-plain- sight strategy.

Written by Douglas David Seifert, DiveMagazine.co.uk