The Tao of Fish Colour
By Shane Paterson
Photos by Matt Weedon

QUESTION: WHY ARE CORAL REEF FISHES SO COLOURFUL?

ANSWER: BECAUSE THEY ARE. THE END.

That answer, arguably as true as any other, reflects the fact that we’re really not entirely certain why many coral reef fishes are so incredibly colourful. We once thought we knew but I, for one, am not so sure about that any more. What I am fairly sure about is that I wouldn’t be much of a scientist if I didn’t take this opportunity to address this question and waffle endlessly about what we do know, and what we think we know.

Still, this is One of Those Questions, and it’s such deceptively simple questions that really highlight how little we know about the oceans and their inhabitants. That, in itself, is quite impressive and that lack of basic knowledge is all the more significant in the context of our species’ frequently demonstrated tendency toward environmental atrocity on land and sea. Let me turn the question around, first, and look at why fishes that don’t live on coral reefs are not so colourful.

If you compare coral reef fishes with fishes of temperate or cold seas — let’s say the Gulf of Maine or Canada’s Gulf of St. Lawrence — you’ll notice that there are typically fewer fishes, certainly far fewer species, and that most of the most obvious species are adorned with colours that could be summed up in one word: drab.

Look more closely at the fishes that hide in rocky recesses and thickets of algae, though, and you’ll start to see some rather vivid colours. Actually, some of these well-camouflaged bottom-dwelling fishes of temperate climes easily rival their tropical counterparts in coloration, and you might be hard pressed to pick which is which if presented with close-up pictures of a blenny from cold Canadian waters and one photographed off Grand Turk’s tropic shore. It makes sense, really — in both cases, the fish is doing its best to match its natural background, and cold water algae and encrusting life can be every bit as colourful as anything that you’ll see on a coral reef. There’s another factor, too: use of colour in courtship, threat, and other displays. That’s where things start to get really tricky, so we’ll come back to that once we’re a bit more warmed up to the topic.

Let’s stick a while longer with our imaginary comparative expeditions to a Turks & Caicos reef and a cooler-water site. If you hook a deep-dwelling fish in those cold waters you might be surprised to see that it’s a brilliant red colour. Same thing in the tropics. This is not mere coincidence.

For most fishes, red — derived from compounds (carotenoids) present in food — is an energetically inexpensive pigment relative to other colours in their repertoire that must be produced from scratch, such as black (melanin pigment). Red fills in nicely for black where light levels are low and may even be less visible than black because it contrasts far less obviously against sand and other light-coloured backgrounds in downwelling moon- and starlight. That’s probably why so many deep sea fishes and so many of the nocturnal fishes you’ll see on coral reefs — like the bigeyes, soldierfishes, and squirrelfishes so common around the Turks & Caicos Islands — are red. It basically boils down to them having nabbed the metabolic equivalent of a really good “sale price” on red paint schemes. After all, if you wanted black paint, but red was on sale (and it was going to be viewed in the dark, anyway), why not save a few dollars? For a fish, those few dollars’ worth of energy may make all the difference in determining what side of the food chain it ends up on at day’s (or night’s) end.

So it seems that our query regarding why cool water fishes aren’t as colourful as those that inhabit coral reefs has proven a bit of a trick question. The same applies to lack of obvious colour. In the same way that a cod’s fairly uniform coloration enables it to blend with its cold water habitat, or with water a lot murkier than we usually see around these islands, so does a tuna’s gunmetal sheen or a big Spanish mackerel’s brilliant silver allow it to blend well with the clear, open water beyond the reef. Although the general pattern most definitely is for coral reef fishes to be far more colourful than their temperate counterparts, the comparison nicely illustrates the universal value of coloration as camouflage. Compare animals from any two marine habitats and you’ll probably find more similarities than differences.

That complex and bright coloration can have camouflage value is apparent just from comparing fishes like benthic (bottom-dwelling) scorpionfishes from temperate and tropical waters, that usually look fairly indistinguishable. These ambush predators also have in common tufts and frills of tissue that break up their outlines and the ability to remain patiently motionless for long periods, both of which enhance their predatory success and — in concert with dorsal fin spines loaded with potent venom — reduce their likelihood of being dispatched by predators. Although many scorpionfishes are often brightly coloured, that doesn’t mean that they can’t match their background as effectively as the muddiest-hued flounder.

That pretty much makes sense, but does it explain the really striking colour schemes of some reef fishes? How could some of the more bizarrely coloured and patterned fishes that swim within the world’s seas ever hope to match with their background? Take a look at photographs of some of these fishes and you’d probably never believe that their coloration and markings could ever be effective camouflage. You’d be right, too. You’d also be wrong. Let’s have a look at how this apparent paradox can be.

CONSPICUOUS CAMOUFLAGE

Without getting too bogged down in the spectral characteristics of light and so on, let’s start with the basic fact that light is essentially absorbed by water one colour at a time, with each increase in depth resulting in another colour disappearing from the visible spectrum. As you descend through the water column, red is among the first colours to go, which brings us back to our deepwater and nocturnal friends that use red as a cheap substitute for black. Cut yourself at a decent depth and you’re liable to bleed green or blue. Blue is the last colour to fade out, and by the time it disappears — at least in clear, tropical waters — you are either in a submarine or way too deep. We fade to abyssal black after blue leaves the building.

The upshot of all this is that the spectacularly-patterned red and white firefishes of the Indo-Pacific region, to look at one of the more extreme examples of apparently conspicuous colours, actually appear more or less black-and-white at any appreciable depth. In other words, the striking colours that you’ll see in photos of the beasts become monochromatic under natural light and what at first appears a very conspicuous scheme becomes great disruptive camouflage — a pattern that breaks up the fish’s outline or otherwise makes it appear like anything but what it is.

Of course, you could propose that the firefishes are advertising their venomous spines to all potential predators and that these striking patterns and colours are a warning signal. That’s something that’s been said often enough and the absolute truth is that both ideas may have merit. There’s no shortage of circumstantial evidence that indicates an aposematic (warning) role for bright coloration in some reef fishes, usually with brilliant hues highlighting spines and other defensive structures or advertising extreme toxicity.

This kind of situation, where we have dueling “Just-So” stories that may both comprise part of the truth, is probably one reason why so many scientists appear so distracted and vaguely uneasy about commitment to one single truth or another. (Another reason why many scientists may seem this way is that they’ve been working too much with statistics or with preservative chemicals, either of which can have fairly adverse effect on the brain.) But I digress. Again. And thus I handily prove my own point about scientists.

A little closer to home, we can see the same kind of non-intuitive camouflage principle at work in the spotted drum. A striking study in stark black and white, this spectacular little fish sports a distinctive pattern of spots and other features that really catch the eye. Take another look, though, and perhaps you’ll see that what all of these bars and blotches do quite nicely is break the fish’s outline and body up into a series of apparently-disconnected globs, none of which look particularly like a fish. The same applies to the aptly-named rock beauty, an angelfish conspicuously adorned in resplendent gold and black. Look beyond the first vibrant impression, though, and what you’ve really got is a thin strip of gold that surrounds a big dark space of what could easily be seen as nothingness.

If a television newscaster or meteorologist wears the wrong shade of blue, there’s a real danger that the camera will basically see through him or her to the projected background — this is the “blue screen” effect used so much in movies. Fishes like the rock beauty have mastered a similar feat of optical magic in having large portions of their bodies rendered basically invisible to potential predators. The rock beauty has essentially made the whole middle of its body disappear. Fishes don’t ordinarily have big holes in their middles so the odds are improved that your average predator may not, at least at first glance or while on the run, mentally process the rock beauty as being a potential meal.

HIDE AND GO SEEK
As we saw with the Pacific firefishes, it’s not at all inconceivable that a given colour scheme could be the result of both a need for camouflage and a need for conspicuousness or, at least, that both of these needs could be expressed through the fish’s coloration. I’ve spent years studying great barracuda and can attest that even their basically silvery coloration has many variants. Any single barracuda’s colour may range from brownish to black to green to very pale gray within a few seconds, perhaps developing bars and other patterns throughout. Some of these dynamic colours and patterns make the barracuda more conspicuous and some make it basically disappear; most can do either, depending on what the prevailing background and light conditions are.

All of these colour changes are probably multifunctional. And this leads me to the really tricky bit: it’s in the arena of displays (whether for courtship, territoriality, or some other need) that things can become even more complex. When we get into considering how coloration plays a part in signalling and so on, the multiple functions of colours and patterns assume multiple layers of significance to the extent that trying to unravel what’s what can be cause for some pretty severe headaches.

Let’s forge onward, anyway. Aspirin in hand, if need be.
Many fishes have prodigious camouflage capability. Species like needlefishes and barracudas can even vanish into the transparency of open water, no small accomplishment in itself. Most other open-water fishes, and many that live closer to the bottom, have obvious countershaded colour schemes with darker tops that match the seafloor and lighter undersides that, viewed from below, allow the fish to blend more with downwelling light and the surface and sky above.

But sometimes you don’t want to blend in to the background. Sometimes being a wallflower just isn’t going to get you very far and, for many fishes, courtship is one of those times. Fishes that rely on camouflage to keep them alive long enough to mate tend to solve the dilemma of reconciling such conflicting survival demands by developing temporary colour or markings schemes that get the message — “I’m in the mood for love” — across without ambiguity.

These temporary colours and patterns can be quite dynamic, perhaps involving flashes of colour or markings that fade in and out to reflect the subtleties inherent to certain parts of the courtship process. Just as many birds follow traditional courtship dances, many fishes go through a species- specific series of fixed movements and postures accompanied by a sound and light show that mixes both visual and auditory signals in an all-out effort to woo the chosen female. The displays’ first order of business, establishing that the two are, indeed, of the same species, is quickly followed by the male doing a great deal of posing intended to persuade the girl of his dreams that he is the reef’s primo studmuffin and most eligible bachelor.

Some fishes that are brightly-coloured on a permanent basis may essentially always be demonstrating their genetic fitness, a behaviour also known as “showing off.” Given that maintaining certain pigments might require designating a significant portion of a fish’s metabolic resources to the task, a fish that is inherently healthier than another is likely to have more robust and glorious colours. By contrast, a fish that is particularly susceptible to parasitism or disease is likely to have duller colours because it’s using all of its energy reserves to battle infection or depletion.

In this way, maintenance of gaudy colours can — like the vigour of his courtship dance — be an indication of a male’s ability to accrue food, territory, and other resources and as good a pointer to his genetic superiority as a female’s likely to get. This isn’t a phenomenon unique to colourful fishes. Look at us. Although prevailing attitudes are shifting in some societies, the default tendency is for women to apply rouge and lipstick to mimic a “healthy” blush and for men to accumulate material wealth to indicate their prowess as providers. And I won’t get into the various chest-bulking plastic surgeries now available for both men and women, except to mention that “cheaters” in the impressing-the-mate process are a feature within fish populations, too. There really is nothing new under the sun. Or under the sea, for that matter.

Multifunctionality is the name of the game when you’ve only got so many modes of expression available and a very common pattern, in fishes and in other animals, is for threat displays to include the same elements as do courtship displays. The display’s context is all-important, and a rapid colour change during a courtship ritual may have an entirely different meaning when accompanying a territorial chase, a predatory strike, or as a fright reaction when predation threatens. If you spend much time on a reef you’ll eventually see predators and their prey, or territorial residents and intruders, flashing through colour or shading shifts. Some of these colour and pattern changes may even be byproducts of a physiological “fight-or-flight” reaction. Again, look at humans: we blush when we’re courting, we blush when we’re embarrassed, we blush when we’re furious, and we sometimes blush when we get a good jolt of adrenaline.

SO, WHY ARE REEF FISHES SO COLOURFUL?
Well, not all reef fishes are colourful. But the bottom-most of bottom lines here is that no scientist can really be 100% sure about why any given coral-reef fish tends to be so colourful or to display such striking patterns. That’s not to say that we don’t have some insights that make sense, though, or that we can’t make a fairly convincing and logical explanation that may even approximate the objective truth.

Still, even something so apparently simple as the question posed at the top of this article tells us a lot about how complex and subtle much of nature is and how little of it we truly understand. We don’t like that, of course. The paradoxes that we’ve examined here can be somewhat frustrating in that it’s hard to point to one thing and say: “This . . . this is the answer.” (Actually, you could, but you may be wrong or — at best — only partly right.)

As long as you keep that in mind and realise that you’re unlikely to ever know the Truth, and take the Taoists’ advice that a belief that you know the Truth automatically disqualifies you from knowing the True Truth, you’ll be okay. Theorise ahead, and darn (this article’s suitable for the whole family) the torpedoes.

Let’s face it: the natural world is a world of paradoxes, which is perhaps why those of us who think only in linear terms may never even begin to understand it. But the coral reef is one collection of paradoxes that works quite well according to its own logic. Whatever that may be.

Shane Paterson, who studied great barracuda while a visiting scientist to the School for Field Studies facility on South Caicos, is currently a Postdoctoral Research Scientist at the Bermuda Biological Station for Research.

Matt Weedon, who worked in the Turks & Caicos Islands as a School for Field Studies intern and as photo pro aboard Peter Hughes’ Sea Dancer liveaboard dive vessel, is a professional photographer now based in Texas.