A healthy coral reef can be compared to the grandest stage production that Broadway or even Bollywood has ever seen – or will ever see. The biodiversity of life that can be found crammed onto those porous slabs of ancient calcium carbonate is unlike any natural system on land. An African savanna, in the peak season of activity, is a sight to behold, but the variety of life forms to be spotted there pales in comparison to any healthy reef in the Indo-Pacific.
And the drama! Every creature is waging a war of survival and just how they throw down their dice can make all the difference between success and failure. The rules of engagement are pretty simple: eat; avoid being eaten; and pass on your genes. But the ensuing game of tactics that follows the fulfillment of these life requirements is what makes the study of marine biology so endlessly fascinating. Television soap-opera fans eat your heart out! And no need for binoculars or microscopes, because everything is right there in front of your face – or is it?
Do you ever get that feeling that something’s watching you? The term ‘crypsis’ refers to the ability of an organism to avoid detection. Very few animals need not worry about being discovered by a larger predator. Even great white sharks have to concern themselves with the giant killer dolphin, the orca. But just as animals may be tasty prey to others, they too may be looking for a meaty meal. Being cunningly camouflaged works in both directions: to kill and to avoid being killed.
As one’s experience observing reef dynamics develops, the ability to detect camouflaged animals improves. Quite simply, nothing beats knowing what to look for. If, for example, one were to go diving in South Australia’s kelp forests without being aware that leafy and weedy sea dragons were living amongst thes waying green fronds, it’s extremely likely that one would swim right past them. Understanding preferred habitats and species-specific behaviour greatly help stopick out the plethora of hidden treasures.
Marine animals use many tactics to avoid attracting unwanted attention and these largely depend on where they live. An open-ocean animal may use transparency, reflection or counter shading.
An organism will achieve transparency if it neither absorbs nor scatters light and this all depends upon the relative refractive indices of both the animal’s biological tissue and that of the surrounding medium. The larval phase of many reef animals and fish are masters of transparent disguises. And this really does represent the most difficult form of crypsis, since there are no surfaces to match or hide behind.
The simple, gelatinous bodies of sea jellies, ctenophores, siphonophores and salps are barely detectable as they float silently through the water column. We Australians know of the devilish invisibility of some of our deadliest killers: the box and irukandji jellies. But even for the most effective of see-through camouflage, there are predators that have evolved to detect the most vague of outlines, especially in the floodlit, shallower waters where remaining truly invisible is almost impossible.
Mantis shrimps are formidable predators that boast the most proficient and sophisticated visual capabilities known in the animal kingdom. Being able to perceive different kinds of polarised light, these animals are ruthless killers of transparent prey, even when they themselves are just juveniles in the planktonic larval phase. Just as the hunted evolve a crafty new way of remaining hidden from sight, so the predator steps up its game …
Developing whole-body transparency is highly restrictive on the complex development of an animal so, as a popular imitation and physiological compromise, the trickery offered by reflective camouflage can be observed in many open-ocean fishes that are silver in appearance. Recent studies have revealed that the skin of sardines and herring are not only highly reflective, but also polarisation-neutral, which essentially means that it acts as a mirror perfectly bouncing back any light that hits it, causing it to disappear from sight. Now that’s great camo!
A great white silently patrols the seafloor, its powerful tail sweeping from side to side. Something as massive and seemingly conspicuous as a great white is also employing a form of crypsis: counter shading. When looking at this leviathan from the side, a distinct line is seen separating the dark upper parts from the paler ventral surface. This method of camouflage is used by many species of fish as it allows them to blend into their background, irrespective of whether predator or prey is observing them from above or below. Looking at them from above, their darker dorsal side matches the darkness of the inky depths, whereas watching the shark from below, their white underbelly blends in with the brightness of the sea’s surface. It may seem simple, but it gives that extra advantage, which may just make all the difference between life and death.
There are countless ways to achieve camouflage, but two broad distinctions can be made: whether the chosen tactics are passive, or active. Animals that employ passive crypsis allow their bodies to do the work. They are, by their very nature, camouflaged.
There are many examples of this to be found on a reef, but perhaps the most famous of these is practised by members of the scorpionfish family. These cunning predators do very little to earn a living, for their disguise does it all for them. They are masters of deception, as their skin’s colour and texture perfectly matches that of their surrounds. All they have to do is sit dead still and wait patiently for a foolish fish to pass dangerously close to their capacious mouths. Once that critical point has been reached, their jaws expand at lightning speed, sucking in their victims before the hapless prey have even realised there was somebody watching.
Stonefish, frogfish and crocodilefish, also known as flatheads, are classic examples of passive camouflage, as is the beautiful and varied leaf scorpionfish, which not only blends in with its background, but sways from side to side as if it were a fallen leaf or clump of algae.
Mimicry is another popular form of crypsis and the leafy and weedy seadragons of the cooler southern waters are adept proponents of this strategy, as is the juvenile rockmover wrasse. The young rockmover wrasse deliberately swims in such a way as to resemble a floating, tumbling, swaying mass of tasteless seaweed. It is common to find the juveniles of many fish species sporting camouflaging colours and patterns to assist them through this most vulnerable stage of their lives.
Seahorses are all slow swimmers, so successful camouflage is paramount. Pygmy seahorses are about the most adorable of all reef creatures, but finding them can be almost impossible. Measuring not much more than a centimetre, these plankton feeders rely on their astonishing camouflage to remain hidden within the polyps of their host gorgonian sea fans.
Another cute con-artist is the crinoid clingfish, which lives its entire life within the protective pinnules of its host, the featherstar. This is a great example of the importance of knowing what to look for, and where. Using a dive torch to carefully inspect the many arms of these crinoids, as well as illuminating the area around the holdfast, usually reveals some form of cryptic treasure. Many of a featherstar’s symbionts have evolved the same beautiful colour combinations expressed by their host, as seen in the different varieties of miniature squat lobsters that take up permanent residence within the protective feeding forest of these sticky echinoderms.
Flounders and sole are masterful agents of crypsis, but possess the added capability of manipulating their skin’s appearance as they move into new feeding grounds or to evade a pursuer. This type of active camouflage is known as background adaptation.
They do this by means of specialised cells embedded within the dermal layer that adapt the expression of pigments to approximately match the hue of the immediate surrounds.
This ability is best demonstrated by the cephalopods. Watching squid, octopus and cuttlefish change colour, texture and posture in front of one’s eyes is surely one of the observational marvels of the animal kingdom. The highly sophisticated nature of this magical transformation is still being fully deciphered by biologists, but the advantages of this skill are patently obvious.
The mimic octopus, found in the warm waters around Indonesia, takes the art of adaptive camouflage to the extreme. Not only is this Machiavellian charlatan able to change its entire appearance as it sees fit, but it has perfected the art of impersonation. It can ostensibly choose to imitate over 15 different animals, most of which are either highly venomous or downright undesirable. Among its impressive repertoire are lionfish, flounder, sea snake, starfish, cuttlefish, sand anemone, jawfish, crinoid, mantis shrimp and jellyfish.
Being able to trick a wary crab into believing an eager mating partner is heading its way is a crafty way to catch a meal, and if the damselfish are attacking, turning into a venomous sea snake will work a treat to put things in order! The fact that this creature knows what animal to impersonate and when is one of the great mysteries of the ocean.
LIGHT AND DARK
One would think that living in the darkness of the deep sea would render the need for camouflage redundant, but trace amounts of light from the night sky filter down through the water column and, if seen from below, a silhouette can be costly. Many open-ocean animals perform daily migrations from deeper layers during the day to feed in the shallower waters at night. Being able to disguise one’s silhouette with light-emitting photophores strategically positioned on one’s underside has proven a very successful nocturnal camouflage for a wide range of deep-sea creatures. Many mid-water cephalopods, decapod crustaceans and fishes capitalise on such counter-illumination tactics.
The shift from day to night is also acutely felt in the upper regions of the ocean. As the sun sets on a reef some resident fishes change their outward colours and patterns to accommodate the shift in lighting conditions. The highly reflective blue-streak fusilier takes on a notable red colour as it beds down in the reef for the night. Red is a difficult colour to see at depth and practically impossible to spot at night. The chevroned butterflyfish also changes into its pajamas, developing two large eyespots along its flank, presumably to convince any prying predators that they have stumbled upon a huge, white-eyed monster.
If the manipulation of one’s skin tone or the creation of light is a little beyond one’s abilities, then gluing bits of surrounding habitat to one’s body is yet another means of achieving invisibility. Decorator crabs carefully select fragments of shell, algae, seaweed, sponge, and more and affix them to their own shells by means of a special adhesive secretion and multiple hooks on the carapace surface. Some take it a step further and kidnap stinging anemones and jellies to serve as their protective blanket.
Disruptive camouflage is another effective way of confusing predators and many reef fish seem to find this useful. Like the black and white stripes of African zebras, bold lines and striking patterns serve to break up the outline of the body, making it difficult to know which end is the head or the tail, especially when the fish concerned are swimming in large, tightly packed schools.
Eyespots are perhaps the most popular guise to disorientate and deceive predators. Interestingly, the bright colours and patterns of many reef fish are themselves effective camouflage, since their specific spectral qualities, when viewed through a fish’s limited colour receptors, yield visual outputs that blend into the overall colour matrix of the reef or the blue open water. The further away such intricate patterns and colours are observed, the more the detailed visual information becomes scrambled, rendering the animal in distinguishable to distant, prying eyes.
MASTERS OF DECEPTION
Deception under the sea’s surface takes innumerable forms, all of them as fascinating and as carefully planned as the next. Remaining hidden while in plain sight allows creatures to go about their daily lives of feeding and finding mates, while remaining safe from hungry mouths.
What works and what doesn’t is never truly established, as predators and prey continually battle it out, forever trying to outwit one another, forever evolving towards the ultimate winner. As a result, the degree of brilliance and intricacy to be observed in this watery kingdom provides endless astonishment. The real question remaining is whether these clever beasties will be able to outlive the most cunning and deadly predator of all … us!