Masters of Camouflage
There is a well known saying humans often apply to the mechanics of the animal world; kill or be killed. While this is an excessively simplified way of viewing things, it is true that most animals do either have to pursue their food, or evade other animals that are pursuing them as food. Some have responded by developing more efficient forms of locomotion, others have developed deterrents in the way of spines or toxic secretions; and many more have become masters of camouflage, blending perfectly with their surrounding environment.
Hunting or Hiding
Camouflage is used as frequently by the hunters as it is by the hunted. In the same way a human bow-hunter on land compensates for his comparatively inefficient weapon by dressing in camouflage clothing, many benthic marine predators utilise camouflage to enable them to approach closer to their prey. Whether they are lie in wait hunters like the Stargazer, stealthy slow moving hunters like the Dory or even fast moving predators such as Sharks and Barracuda, camouflage plays a big part in the success rate of their hunting activities.
Stargazer and Mantis shrimp, two benthic predators that rely on camouflage
Conversely, many non-predatory species that are neither fast-moving nor afforded the protection given by spines or toxicity, rely heavily on camouflage for concealment against predators. Much like an arms race between Superpowers, predators and prey are continually evolving more efficient ways of either approaching undetected or evading each other, and the underwater world is one of the best places to see the results of this evolution.
There are some extraordinary examples of animal camouflage in the marine environment. Crypsis, or resemblance to ones background is limited not only to fish species, but can be observed also in crustaceans, cephalopods, elasmobranches, gastropods in fact representatives of all marine Phyla. [There's a glossary at the bottom of the page!]
LEFT : Harlequin shrimp against coral _ RIGHT : Seahorse imitating twig
Blending into an environment usually involves an animal taking on the colour of its surroundings. Sessile (non or near non-moving) creatures which live in one environment often have fixed colouration and markings to suit their particular micro-habitat, (for example the Harlequin Shrimp on Pocillapora corals), but mobile organisms need to be able to change their colours at will as they move across varying coloured backgrounds.
The Mechanics of Colour-change
Colour change in animals is carried out by special cells known as Chromatophores. Chromatophores are cells containing pigments that are controlled or stimulated by either nerves or hormones, depending on the organism. This pigment can either be concentrated at the core of the cell, reducing the effect of the colour, or spread throughout the cell intensifying the colour. Layers of differently pigmented chromatophores effectively produce different colours, responsible for the myriad combinations of hues we see in tropical reef fish. Other cells called iridophores which reflect light and produce a mirror effect, are responsible for the silvery sheen of many mid-water fish species.
Photophores are yet another type of cell which actually produce light, and which are used by some cephalopods to prevent predators from picking out their silhouette against a sunlit surface. By illuminating their underside their own shadow is eliminated, an effective camouflage technique known as counterillumination.
Colour for Camouflage
A similar principle to counterillumination is employed by pelagic and mid-water fish species, known as countershading; only using colour rather than light to achieve the same effect. The dorsal (upper) surface of such fish is a dark colour that when seen from above blends with the blue of deep water, whereas the ventral (underside) is white to merge with the bright surface light apparent from below.
Fish such as Flounder and Scorpionfish not only change their colour, but also their pattern to match the background perfectly. Certain Pipefish and Frogfish also have small fleshy protuberances on their body that can mimic the algaes (seaweeds) they hide amongst. Without a doubt though, it is the cephalopods that are the real masters of colour change, being able to change colour and pattern in seconds, and having an immense range of hues available to them.
A Cuttlefish in 'mid' colour change
Certain diurnal (daytime) species that rest at night, adopt blotchy colouration during the hours of darkness which helps break up their outline; known as disruptive colouration this form of camouflage is widely employed by diurnal fish as well. Bold stripes or patterns effectively break up the shape of the fish making them hard to pick out from the background.
Light levels also affect the use of colour for camouflage; often nocturnal or deepwater fish species are red in colour, as this is the first colour of the spectrum to disappear when light levels drop. Because of this, at depth red appears dark, and is hard to pick out in the gloomy light.
There is an even more refined use of colour markings for camouflage purposes in fish which acknowledges the importance of visual acuity in predatory species; the development of eyespots (imitation eyes).
Twinspot Lionfish displaying its rear 'eyespot'
Predatory fish often fixate on the eyes of their prey to determine their intentions and flight-path. Some prey species have dark patches or stripes across their eyes (eyebars) to make them less obvious, and an eyespot near the rear of their body. This may cause the predator to misjudge the direction of the prey fishes flight; or at worst attack the less vital end of the animal. Large prominent eyespots may actually deter a predator altogether by causing an over-estimation of the preys size.
While not strictly camouflage, it is worth mentioning another way colour is used for protection in the marine environment; as a warning to would be predators that the owner is dangerous, toxic, or inedible. Species employing this strategy include most Nudibranchs, (sea-slugs), the Lionfish, sea snakes, and several species of Cone shell to name but a few.
The nudibranch Chromodoris magnifica advertises its unpalatability
with bright warning colouration
There is a species of Parrotfish that has gone a step further than just visual camouflage; the Queen Parrotfish actually surrounds itself with a transparent mucous cocoon, which is secreted from its mouth. This effectively hides its scent from nighttime predators that rely on scent, such as moray eels.
Parrotfish inside its 'scent-proof' mucous envelope
Squid, Octopus and Cuttlefish also use a type of scent camouflage. All share the ability to squirt a blob of ink in defense, which as well as acting as a smoke screen, is believed to inhibit the sense of smell in predators
Morphology; or Body Shape Camouflage
To blend most effectively with their surroundings, animals often have a modified body shape; for example on flat sandy bottoms we find the flounder, a fish species that is so laterally flattened as to appear run over by a steamroller!
LEFT : A Flounder blending in with the substrate _ RIGHT : Harlequin Ghost Pipefish looking like a twig
The flounders body shape is adapted to its benthic environment to the extent that both eyes have migrated to one side of their body, (the top), and the mouth is on the other, (the bottom) allowing this species to lie flush on a sandy substrate with only its darting eyes giving it away. Ghost pipefish on the other hand have a thin, more cylindrical body, which is particularly suited to their habitat of seagrasses, vegetation, and debris.
Yet another morphological adaptation for the purpose of camouflage can be seen in the Razorfish, which has a thin, very flattened body that it holds in a vertical position. When amongst seagrass it becomes virtually invisible. Again though, the prize for best morphological camouflage goes to a cephalopod; the Octopus is capable of changing its body shape to resemble a pile of rocks, a sandy hump or whatever shape is requires to blend into its background.
LEFT : The versatile Octopus _ RIGHT : Cockatoo Waspfish imitating a dead leaf
Areas of muck and detritus often yield some of the most interesting examples of camouflage amongst marine organisms; spiny, tassellated fish such as the Crocodilefish, leaf-shaped species like the Cockatoo Waspfish, and a host of bizarre crustaceans spider crabs, decorator crabs, etc.
The latter species actually utilise bits of their background environment to camouflage themselves, attaching small sponges or seaweeds to their carapace making them doubly hard to distinguish!
LEFT : Head of the cryptic Crocodilefish _ RIGHT : Sponge covered Decorator Crab
Camouflage and Balance
Obviously what is suitable camouflage for one habitat is inappropriate for another, however nature has a way of getting it right, and only those whose camouflage is effective will survive to propagate their species.
Evolutionary progression will see to it that the arms race between hunters and hunted will continue giving rise to more weird and wonderful camouflage adaptations; while throughout that process a balance is maintained. In the natural order of things competition results in innovation and biodiversity rather than extinction that is, barring interference from Man.
No matter what the environment; be it weed forest or sandy bottom; coral reef or open water; there will exist a diverse range of species within that habitat perfectly adapted to disappearing before our very eyes Masters of Camouflage!
Dip. Marine Studies [NZ]
benthic, bottom dwelling
carapace, external skeleton covering body
cephalopods, squids, octopus and cuttlefish
crustaceans, crabs, shrimps
diurnal, active during the night
dorsal, upper surface
elasmobranches, sharks and rays
gastropods the phylum to which shellfish and cephalopods belong
morphological, relating to the body
nocturnal, active at night
phyla. Primary subdivision of taxonomic classification
sessile, not moving
substrate, bottom material
tasselated, having small skin tassels
ventral, lower surface or underside
Deloach N, Reef Fish Behaviour, 1999, New World Publications;
Allen GR & Steene R, Indo-Pacific Coral Reef Guide 1999, Tropical Reef Research;
Debelius Ellis R, Monsters of the Sea, 1996, Doubleday