Camouflage under water
the art of not being seen
by Dr Floor Anthoni (2009)
www.seafriends.org.nz/indepth/camouflage.htm
in-depth logo Floor Anthoni

 

Sea organisms have developed an amazing array of methods to seem invisible. They use camouflage and mimicry to pretend what they are not. This article shows the many ways in which they can disguise themselves to fool predators and prey.

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Introduction
The ability of being invisible has always been a long-time dream of people who are perhaps the most visible of all creatures on Earth, with our odd angular shapes, flailing extremities and upright way of walking. It is not surprising then that we aim to hide our ugly features by means of clothing and paraphernalia. But wouldn't it be nice to be entirely invisible at will, so we can watch others without being watched in return? So that we can gain untold advantage over those who can't?
The closest we can come to this ideal is by means of camouflage. Soldiers, hunters and photographers do this by wearing clothing with random patterns resembling a near-chaotic natural environment. They paint their faces such that the shapes of eyes, nose, mouth, chin, forehead are hidden, and then they place bits from the environment on their clothing to soften their angular shoulders and heads. They do something similar to their rifles and backpacks. Then they hide behind natural objects such as trees and stones, and in trenches.

Mimicry is the act of mimicking (pretending to be) some other organism, usually one that is bigger or dangerous or inedible or even poisonous. It may fool predators often enough to extend life sufficiently to reproduce adequately, in order to maintain viable populations. So neither mimicry nor camouflage needs to be 100% fool-proof.

In general one can say that the most amazing feats of camouflage and mimicry are performed by small animals that live in shallow water where exact colours matter. Since camouflage is a form of adaptation (evolution), it follows that the most amazing acts of camouflage are found where biodiversity is highest: in the warm tropical seas, particularly in sheltered places that are not destroyed frequently by large storms. Unfortunately we do not have any photographs from such places, and will therefore restrict this article to New Zealand and Niue. Fortuneately Teresa Zubi made some astounding photos available from www.starfish.ch.
 
 

Antennarius pictus. The painted frogfish
Antennarius pictus. The painted frogfish, here in an even grey costume, can change its colours slowly to mimic its temporary environment, even with black spots as if they were sponge pores. Copyright Teresa Zubi.
Novaculichthys taeniourus. The dragon wrasse or rockmover wrasse
Novaculichthys taeniourus. The dragon wrasse or rockmover wrasse looks like a broken piece of seaweed, and even swims like one, erratically, with its head down. It indeed flips rocks to eat what's hiding underneath. Copyright Teresa Zubi.
Solenostomus cyanopterus. The robust ghost pipefish
Solenostomus cyanopterus. The robust ghost pipefish has adapted both size, shape and colour to live unseen between the arms of a feather star. Copyright Teresa Zubi.
Solenostomus halimeda. The halimeda ghost pipefish
Solenostomus halimeda. The halimeda ghost pipefish looks indistinguishable from the green halimeda seaweed. Copyright Teresa Zubi.
Hippocampus bargibanti. the pygmy seahorse
Hippocampus bargibanti. the pygmy seahorse is very small with warts on its skin resembling the coral polyps where it lives. Copyright Teresa Zubi.
Hippocampus bargibanti. the pygmy seahorse, a crowd
Hippocampus bargibanti crowd. In this picture you should see eight pygmee seahorses, twined together and around the branches of this fan coral. Copyright Teresa Zubi.

What tricks does nature have in store for so many creatures threatened by predation? How can they live while being invisible? Let's have a look at camouflage in the underwater world. But first some background knowledge about how light behaves under water.
 
 
How colours change with depth
light absorption with depthOne may have noticed that clean water looks blue, because blue light is bent by water molecules. For the same reason a clear sky looks blue, because blue light is bent by the molecules in air. White sunlight is not a fixed quality, but consists of a continuous and endless number of colours ranging from violet (near ultra-violet) to red (near infra-red). As light penetrates the water, some of these colours are hindered more than others, as shown in this diagram. The blue and cyan colours reach deepest as they are attenuated (diminished) least, whereas the orange-red colours are attenuated most. Infra-red which is a colour our eyes cannot see, penetrates only a few centimetres as it is immediately absorbed by the water molecules. This selective filtering by water is something underwater photographers try to compensate for by lighting their underwater subjects with artificial white light. Even then the total light path a+b can often be enough to require extra filtering with photographic colour filters. For more about this, visit our extensive course in underwater photography.
What you should remember is that many camouflage photos shown in this article have been enhanced with artificial light. The angle at which one looks under water is also important because (blue) light appears to come from all directions, softening shadows and contours, especially when looking straight down.
 

northern scorpionfish (Scorpaena cardinalis)
f030723: a northern scorpionfish (Scorpaena cardinalis) lies in ambush in a colourful environment of sponges. Note its almost random skin patterns. The photographer used a strobe (flash light) to bring out the environment's natural colours, but this is not what its victims see.
northern scorpionfish (Scorpaena cardinalis)
f030723: this is what the scorpionfish would look like at about 15m depth where this photo was taken. As you can see, by removing the red-orange components of visible light, even approximate camouflage becomes near-perfect. However, notice the harsh shadows which are not possible at this depth.
invisible goatfish
f009427: when looking straight down, these goatfish (Upeneichthys lineatus) are almost entirely invisible. The photo shows 33 fishes. Notice how shadows disappear.
invisible goatfish
f001127: the same goatfish seen from the side and lit by a torch, the light of which reflects from their pearly skins, making some clearly visible. But can you count 14 here?

 
The bottom that shook
One day, near the beginning of my dive, I slowly swam through the Goat Island Channel, about 1.5m from the bottom, when suddenly the whole bottom moved, causing a sensation of dizziness. I had been swimming over a shoal of some 400 goatfish, quietly sitting on the bottom, all perfectly lined up into the mild current. Then one of them startled, and the whole school startled, soon to bed down again. I was so ashamed of not having seen them, so many from so nearby. Had the fish stayed put, I would not have noticed them!



 
Transparent scales
Most bony fishes have transparent scales, which let the skin colours shine through. Some fish can change the pigments in their skins, either very slowly over several months, or immediately, or by storing the pigments of their food in their skins. Some have no apparent scales, like eels and blennies (slimefishes), so their skins are not obstructed from view. Some organisms are completely transparent, which makes them nearly invisible. It is so amazing that their muscles cannot be seen, being entirely transparent.
rock skipping blenny (Istiblennius edentulus)
f043535: a tiny rock skipping blenny (Istiblennius edentulus), living half out of the water. It can move very fast over the surface of the water bent in this u-shape which forms two 'legs': head and tail.  Niue.
sand flounder (Rhombosolea plebeia)
f036411: a sand flounder (Rhombosolea plebeia) hides by changing the colours and patterns in its skin and by covering itself with sand. In our aquariums these flounders can change their colours from near-white to near-black, and they can even change the colours of their fins!
school of whitebait (Galaxias sp.)
f002203: a school of whitebait (Galaxias sp.) is almost invisible because they are transparent. The fish are born in salt water and migrate upstream into fresh water rivers.
comb jelly
f002804: wind and eddies have gathered these comb jellies of no more than 3cm into a dense patch. Because they are transparent, they are visible only close to the surface. Note how they quickly vanish in the distance, and further down.
common shrimps (Palaemon affinis)
f213218: common shrimps (Palaemon affinis) are almost invisible because they are so transparent. How many do you see?
common shrimp (Palaemon affinis)
f032021: closeup of a common shrimp (Palaemon affinis) shows that one can see right through its body. How can muscles and blood be invisible?


 
hiding behind mirrors
The best cloaking device (cloak = mantle) is one that lets the light shine right through such as being completely transparent. The next best solution is a mirror that shows something else instead of the real thing. Pelagic fishes that swim in the open sea have perfected a silver mirror which is fuzzy, thus showing the sea around, without any clear objects - an almost perfect cloaking device. To make this deception even more perfect, their mirrors are shaded on their backs where most of the light comes from (counter-shading) and whitish on their bellies.
school of piper (Hyporhamphus ihi)
f049305: a school of piper (Hyporhamphus ihi) swimming close to the surface where their silvery bodies make them almost invisible.
group of trevally (Pseudocaranx dentex)
f001314: a peer group of young trevally (Pseudocaranx dentex) foraging the sandy bottom. Note how their silvery sides still make them less visible.
silver trevally (Pseudocaranx dentex) and kahawai (Arripis trutta)
f051930: a mixed school of silver trevally (Pseudocaranx dentex) and kahawai (Arripis trutta) fading in the blue sea. These silvery fish assume the colour of the water because their silvery skins reflect it. Notice how quickly they fade away in the distance. It would be very difficult to say how many fish are in this picture.


blue like the sea
Fish living in clear blue waters can become invisible when their skins match the colour of the water, plus some counter-shading to make their backs darker than their sides, and their bellies lighter. Fish living mainly in kelps of a certain colour, merely need to be of similar colour, in order to vanish from view.
blue maomao (Scorpis violaceus)
f024623: because they are blue above and silver-white below, blue maomao (Scorpis violaceus) quickly disappear in a blue sea. These fish cannot change their colour at will but become more deep blue with age.
parore (Girella tricuspidata)
f001934: parore (Girella tricuspidata) can change their costumes from almost white to black. Here they are in their grey active wear with thin black stripes, ready to bed down when they change into their pyjamas. Can you see two sleeping fish near the centre of this picture?

 


Born with the right colours
Life on earth has existed for eons (1000 million year epochs), and fishes are some of the earliest advanced forms of life (with a spine). It is not surprising then that the process of natural selection has produced many forms of underwater life with near-perfect camouflage for where they normally occur. The surface waters where the red colours are still adequate, is most demanding of perfect camouflage. One finds triplefins, blennies and crabs with near-perfect camouflage obtained at birth.

 
brown stargazer (Gnathagnus innotabilis)
f021616: a brown stargazer (Gnathagnus innotabilis) sits motionless on the sandy bottom, being invisible due to its inherited skin colour.
sand goby
f039224: a sand goby (?) is almost invisible on the sand, and usually they burrow as well, leaving only their topside eyes protruding above the sand. 
marbled hawk fish (Cirrhitus pinnulatus)
f046917: a sleeping marbled hawk fish (Cirrhitus pinnulatus) is well camouflaged in its commando action suit. Niue.
juvenile banded flag-tails (Kuhlia taeniura)
f043530: juvenile banded flag-tails (Kuhlia taeniura) in a small rock pool. . Their black and white colours with patterns that disrupt their shapes, make these fish almost invisible when sitting still. Niue.

 
Crustaceans have thick external skeletons that are formed as part of their skins, but then remain fixed in size, shape and colour until they are shed during moulting. Crabs and shrimps thus cannot change the colour of their carapaces, and for most species these remain the same during their entire lives. However, some have transparent exoskeletons.
young mantis shrimp (Lysiosquilla spinosa)
f034503: a young mantis shrimp (Lysiosquilla spinosa) is about to begin its burrow that can reach many times its length. Because of its half-round body shape, it can turn around inside its narrow burrow. Notice its false head on its tail.
tiny sand shrimp
f215336: a tiny sand shrimp (?) is almost entirely invisible on the sand due to its inherited colours. Notice the black tail spot which mimics its eye. When burrowed, it leaves both antennas flat on the surface.
mysid shrimps hidden on the sand
f039631: two mysid shrimps sitting on top of the crystal clear sand. Can you see the one in the centre?
female shore crab (Plagusia depressa tuberculata)
f046519: female shore crab (Plagusia depressa tuberculata) is well camouflaged, even though it can move quite fast with its long spidery legs. Niue.
blue ocean wanderer (Planes cyaneus)
0703076: this blue ocean wanderer (Planes cyaneus) lives on floating seaweeds as it wanders the oceans. We saw it change colour from bright yellow to olive while we watched, and it can also become blue. Apparently some crustaceans are able to change their colours at will, through a transparent carapace.
young killer prawn (Ibaccus alticrenatus)
f028925: a young killer prawn (Ibaccus alticrenatus) has a green skin and transparent abdomen when young, but later changes to brownish-red. Some crustaceans can change their skins from moult to moult.
mature killer prawn (Ibaccus alticrenatus)
f040436: a mature killer prawn (Ibaccus alticrenatus) is well camouflaged in its velvet external skeleton which looks quite different from its juvenile skin.

 
Predators enjoy the camouflaging advantage twice: once while young, when they are preyed upon, and later in life when they can ambush others. The scorpionfishes are champions in disguise as they have many different costumes. Because of this, it is thought that they can actually change colour, even though they do not do so frequently.
northern scorpionfish (Scorpaena cardinalis)
f049236: the northern scorpionfish (Scorpaena cardinalis) is a big fish (40cm) with so many colour patterns that it must be able to change them at will.
northern scorpionfish (Scorpaena
f041618: a northern scorpionfish or granddaddy hapuka in a brown-red costume to match its environment.
Cook's scorpionfish (Scorpaena cookii)
f030822: the Cook's scorpionfish (Scorpaena cookii) is more slender than the northern scorpionfish, but equally adept at camouflaging itself.
dwarf scorpionfish (Scorpaena papillosus)
f018325: the dwarf scorpionfish (Scorpaena papillosus) is a true master of camouflage, reason why we are showing several images here.
dwarf scorpionfish (Scorpaena papillosus)
f049422: dwarf scorpionfish, almost invisible.
dwarf scorpionfish (Scorpaena papillosus)
f040417: dwarf scorpionfish can even do purplish colours.
dwarf scorpionfish (Scorpaena papillosus)
f039229: dwarf scorpionfish in brown colours on a muddy/sandy bottom with brown rocks.
baby dwarf scorpionfish (Scorpaena papillosus)
f052529: this baby dwarf scorpionfish is only 5cm long and must hide carefully, being so small.
lizardfish (Saurida nebulosa)
f047733: a lizardfish (Saurida nebulosa) is a fierce predator but lies quietly in ambush, like scorpionfishes do. Niue.
devil scorpionfish (Scorpaenopsis diabolus)
f047701: the devil scorpionfish (Scorpaenopsis diabolus) is well camouflaged. Its body has humps making it look less like a fish, and it allows algae to grow on its scales. Niue.

 
changing costumes
When living in shallow water where all colours of the rainbow are present, camouflage must be perfect. The little fishes, the blennies and triplefins, have the ability to change colours and patterns with ease.

 
cryptic triplefin (Cryptichthys jojettae)
f034521: the cryptic triplefin (Cryptichthys jojettae) lives on wave-washed rocks, swimming fast in the surging waters, while bracing itself in the backwash. It adapts its colouring to suit.
variable triplefin (Fosterygion varium)
f036527: the variable triplefin (Fosterygion varium) has a wide range of costumes, but seldom appears in this reddish suit, which made it quite difficult to spot amongst the red vegetation.
blue-dot triplefin (Notoclinops caerulepunctus)
f048116: the blue-dot triplefin (Notoclinops caerulepunctus) is very small (4cm) and rather difficult to see without a torch, as its gaudy patterns hide its shape.
f034528: the scaly-head triplefin (Karalepis stewarti) has many costumes, even though it hunts mainly at night. It is usually found with its head pointing downward.

 
adapting skin colour and texture
In order to adapt both skin colour and texture, one cannot have scales or external skeletons. It is an act performed only by inkfishes, particularly those living in shallow water, like octopus and cuttlefish. Read our special chapter about these adorable cephalopods, but here we show what the octopus is capable of.

 
sand octopus (Octopus gibbsi)
f009406: a sand octopus (Octopus gibbsi) has made itself look like an encrusted reef wall., complete with protrusions of its skin.
sand octopus (Octopus gibbsi)
f009134: a sand octopus mimicking encrusted stones. Can you find its eyes? Its arms?
sand octopus (Octopus gibbsi)
f033529: a sand octopus has crept half out of its den for a good view of the world, imitating its sand-covered environment.
tropical octopus
f046419: a tropical octopus (?) resting on an encrusted urchin barren. Niue.
young broad squid (Sepioteuthis australis)
f007237: a gang of young broad squid (Sepioteuthis australis), changing colour as they pass over various coloured objects.

 
burrowing
Burrowing in sand or mud is a good strategy to make oneself disappear. It is a skill that almost all bottom dwellers possess. Some burrow shallowly such that their eyes (often on stalks), protrude above the sand, to watch the goings-on. Others burrow by day, when they can be seen, in order to forage in the night when they cannot be seen.

 
 
snapper biscuits (Arachnoides zelandiae)
f003008: snapper biscuits (Fellaster zelandiae?) or sand dollars are flat sea urchins that live just beneath the surface of estuarine sands at low tide level, where they eat detritus. On left one cleaned and placed on top; on right one burrowed.
mottled sand star (Luidia varia)
f048809: the mottled sand star (Luidia varia) about to disappear straight down into the sand. These sea stars are active mainly at night.
beautiful Hydatina physis
f021215: a beautiful Hydatina physis decides to disappear into the sand after seeing the photographer's light during a night dive.
comb star (Astropecten polyacanthus)
f048810: a comb star (Astropecten polyacanthus) also forage mainly at night. this one was found marching during the day.
short tail stingray (Dasyatis brevicaudata)
f002215: a short tail stingray (Dasyatis brevicaudata) has carefully burrowed herself under a thin layer of sand, which makes her invisible to her worst predator, the orca.
short tail stingray (Dasyatis brevicaudata)
f020027: because killer whales 'see' with ultrasound echolocation, the short tail stingray hides with a shape resembling a stealth fighter, covered in sound-absorbing velvety skin. It does not have a swim bladder which would give a strong sound echo.

 
In general, prey animals need to camouflage themselves only according to the ability of their predators to see them. Some see with eyes sensitive to many colours. Some are colour blind. Orca and dolphins see with sound and some sharks feel electrical patterns. Others are attracted by smells. So the act of camouflaging has a large invisible dimension.

 
sand flounder (Rhombosolea plebeia)
f035823: this sand flounder (Rhombosolea plebeia) has its eyes perched high above so that it can cover itself with sand, yet observe the goings on. When moving along on its fins, it often keeps a thin layer of sand over its back.
sand sole (Peltorhamphus sp.)
f007808: the small sand sole (Peltorhamphus sp.) is a superb master of camouflage, seen here lying on top of the sand. It moves in short bursts, often in the direction and with the speed of the light patterns on the sand. So it remains invisible, even while moving.
peacock flounder (Bothus mancus)
f047716: the peacock flounder (Bothus mancus) is so well camouflaged as to be almost invisible. Rather than swim, it crawls forward and backward on its fins, while continuously changing colours and patterns. Niue.



 
hairs for camouflage
A hairy skin is ideal for growing dirt and algae on, until the animal looks and tastes like the mud or sand it lives in. Particularly crabs have developed this form of camouflage.
bighand crab (Heterozius rotundifrons)
f033304: an old bighand crab (Heterozius rotundifrons) or pebble crab has camouflaged its legs with dirty hairs, which makes the animal look like a grey pebble without legs.
hairy crab (Pilumnus sp.)
0609153: a hairy crab (Pilumnus sp.) is completely covered in hairs trapping dust and detritus, making it look like the dirt it lives in. Here it was placed on a barren rock to expose its camouflage.
hairy crab (Pilumnus sp.)
f051611: a hairy crab (Pilumnus sp.) has just moulted and its clean hairs are ready to trap whatever wants to stick to it.
f050405: an unknown species of crab shows how it becomes almost invisible due to its dense hair. The hairs disguise its shape and eventually will become fouled.
hat urchin
f029301: a hat urchin (Clypeaster australasia?) holds sand and coral debris onto its back with its many sticky tube feet. It catches the articles at sand level and moves them up over its hat.
red sponge covered in chalky sand
f018132: a red sponge (?) has selectively caught limestone sand particles to hide behind. The middle bit has been cleaned by the photographer in order to show its disguise.

 
One of the most adorable creatures is the camouflage crab or decorator crab. Being entirely defenseless, it covers its hairy legs with whatever it can find, preferring living organisms like plants. As long as it remains in the light, these plants grow to enhance its camouflage. When it moves to a new area (or when we place new seaweeds in our aquariums), it pulls the old ones from its back and replants its back garden with the newer variety. After moulting, it often plucks the vegetation from its old shell and re-uses it on its new shell.

 
camouflage crab (Notomithrax sp.)
f028916: a camouflage crab (Notomithrax sp.) living in a dark archway, has covered its back and legs with dead pieces of bryozoan coral and live seasquirt colonies. 
lesser decorator crab (Notomithrax minor)
f032311: a lesser decorator crab (Notomithrax minor) has covered its hairy back and legs in red seaweeds which make it entirely invisible amongst these weeds. 
lesser decorator crab (Notomithrax minor)
f052125: a lesser decorator crab (Notomithrax minor) has just moulted, which shows the long hairs on its carapace, ideal to attach organisms to. There's no need to hurry.
young sponge crab (Petalomera wilsoni)
f051634: a young sponge crab (Petalomera wilsoni) could not find a suitable sponge and hides under a kelp leaf instead, which it has trimmed to suit its size. Sponge crabs have four modified legs to hold a sponge on its back. The sponge then grows and is reused after moulting.
slender pillbox crab (Halicarcinus sp.)
f039118: the slender pillbox crab (Halicarcinus sp.) collects dust on its back but not on its slender legs with which it can swim quite effectively. 
hermit crab
f049404: by living in a foreign house into which it can withdraw completely, a hermit crab is very effectively camouflaged and protected. But it needs to trade it for a bigger one as it grows.
large hermit crab (Dardanus sp.) with anemones
f043316: a large hermit crab (Dardanus sp.) has gone to extremes finding two large anemones and planting these on its shell. When it moves house, it also moves the anemones over. But where does it find these anemones, as I've never seen one like these? The thin shell suggests that these hermit crabs come from deep water beyond diving depth, where they also find these anemones? Photo taken around mid night, Poor Knights, 20m deep
large-eyed hermit crab (Dardanus gemmatus) with anemones
f047114: a large-eyed hermit crab (Dardanus gemmatus) collects anemones on its shell. This one has three big ones and a small one. The anemones benefit by being moved around while taking part in the hermit crab's meals, while the crab is protected by the stinging, bad-tasting anemones. Niue.This strategy must fool its worst predator, the octopus whose exploring arms can taste. Niue.



 
Inviting lodgers
Some organisms actively invite other organisms to grow all over them, making themselves invisible and also hiding their taste and form.
edible warty seasquirt (Microcosmos kura)
f052403: the edible warty seasquirt (Microcosmos kura) invites other organisms to grow all over it, like seasquirts and stinging anemones. These organisms are effective camouflage and protection. Here two seasquirts are stuck side to side.
f045530: a golfball sponge (?) invites hard crustose coralline algae to cover it completely, but manages to keep breathing holes open.

 



 
cryptic behaviour
Most fishes need to move in order to find food, but as soon as they move, their camouflage fails. The sole moves in rhythm with the ripples of the light, and the peacock flounder changes colour as it crawls along on its fins but there are more ways in which movement or other behaviour helps being invisible. Scorpionfishes sit perfectly still, while waiting in ambush.
male and female potbellied seahorse (Hippocampus abdominalis)
f003324: a male and female potbellied seahorse (Hippocampus abdominalis) entwined around a branch. The reddish male is pointing this way and the female the other way. This arrangement helps their disguise.
female network pipefish (Corythoichthys flavofasciatus)
f047531: a female network pipefish (Corythoichthys flavofasciatus) is well camouflaged by her markings, but also takes care to move in short bursts when a wave moves the short algae around her. Her swim fins are almost invisible. Niue.
john dory (Zeus faber) hunting
f017714: a john dory (Zeus faber) lies flat on it side, doing its very best to look like a stone, as it slowly moves in on its prey. It swims by fanning its almost invisible back fin and anal fin.

 



Consider also these forms of camouflage for which we have no pictures yet: changing skin through diet (nudibranchs, shrimps), disguise as something dangerous or poisonous (mimicry).

For corrections and suggestions, e-mail the author.
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