Camouflage under water the art of not being seen by Dr Floor Anthoni (2009) www.seafriends.org.nz/indepth/camouflage.htm
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.
introduction: what is
camouflage and what is mimicry? Examples from the bio-diverse tropics.
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, 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 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
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
looks indistinguishable from the green halimeda seaweed. Copyright Teresa
Zubi.
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 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 One
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.
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.
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.
f009427: when looking straight down, these goatfish (Upeneichthys
lineatus) are almost entirely invisible. The photo shows 33 fishes.
Notice how shadows disappear.
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.
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.
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!
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.
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.
f213218: common shrimps (Palaemon affinis) are almost
invisible because they are so transparent. How many do you see?
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.
f049305: a school of piper (Hyporhamphus ihi) swimming
close to the surface where their silvery bodies make them almost invisible.
f001314: a peer group of young trevally (Pseudocaranx
dentex) foraging the sandy bottom. Note how their silvery sides still
make them less visible.
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.
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.
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.
f021616: a brown stargazer (Gnathagnus innotabilis)
sits motionless on the sandy bottom, being invisible due to its inherited
skin colour.
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.
f046917: a sleeping marbled hawk fish (Cirrhitus pinnulatus)
is well camouflaged in its commando action suit. Niue.
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.
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 (right).
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.
f039631: two mysid shrimps sitting on top of the crystal
clear sand. Can you see the one in the centre?
f046519: female shore crab (Plagusia depressa tuberculata)
is well camouflaged, even though it can move quite fast with its long spidery
legs. Niue.
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.
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.
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.
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.
f041618: a northern scorpionfish or granddaddy hapuka in
a brown-red costume to match its environment.
f030822: the Cook's scorpionfish (Scorpaena cookii)
is more slender than the northern scorpionfish, but equally adept at camouflaging
itself.
f018325: the dwarf scorpionfish (Scorpaena papillosus)
is a true master of camouflage, reason why we are showing several images
here.
f049422: dwarf scorpionfish, almost invisible.
f040417: dwarf scorpionfish can even do purplish colours.
f039229: dwarf scorpionfish in brown colours on a muddy/sandy
bottom with brown rocks.
f052529: this baby dwarf scorpionfish is only 5cm long and
must hide carefully, being so small.
f047733: a lizardfish (Saurida nebulosa) is a fierce
predator but lies quietly in ambush, like scorpionfishes do. Niue.
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.
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.
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.
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.
f009406: a sand octopus (Octopus gibbsi) has made
itself look like an encrusted reef wall., complete with protrusions of
its skin.
f009134: a sand octopus mimicking encrusted stones. Can you
find its eyes? Its arms?
f033529: a sand octopus has crept half out of its den for
a good view of the world, imitating its sand-covered environment.
f046419: a tropical octopus (?) resting on an encrusted
urchin barren. Niue.
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.
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.
f048809: the mottled sand star (Luidia varia) about
to disappear straight down into the sand. These sea stars are active mainly
at night.
f021215: a beautiful Hydatina physis decides to disappear
into the sand after seeing the photographer's light during a night dive.
f048810: a comb star (Astropecten polyacanthus) also
forage mainly at night. this one was found marching during the day.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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).
One
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.
