IF YOU’RE snorkelling on a coral reef this summer, you’ll see the local
inhabitants in their carnival colours. The show clearly isn’t just a tourist
attraction. For the fish that live on the reef, dressing up is a matter of life
and death. Like any other animal, a fish’s survival depends on just two
things—food and sex. The trouble is that eating, and not being eaten, need
stealth so it helps to blend into the background. To attract a mate, on the
other hand, requires a certain flamboyance. This is the fish’s dilemma.
Seeing a coral reef in all its glory you can’t help feeling that fish have
completely failed to solve this dilemma. The picture only comes into focus when
you take the fish’s-eye view. For fish see things differently, says Justin
Marshall from the Vision, Touch and Hearing Research Centre at the University of
Queensland in Brisbane.
“Our visual system is a primate one,” he says. “It’s very good at seeing
yellows and reds versus greens, because our main job in life is spotting ripe
fruit, or females’ bottoms.” But long wavelengths don’t penetrate water well.
Below 30 metres, there is no red light. So fish colour vision tends to be tuned
to the shorter end of the spectrum—blue and ultraviolet—and to be
less sensitive to reds and yellows. This means that the carnival looks quite
different to the locals.
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Marshall has designed a unique underwater spectrophotometer which analyses
the colours of things objectively in terms of their physical reflection. He is
also measuring the light available in different microhabitats. Together with
information about the visual sensitivity of individual fish species and their
behaviour, this lets him begin seeing things as the fish do. And it is starting
to reveal how both the showy and the shy can make use of the same bright
colours.
The general shift towards the blue end of the spectrum underwater explains
why most nocturnal reef fish, such as the soldierfish, squirrelfish and bigeyes,
are mainly red. During the day these fish hole up in reef crevices. They may
look obvious to human eyes, but to a fish they blend into the dark background.
“Some reef fish might see red,” adds Marshall. In which case they could
capitalise on the colour blindness of others and use red markings for private
communication. But in most cases reds and pinks are inconspicuous.
The other effect of a visual system that works best at short wavelengths is
that fish that look drab to us—particularly those with dark blue or black
patterns—may stand out to their own kind in the ultraviolet range. But
even though this lets the fish attract attention when it wants to, it needn’t
give the game away when the fish needs to hide.
“UV is potentially a very good colour to use as a close signal,” says
Marshall. “This is because of the way in which water scatters light.” Short
wavelengths are scattered more than long wavelengths, which makes ultraviolet an
excellent colour to use as a private signal over distances of a few centimetres.
Beyond this distance, the signal is lost, so predatory fish can’t eavesdrop.
Mike Land, a vision expert at the University of Sussex, says that many fish do
have separate UV receptors.
What hits a snorkeller immediately is how many reef fish are kitted out in
eye-catching ensembles of highly contrasting colours. “Lots of reef fish are
yellow and blue,” says Marshall, “which is a very good example of complementary
colours.” In other words, blue reflects light in the spectrum that yellow
doesn’t and vice versa. These sorts of colour combination are conspicuous in
close-up, and look to us as though camouflage is the last thing on the fish’s
mind. But they fade to grey at a distance, when the stripes are too close
together to resolve, and the colours merge together. Marshall has calculated
that in fish sporting fine bands of colour, such as parrotfish and wrasse, the
different shades are distinct for only one metre and certainly no more than
five. Beyond this, they blend in to the sea colour around the reef.
Wider colour bands will be visible much further away, of course, but even
here, the fish’s-eye view is different from ours. Most recently, Marshall has
discovered that fish may see hardly any contrast between the blue of many
species, such as tropical angelfish, and the colour of the water around a
tranquil reef. More surprisingly, says Marshall, a yellow fish is well
camouflaged against the reef.
If fish really can’t see the difference, then it looks as though they have
only two types of photoreceptors for colour, suggests Marshall. This is a
controversial claim. As Land points out, some fish actually have four types of
colour receptor (they’re “tetrachromatic”), and may have even better colour
discrimination than humans, with our three different photoreceptor types. What’s
more, colourful animals usually have good colour vision. But Marshall says that
it’s easiest to assume that reef fish are dichromats because of the way they
behave—and there’s already some evidence. “This information is actually
coming out as we speak,” he says.
If Marshall is correct, then a fish with bold blue and yellow markings can
either advertise or hide itself by simply adjusting its behaviour. The striking
bands shout “come and get me” to a potential mate when displayed against a plain
background. But against a background of blocky contrasting colours they work as
disruptive camouflage, on the same principle as military camouflage. This
explains why angelfish tend to retreat to areas of branching coral when
predators approach. Such caution is not displayed by the poisonous boxfish,
which flaunts its bright colours to ward off potential diners.
Blue and yellow fish aren’t the only extroverts with highly contrasting
stripes. “Black and white striped fish—which are very common—are
more likely to want to be seen from a distance,” says Marshall. But even this
distinct pattern will merge into some backgrounds. When the fish are all
together in a shoal, it’s hard for a predator to spot where one individual
starts and another ends. “This is the zebra effect,” says Marshall. Similar
subterfuges are used by fish that disguise parts of their bodies with patches of
colour, or have false eyes near their tails.
So, one set of colours can send out very different signals depending on the
setting. To complicate things further, most reef fish can vary their colours.
Triggerfish, for example, are chameleon-like, changing their colour to suit
their background. Others change colour from night to day or as they grow older.
Colours even change with a fish’s “mood”—whether it is fighting or fleeing
from predators. Marshall’s insight has been to look at fish colours in their
natural context. Failing to do this is a mistake that’s been made time and
again, he says. “And it’s still being made by very eminent scientists who go out
and hold up bits of coloured paper against animals and they say `oh, it’s that
ł¦´Ç±ô´ÇłÜ°ů’.”
Â鶹´«Ă˝ readers with a love of snorkelling, will now know better than
to fall into that particular anthropocentric trap.
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Further reading:
Communication and camouflage with the same `bright’ colours in reef fish
by Justin Marshall, Philosophical Transactions of the Royal Society (in press)