Bony flat-fishes, like plaice and sole, have a remarkable power of
adjusting their hue and pattern to the surrounding gravel and sand, so
that it is difficult to find them even when we know that they are there.
It must be admitted that they are also very quick to get a sprinkling
of sand over their upturned side, so that only the eyes are left
showing. But there is no doubt as to the exactness with which they often
adjust themselves to be like a little piece of the substratum on which
they lie; they will do this within limits in experimental conditions
when they are placed on a quite artificial floor. As these fishes are
very palatable and are much sought after by such enemies as cormorants
and otters, it is highly probably that their power of self-effacement
often saves their life. And it may be effected within a few minutes, in
some cases within a minute.
In these self-effacing flat-fishes we know with some precision what
happens. The adjustment of colour and pattern is due to changes in the
size, shape, and position of mobile pigment-cells (chromatophores) and
the skin. But what makes the pigment-cells change? The fact that a blind
flat-fish does not change its colour gives us the first part of the
answer. The colour and the pattern of the surroundings must affect the
eye. The message travels by the optic nerve to the brain; from the
brain, instead of passing down the spinal cord, the message travels down
the chain of sympathetic ganglia. From these it passes along the nerves
which comes out of the spinal cord and control the skin. Thus the
message reaches the colour-cells in the skin, and before you have
carefully read these lines the flat-fish has slipped on its Gyges ring
and become invisible.
The same power of rapid colour-change is seen in cuttlefishes, where it
is often an expression of nervous excitement, though it sometimes helps
to conceal. It occurs with much subtlety in the Æsop prawn, Hippolyte,
which may be brown on a brown seaweed, green on sea-lettuce or
sea-grass, red on red seaweed, and so on through an extensive repertory.
According to the nature of the background, [Professor Gamble writes]
so is the mixture of the pigments compounded so as to form a close
reproduction both of its colour and its pattern. A sweep of the
shrimp net detaches a battalion of these sleeping prawns, and if
we turn the motley into a dish and give a choice of seaweed, each
variety after its kind will select the one with which it agrees in
colour, and vanish. Both when young and when full-grown, the Æsop
prawn takes on the colour of its immediate surroundings. At
nightfall Hippolyte, of whatever colour, changes to a transparent
azure blue: its stolidity gives place to a nervous restlessness; at
the least tremor it leaps violently, and often swims actively from
one food-plant to another. This blue fit lasts till daybreak, and is
then succeeded by the prawn's diurnal tint.
Thus, Professor Gamble continues, the colour of an animal may express a
nervous rhythm.
[Illustration: _Photo: J. J. Ward, F.E.S._