The female common market squid can change its appearance to make it look as though it has
testes in order to spurn the advances of males.
The cephalopod -- AKA Doryteuthis opalescens -- has a
pair of stripes that can sparkle with rainbow iridescence. These
stripes can be found either side of another single strip, which can
switch between transparency and bright white.
It is the central transparent-to-white stripe that allows for
the devious masquerade. The white stripe can be turned on at will
to look like the male squid's bright white testis.
"Our best supposition is that the female can masquerade as a
male to discourage multiple matings," said Daniel DeMartini, the
doctoral student at UC Santa Barbara, who discovered this feature.
"The white stripe is turned on so it looks like the female has a
testis. She may do this to protect the survival of her fertilised
eggs, but that is just a suggestion."
The authors of the study theorise that the ideal number of
matings might be more than one, so the females wouldn't immediately
switch on the bogus gonads. They might mate multiple times so that
their eggs are fertilised by a few different fathers to increase
their genetic diversity and make the offspring more likely to
survive under a range of conditions.
In addition to observing this defensive display, this was the
first time that researchers have observed switchable white cells
based on reflectins -- the proteins responsible for reflecting
light as colour.
Although the resulting colours are different (iridescent or
white), the underlying mechanism is the same: the cells become
switchable when activated by a neurotransmitter called
acetylcholine, explains study co-author Daniel Morse, professor of
molecular genetics and biochemistry in the university's Department
of Molecular, Cellular and Developmental Biology.
The switchable white cells (eucophores) contain some of the same
reflectins and are triggered by the same neurotransmitter, which
causes the reflectins to produce a biological nanostructure that
reflects light in such a way that we see a white colour.
The nanostructure created in the switchable white cells is
different from the switchable multi-coloured iridescent cells. In
the switchable colour cells (called iridiocytes), the arrangement
of the reflectins creates an accordion-like structure. The
switchable white cells also contain reflectins, but these are found
in a thousand or more small membrane-enclosed spherical vesicles of
different sizes and shapes. When the reflectins in both eucophores
and iridiocytes are made to condense (by expelling water), their
refractive index increases, which leads to light being reflected in
two different ways: Bragg reflection (iridescence) and Mie
Bragg reflection is similar to the light from the sheen of oil
on water, where you can see multiple colours. Mie reflection
scatters all wavelengths of light, producing white.
The discovery provides further evidence that transparent
polymers (in this case the polymers are proteins) can be used to
produce nanostructures that control the behaviour of light. By
harnessing this phenomenon, new advanced materials with optical and
electronic applications could be developed.
Morse concluded: "So the focus of our work is directed toward
new applications that translate the mechanism from the world of
proteins and living cells to the world of practical engineering
with synthetic polymers."
This is not the first attempt to harness the squid's
colour-switching properites. A team at UC Irvine has been working
to develop a squid-inspired infrared camouflage that could one day
help soldiers escape detection on night-time