This is the blue-rayed limpet.
It is a tiny mollusk that lives
in kelp beds along the coasts of
Norway, Iceland, the United
Kingdom, Portugal and the
Canary Islands. These organisms
may be small, their size is
comparable to that of a human
fingernail, but they have a
unique and noticeable feature:
bright-blue dotted lines that
run in parallel along the length
of their translucent shells.
Now, scientists at MIT and
Harvard University have identified
the two optical structures within
the limpets shell that give its
blue-striped appearance. First,
the researchers scanned the
surface of the limpet shell and
found no structural differences
in areas with and without the
stripes; an observation that led
them to think that perhaps the
stripes arose from features
embedded deeper in the shell.
To get a picture of what lay
beneath the researchers used
a combination of high-resolution
2-D and 3-D structural analysis
to reveal the 3-D nano-
architecture of the photonics
structures embedded in the
limpet's translucent shells.
In the regions with blue stripes
the shells top and bottom layers
were relatively uniform, similar
to the shell structure of other
mollusks. However, about
30 microns beneath the shell
surface the researchers noted a
stark difference. In these
regions the regular platelets of
calcium carbonate morphed into
two distinct structural features:
a multilayered structure with
regular spacing between calcium
carbonate layers, resembling a
zigzag pattern, and beneath this
a layer of randomly dispersed
spherical particles. Through a
series of measurements and tests
the researchers found that the
zigzag pattern acts as a filter
reflecting only blue light.
As the rest of the incoming
light passes through the shell
the underlying particles absorb
this light, an effect that makes
the shell's stripes appear even
more brilliantly blue.
The researchers say such natural
optical structures may serve as
a design guide for engineering
color-selective, controllable,
transparent displays that require
no internal light source and
could even be incorporated
into windows and glasses.
