Typically in the reflector there are tens to a hundred of these multiple layers. A reflector like that should act just like the surface of water or a wet road – when it reflects light it should polarise it.
Dr Nicholas Roberts
As light waves travel from the sun, they can oscillate in all directions; there’s an equal chance that this oscillation may be horizontal or vertical. However, when it hits a reflective surface, the waves tend to bounce off this surface only oscillating horizontally. This phenomenon is described as polarisation, and is one of the basic laws of physics.
A study reported this week in the journal Nature Photonics
describes how silvery fish such as sardine and herring have developed an adaptation which circumvents this rule in order to escape the attention of predators in the ocean.
The work was conducted by PhD student Tom Jordan under the supervision of Professor Julian Partridge and Dr Nicholas Roberts from the Bristol Centre for Complexity Sciences (BCCS)
at the University of Bristol
. It has uncovered the specific optical properties of the skin of silvery fish which make abnormally high reflectivity possible.
I spoke to Dr Roberts in an effort to find out more about the background to this study, the structures that allow silvery fish to defy physics, and the potential usefulness of these findings in manmade optical devices. Since the 1960s, some research has been conducted into why silvery fish are silvery and into understanding how their appearance helps to camouflage them in open water.
"It was noticed in the early measurements that the way silvery fish reflect light isn’t quite typical," Dr Roberts began. "It doesn’t behave the way one would normally expect of that type of reflector, but this was never properly investigated. We wanted to explain why the reflectors have the optical properties that they do and what the underlying mechanism is."
The skin of these types of fish is made up of extremely thin layers of guanine crystals – at several hundred nanometres thick, these are approximately the same width as the wavelength of light – with cytoplasm-filled gaps between. Although the team were aware of this particular structure, it was originally thought that this structure would fully polarise light and cause a reduction in reflectivity.
"Typically in the reflector there are tens to a hundred of these multiple layers," Dr Roberts explained. "A reflector like that should act just like the surface of water or a wet road – when it reflects light it should polarise it.
"When light is reflected from the side of a fish, however, it doesn’t polarise like a normal reflector. Any light that goes in comes out exactly the same way, so the term that we use to describe this surface is ‘polarisation-neutral’. We didn’t know how this multilayer structure worked and what the mechanism was that allowed it not to polarise light."
The researchers found that the skin contains two different types of guanine crystal, each with different optical properties.
"The skin of fish is made up of multiple layers," Dr Roberts went on. "The scales sit on the top and sometimes there is a layer of dermis over them. There are some guanine crystals on the back of the scales, but if the scales are pulled off it still looks just as silvery. The layer that we’ve been looking at is actually at the back of the skin and is called the ‘stratum argenteum’. That’s where these multiple layers of guanine and cytoplasm exist."
The guanine crystals in the stratum argenteum possess different properties compared with some of the crystals on the back of the scales. By mixing the two types of crystal, the fish's skin no longer polarises reflected light but rather maintains its high reflectivity. When non-polarised light lands on a reflector and becomes polarised as it is reflected, there is a subsequent drop in reflectivity: not all of the light is returned.
"If you put a normal handheld mirror under water, if the light reflected off the front of it is exactly the same as the light coming from behind it, it disappears," said Dr Roberts. "It acts as a perfect form of camouflage."
For a fish trying to evade a predator that could potentially catch a glimpse of it from all sorts of angles, emulating a mirror is a much better way of trying to hide in open water than acting as a normal reflector would be. There is a strong evolutionary basis for selection for this kind of reflector over and above the normal multilayer reflectors of the type found in iridescent beetles and butterflies, for example.
"For a fish trying to camouflage itself in open water, it wants to be able to be a perfect reflector from any angle. The normal polarising mode of reflection doesn’t allow that. For a polarisation-neutral reflector, then anything that comes in is reflected off and there is no associated drop in reflectivity – it can act as a perfect mirror."
Matching the light environment of open water in this way could provide an effective mode of disguise for small fish trying to evade those higher up the food chain, such as dolphin and tuna. It could also provide clues for the development of more efficient and effective optical devices. Manmade reflectors such as low loss optical fibres often use similar non-polarising reflectors, but at the moment they use materials with specific optical properties that are not always ideal. I asked Dr Roberts about the potential applications of their fish-related findings.
"I should say this with the caveat that I’m a biologist, not an engineer, but these types of reflectors are already in use as, for example, back-reflectors on light-emitting diodes (LEDs)," he replied. "Just as a fish wants to maximise its reflectivity over all angles of instance, we want exactly the same from an LED to get the most light and most efficiency out of it.
"However, there is a certain design constraint that requires the optical properties of whatever is outside the actual LED part to be different from the equivalent cytoplasm layers in the fish skin structure. This effectively screens out some of the angles of instance of the light coming in; it reduces the polarising effect that way. The implication of the different mechanism that the fish have for doing this is that you can build devices with the same material on the outside as on the inside."