<http://news.discovery.com/animals/fish-break-law-of-physics-become-in...
e-121021.html> Fish can cloak, become invisible to predators

Silver-colored fish, such as herring, sardines and sprat, are bending the
laws of physics, according to a new study published in Nature Photonics. The
ability allows the fish to become invisible to predators.

As researchers Tom Jordan and Julian Partridge from Bristol University
explain, reflective surfaces polarize light, a phenomenon that fishermen or
photographers overcome by using polarizing sunglasses or polarizing filters
to cut our reflective glare.

Jordan and Partridge, however, found that silvery fish have overcome this
basic rule of reflection.

The fish's skin contains multilayer arrangements of reflective guanine
crystals. (Here's a factoid: guanine is also one of the key components of
guano, aka bird and fish poop. The terms originate from an ancient word for
dung, "wanu.") It was previously thought that fish skin would fully polarize
light when reflected. As the light becomes polarized, there should then be a
drop in reflectivity.

But that's not what always happens, as it turns out.

The researchers found that the skin of sardines and herring contain not one
but two types of guanine crystal. Each has different optical properties. By
mixing these two types, the fish's skin doesn't polarize the reflected light
and maintains its high reflectivity.

The result is an optical illusion that can make the fish at times seem
invisible to other marine dwellers.

"We believe these species of fish have evolved this particular multilayer
structure to help conceal them from predators, such as dolphin and tuna,"
Roberts was quoted as saying in a press release. "These fish have found a
way to maximize their reflectivity over all angles they are viewed from.
This helps the fish best match the light environment of the open ocean,
making them less likely to be seen."

In future, fish skin might inspire inventors to create better optical
devices.

Jordan explained, "Many modern day optical devices such as LED lights and
low loss optical fibers use these non-polarizing types of reflectors to
improve efficiency. However, these man-made reflectors currently require the
use of materials with specific optical properties that are not always
ideal."

He concluded, "The mechanism that has evolved in fish overcomes this current
design limitation and provides a new way to manufacture these non-polarizing
reflectors."

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