If the squid had her way, nothing!
Cephalopods are good at camouflage – their color and texture-changing skin is one of their claims to fame. This is all well and good for species of animals who live on or near the ocean floor, where there are things to blend in with – in the open ocean, however, different camouflage techniques are needed to keep out of a potential predator’s sights.
In the case of some squid (Loligo opalescens, and probably other species in the family of squid they belong to,) the answer to this problem is, in part, their ability to reflect light.
One of the ways that cephalopods can match the color of their surroundings so well is by simply reflecting light from their surroundings – by reflecting the colors of their surroundings using a layer of white pigment-filled organs that sit in one of the lowest layers of their skin, octopuses can blend into a green or blue background, even though their skin has no green or blue pigment.
Lolignid squids utilize this strategy, too, but they have something that is a bit more difficult to hide.
They have large eyes (compared to their body size, some of the biggest in the animal kingdom), and because they need to see out of them, they can’t have an opaque covering over them to reflect light. The authors of a new paper have described how the covering of a squid’s eyes reflects light in such a way as to camouflage them, helping to keep what would otherwise be big black spots from giving away the squid’s position to predators.
By removing the tissue that covers the squid’s eyes (which the authors call “silver tissue”), shining light on it, and measuring the reflectance, the authors found that it reflects a large amount of light (though not all of it.) In addition, it reflects the same amount of light at any angle, meaning that if a predator circles a squid, it won’t see any changes in the brightness of the squid’s eye – ideally, the amount of light reflecting off of the squid’s eye will match the light filtering through the water around it, and the eye will blend into the water.
The researchers looked at the structure of this tissue under a microscope, and found something interesting – the cells in this tissue basically protein-filled spindles that pack together in a structure that is specialized for reflecting light in a certain way. As with the cells that make up the lens of the human eye, they have no nuclei (which are normally found inside the cell) – there were nuclei in the tissue, but they were outside the cells! The authors speculate that these nuclei were pushed out of the rod-like cells in order to help them serve their purpose as reflectors. In the microscopic picture below, you can see the cells (those rod-like things in the upper part of the picture) and the nuclei they have presumably pushed out (those green blobs in the lower part of the picture.)
The dense proteins of the cells and the watery fluid they sit in form a mosaic, in which different pieces have different optical properties, creating what’s called a Bragg reflector. Each time light passes from one medium to another (from the cells to the fluid, or vice-versa,) some of it is reflected – the result of this is that light of a wide range of wavelengths is reflected almost perfectly from the structure, while some light is still allowed to pass through.
The authors are sure to point out why the reflective properties of squid eyes are useful things to study – it turns out that Bragg reflectors, which are made of very delicate assemblies of various materials, are somewhat difficult to make. The squid, however, manages to put together a large, virtually flawless reflector using what appears to be only a few materials in a very simple arrangement. If we can figure out how they pull this off, we might find a better way to go about building Bragg reflectors of our own.
Holt AL, Sweeney AM, Johnsen S, & Morse DE (2011). A highly distributed Bragg stack with unique geometry provides effective camouflage for Loliginid squid eyes. Journal of the Royal Society, Interface / the Royal Society PMID: 21325315