Closer look reveals how deep ocean squid uses 'leaky' optical fibers to disappear into the background

June 8, 2016 by Bob Yirka report

(Phys.org)—A pair of researchers with the University of Pennsylvania has discovered how a type of deep ocean squid is able to remain unseen by predators despite having clearly visible eyes. In their paper published in Journal of the Royal Society Interface, Amanda Holt and Alison Sweeney describe their study of glass squid and how it has evolved to hide itself from predators that lurk in any direction.

Glass squid live in the deep ocean and every part of their body except their eyes is clear, which helps to avoid being seen and eaten by predators from below. Such predators look up to find animals that create silhouettes against the distant light striking the surface of the water. Prior research had shown that the squid use what is known as counter-illumination to keep their eyes from being seen as part of a silhouette—they have organs below their eyes (photophores) that emit just enough light to match the light that is broadcast from above, preventing the eyes from being seen from below. But what about prey that live at approximately the same depths?

That is what the research pair wanted to know, so they obtained some samples of the squid and put them under a microscope. They found that the photophores were actually made of cells that have a bend in them and that the walls were reflective—the two attributes together caused light to be channeled, like fiber optic cables. Looking even closer they discovered that the reflecting ability was not very efficient, which meant a lot of the light that was supposed to be channeled would leak out. At first, the researchers found this baffling, but then discovered that it actually served a very real purpose. In leaking small amounts of light, which turned out to be in many directions, the squid's eyes became invisible to creatures that were at or near the same water level.

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