Fiber optic cables prove eyes of glass squids are like invisibility cloaks

Researchers from the University of Pennsylvania have used fiber optic cables to reveal how glass squids turn themselves invisible, according to a paper published in the Journal of The Royal Society Interface.

Glass squids are a type of cephalopod – a group which also includes the octopus – and encompass approximately 60 species. They lurk in the dark depths of the sea and are transparent except for a pair of visible eyes.

Prior research has managed to explain how the lack of color allows the squid to shield itself from predators swimming beneath it.

Using a technique called counter-illumination, the eyes of the squid can also appear transparent, so it does not cast a shadow when light shines onto the surface of the water. Predators look for silhouettes to spot prey swimming above them.

The new research shows that the Galiteuthis squid can also completely mask itself from predators swimming at the same depths by channeling light in a similar way that fiber optic cables do, effectively throwing up a cloaking device.

The researchers studied the squid's photophores – light-producing organs beneath its eyes – through a microscope and found they are made out of cells that have a bend in their structure, allowing light to be reflected from their surface. Any light channelled onto these cells leaks out, since the reflectivity isn't perfect, much like the inside of fiber optic cables.

Fiber optic cables behave like mirrors, with high reflectivity to carry light across without losing any signal, but are not completely leak-proof.

The researchers predicted that the squid's photophores act like an invisibility cloak, scattering light rays away from its eyes. Their hypothesis was proved correct when they decided to simulate the squid's photophores using fiber optic cables under water.

First, they built fiber optic cables with a similar reflectivity as the squid's photophores so they leaked the same amount of light. Next, they changed the light levels in a tank of water to simulate the squid's surrounding environment.

The researchers found that the light signals from the fiber optic cables and water matched, which proved that light leaking from the photophores acts like a camouflage for the squid's eyes – so that its whole body is then transparent. ®