Squid don’t just see with their eyes
Squid, those slippery denizens of the deep, may not only see through their enormous round eyes but it seems they can also detect light all along their bodies as well.
Researchers from the University of Wisconsin-Madison have been studying the Hawaiian bobtail squid. These 3cm long squid have ink sacs on their bellies that don't just squirt out ink but also glow - a process known as counterillumination. When predators look up at the squid from below, the outline of the squid doesn't show up as a dark silhouette but instead blends into the bright background of the oceans' surface. Margaret McFall-Ngai and her team publishing in the journal PNAS have discovered that these ink sacs are capable of not just emitting but also detecting light.
A type of bioluminescent bacteria called Vibrio fischeri live inside the squid ink sacs in a two-way symbiotic relationship that benefits both squid and bacteria. The bacteria help the squid camouflage themselves against the bright ocean surface and in return, the bacteria get a safe place to live with all the nutrients that they need.
The bacteria emit a constant glow but the squid can control how much of that light escapes by changing the shape of the ink sac tissues, rather like the iris can let more or less light into an eye. The squid ink sac even has a transparent layer across the surface that acts like a rudimentary lens controlling the direction of the emitted light. By letting more or less light through from the bacteria, squid can match themselves to the brightness of the sea surface that varies depending on how deep down they are and what time of day it is.
The question is, how do the squid know how light the ocean is around them? It now seems that they might not only use their eyes, but the ink sacs also have a role to play in detecting ambient light.
McFall-Ngai have discovered that the ink sack tissues housing the luminous bacteria contain genes that produce proteins associated with light detection, including some similar to ones found in the retina - the light detecting layer of cells in the eye. They also hooked up an ink sac to an electroretinogram - electrodes that are usually used to measure the electrical responses of a retina when light is shone on it. They found similar electric signals were generated by the squid ink sac, indicating that it is sensitive to light.
The researchers don't yet know exactly how these second light-receptors evolved. It could be a form of genetic or evolutionary "tinkering", a technical term in which existing components of a living system are reassembled and tinkered and put to use in new combinations or locations. More studies are needed to delve deeper into exactly what is going on.
This study sheds light into a remarkable symbiotic relationship between bacteria and large animals, something that is particularly important for us humans to understand since we rely on trillions of bacteria living inside us to keep us healthy. We may not glow brightly at night, but eight out of ten of our major organs have some sort of bacteria living in them.