How bobtail squid vanish before your eyes
In our rundown of top ocean light-makers, the adorable bobtail makes an appearance before disappearing before your very eyes. This glowing critter uses bioluminescence to make itself invisible...
The problem with living the 3D environment of the ocean is that a potential predator could come at you from the sides, or above or below. Many species in the oceans show countershading - where the underside of the body is coloured lighter than the top of the body, to counteract the shading from being lit from above. But some species go further. Several species of squid use bioluminescence on the underside of their body to try and match the colour of the light coming down from above, so if a predator sees them from below, they don't see a dark shape against the light ocean surface, but the squid become almost invisible.
The idea of ventral bioluminescence as a form of countershading was supported by evidence from several species but it was only when an on-board study of squid caught in the open ocean in the 1970s and exposed to light from above showed that they fluoresced on their undersides to match the light level coming from above proved that it was a strategy being used. These results have been confirmed several times by other researchers.
Some of the most recent research has been into the Hawaiian bobtail squid, which I have to say is one of the cutest squid I've ever seen - it's like Disney have drawn it specifically to look round and cute - they're also known as dumpling or stubby squid! Well, these squid have a symbiosis with the bioluminescent bacteria Vibrio fischeri, which live in special crypts in two paired areas of the squid's body known as the light organs. The light organs have a reflective inner surface close to the body, so all the light gets directed away from the inside, and they also have a lens-like structure on the outside to help diffuse the light.
By using the muscles that are also used to control its ink sac, the squid can change the size and shape of the organ and so how much light is emitted, so it can match the light coming from above and disappear!