The eyes have it for deep sea squid’s survival
23 Feb 2017 by Evoluted New Media
The first behavioural evidence for why a deep sea squid has developed an ocular oddity has been collected by researchers in the US.
The first behavioural evidence for why a deep sea squid has developed an ocular oddity has been collected by researchers in the US.
Histioteuthis heteropsis – also known as the strawberry squid – has one ‘normal’ blue eye and another giant bulging yellow eye. Histioteuthis lives in the mesopelagic zone of the ocean, 200-1000m below the surface. The squid drift through the ocean with their head down and tail up, almost vertically, with their big eye orientated upwards and the small eye looking downwards.
Professor Sönke Johnsen, from Duke University and senior author of the study, said: “The deep sea is an amazing natural laboratory for eye design, because the kinds of eyes you need to see bioluminescence are different from the kinds of eyes you need to see the basic ambient light. In the case of Histioteuthis, they chose one eye for each.”
By combing through more than 150 undersea videos and creating visual simulations, the researchers believe that the large eye is specifically adapted for gazing upwards. The squid’s other smaller eye scans deep, dark water for bioluminescence. The light that reaches these depths is extremely dim, monochromatic blue and often flashes of bioluminescence are brighter than the light from above.
Kate Thomas, a biologist at Duke involved in the research pored over 30 years of videos collected by the Monterey Bay Aquarium Research Institute (MBARI). This video footage, collected by remotely operated vehicles, had documented 152 sightings of Histioteuthis heteropsis.
Using computer models to understand how changes in eye size might be beneficial, Thomas increased the size of its upward-facing eye by a small amount – she found this greatly improved its sensitivity to dim light. But this was not repeatable when the size of the downward facing eye was increased.
Thomas said: “The eye looking down really only can look for bioluminescence. There is no way it is able to pick out shapes against the ambient light. And once it is looking for bioluminescence, it doesn't really need to be particularly big, so it can actually shrivel up a little bit over generations. But the eye looking up actually does benefit from getting a bit bigger.”
The research was published in Philosophical Transactions B.
