The asymmetric eyes of strawberry squid (Histioteuthis heteropsis) spot different sources of light in the deep sea. [Image: Kate Thomas/Duke University]
Behavioral studies conducted by U.S. researchers indicate cockeyed squid evolved dimorphic eyes to see two different sources of light—bioluminescent pinpoints from the dark ocean below and shadowy outlines in the diffuse sunlight above (Phil. Trans. R. Soc. B, doi: 10.1098/rstb.2016.0069). The team, made up of scientists from Duke University, USA, and the Monterey Bay Aquarium Research Institute, USA, based its conclusions on more than 150 marine videos of the squid, and tests using visual simulations.
An eye for a purpose
Cockeyed squid have a unique look. “You can’t look at one and not wonder what’s going on with them,” says lead author Kate Thomas. The squid’s left eye is large with an elongated, tubular shape, while the right eye is small and more inconspicuous.
In hopes of finding out “what’s going on” visually with the squid, Thomas and colleagues from Sönke Johnsen’s sensory biology lab at Duke viewed in situ video footage of the creatures collected over the past 25 years from remotely operated vehicles in the Monterey Submarine Canyon off the coast of California. There they observed the eye orientation of 152 Histioteuthis heteropsis and nine Stimatoteuthis dofleini squid.
H. heteropsis and S. dofleini dwell in the ocean’s mesopelagic region, 200 to 1000 meters below the surface. Their dimly lit home has two very different sources of light: sunlight and bioluminescence. Johnsen’s team says that in the mesopelagic region, light is either coming down through the water from the sun, or up from the dark depths of the ocean from bioluminescent organisms. This directionality makes viewer orientation and eye structure important factors in mesopelagic-region vision.
In watching the videos, the scientists noticed that the squid positioned themselves to have their larger left eye facing upward, while their smaller right eye faced downward. The researchers say that this orientation makes sense visually, as images produced by sunlight filtered down through the water are easier to detect with the larger eye that is oriented upward. Bioluminescence detection is optimal where sunlight is minimized, so downward viewing increases the contrast of bioluminescent signals against the dark waters.
Visual modeling supports asymmetric vision
Visual modeling results support the scientists’ conclusion that the dimorphic eyes of these two species of squid are a response to the multidirectional light field of the mesopelagic region, and not an adaptation to variations in light brightness.
Interestingly, the team’s models showed that a larger eye oriented downward can detect bioluminescence as well as the smaller eye. However, the smaller eye functions at a fraction of the “metabolic cost.” (Eyes are biologically expensive to grow, maintain and use.) “So while larger eyes can improve both sensitivity and resolution,” say the authors, “selection probably favors an eye just large enough to perform a necessary visual task but no larger.”