Octopus Eyes Lack Blind Spot Due to Unique Nerve Placement, Highlighting Convergent Evolution

A recent social media post by Kane 謝凱堯 highlighted a fascinating aspect of evolutionary biology: the independent development of eyes in humans and octopuses, with a key difference in their optical nerve arrangements. "Humans and octopuses evolved eyes independently, but we got ours with optical nerves attaching to the front of the retina and octopuses through the back so we have a random blindspot and they don’t," the tweet stated. This anatomical distinction results in octopuses possessing a visual field free of the blind spot inherent to vertebrate vision.

The human eye, a product of millions of years of evolution, features photoreceptor cells that are oriented away from incoming light, necessitating that the optic nerve fibers run across the front of the retina. These nerve fibers then converge at a single point, the optic disc, to exit the eye and transmit signals to the brain. This exit point lacks photoreceptors, creating a physiological blind spot in our vision.

In contrast, cephalopod eyes, including those of octopuses, developed with photoreceptor cells oriented towards the light, allowing their nerve fibers to route behind the retina. This posterior arrangement means the optic nerve does not need to pass through the photoreceptor layer, thus eliminating the blind spot found in vertebrates. Both human and octopus eyes are classic examples of "camera-type" eyes, featuring an iris, lens, and retina, yet their developmental pathways and internal wiring differ significantly.

This phenomenon is a prime illustration of convergent evolution, where unrelated species independently evolve similar traits to adapt to similar environmental pressures. Despite their last common ancestor existing over 500 million years ago, long before the development of complex camera eyes, both lineages arrived at a highly effective visual organ. Scientists continue to study these differences, noting that while human eyes offer color vision, many cephalopods are believed to be color-blind, perceiving the world in black and white.

Research into cephalopod eyes also offers insights into eye development and ocular diseases, as their ability to regenerate eyes provides unique opportunities for studying how such complex structures are formed and maintained. The distinct evolutionary paths underscore the diverse solutions nature finds to solve similar biological challenges, with the octopus eye often cited as a more "efficient" design in terms of nerve placement.