The octopus is an extraordinary animal in many ways, but the eyes of this creature are especially incredible. When you consider that they, and their relatives the squid and cuttlefish (collectively known as Cephalopods), are related to mollusks such as clams and snails, this structure is even more amazing.
Looking at the eyes in the photo above you can see they are very similar to mammalian eyes. Although in an octopus the slit-like pupils are horizontal rather than round (like ours) or vertical (like cats). And the way these eyes work, and indeed the parts that make up the eye, are also similar to ours. This is a wonderful example of convergent evolution.
Convergent evolution occurs when two unrelated groups of animals develop similar structures independent of each other. One example is the wings of bats and those of birds. But the similarities between the eyes of mammals and cephalopods are so close it would be like if bats had independently developed feathers and hollow bones. You may wonder how we know the octopus and mammalian eye is truly convergent evolution and not just common traits inherited from common ancestors. This is because the vertebrate/invertebrate lines diverged around 500 million years ago, before either branch had eyes at all.
Very briefly, here’s how our eyes work and then we’ll look at the few differences in the eyes of these completely unrelated mollusks. When light hits our eyes it enters through the cornea and into the pupil. Surrounding the pupil is the iris, the colored part of the eye, which is blue if genetically you have little pigmentation in the iris and brown if you have more. This iris can expand or contract depending on the intensity of the light to allow more or less light into the pupil. Note, the pupil doesn’t change size, the iris does.
Behind the pupil is the actual light-capturing layer, called the retina. The retina contains two types of cells; rods, which capture the light energy, and cones, which are used to detect detail and color. Now all this information gathered by the eyeparts needs to be transferred to the brain. This is done by the optic nerve. In our eyes the optic nerve passes through the retina, which means that there are no receptors at this point on the retina and therefore this optic nerve causes a blind spot to occur. Our brains adapt to this blind spot by making us ignore it. But it’s there. Don’t believe me? Close your left eye and stare at the dot. Slowly move your head closer to the screen and when the image of the cross hits your blind spot it will disappear.
Ok, I went through all this because although an octopus shares all of the eye structures mentioned above with us, its optic nerve goes around the outside of the retina instead of through it. An octopus has no blind spot.
Does this make an octopus’ eyes actually superior to ours? No. It can get away with this setup because the rods and cones face toward the light instead of away from the light like ours. Which means to get to these cells of ours, light has to pass through much more nerve and blood tissue as it is reflected off the back of the eye, which is what helps to increase the acuity of our sight. Although an octopus has very good eyesight at close range, it is very nearsighted and cannot see a thing past about eight feet.
One other very cool characteristic of the octopus’ eye is that the placement of the pupil remains constant regardless of the position of the body. Statocysts in the brain keep track of body oriention and always keep the pupil in a horizontal position.
Octopuses are predators, mainly on crabs and shrimp, and they rely on both touch and sight to find the prey. They also are very curious animals and by observing their behaviors you can eventually get a feel for moods and emotions. The images here are of my octopus, named "violet". She got the name because a few weeks after capturing her I noticed that she would get a purple streak in the tissues surrounding her eyes whenever anyone she recognized approached her aquarium.
Another sign of recognition is when she stretches her eyeballs up into stalks, which is what she is doing in all these photos because she knows the photographer. If a stranger approaches those eyes would flatten down against her head to about the level of the lip of the jar. Those first two photos were close-ups, but here she is sitting in her favorite glass bowl. Being very secretive, these animals feel most comfortable wedged into small, cramped quarters (I found one one time stuffed into a coke bottle). They are also meticulously clean and remove all debris, crab shells and stones from their dens daily and often venture out to find special shells to hold against the opening like a door.
Because of their high intelligence, octopuses can get bored easily. For this reason I’m constantly providing new den materials for her to keep her busy. This jar, which one of my coworkers found in an antique shop, is currently her favorite. When she gets bored with that one we’ll switch it for another.
I didn’t get into the color changing ability, which is like nothing you’ve ever seen. And this is related to the eyes since this animal is able to conciously manipulate its skin color and texture at will, based on observing its surroundings, in order to camouflage or express emotions. Let me know if this interests you and I’ll add it to the poll some week once I get some good footage of her changing color.
Movies of Violet you may have missed:
Violet Kills A Crab
Stupid Octopus Tricks
Other diaries in this series can be found here.