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The eyes of octopuses and squids are a wonder to scientists and evolutionary biologists, because they are similar to the eyes of vertebrates and yet they have been shown to have evolved independently. Both eyes have evolved lenses and similar muscle structures, however, the eyes of octopuses and squids have no blind spot (a trait only expressed in vertebrate eyes, and shall be discussed on the next page). So how does a lens-containing eye evolve from a cup-like or pinhole eye? What are the advantages and disadvantages of the eyes of squids and octopuses?



The Evolutionary Theory of Lens Formation

Many aquatic organisms have cup-like eyes, which have a vitreous mass which helps to protect their photoreceptors from seawater, and damaging particles in the seawater (Patton). Now let us consider a mutation that causes this vitreous mass to have a higher refractive index than the water (this can happen when a more dense protein is produced than normal). This would allow for slightly better vision, which could be a big advantage in a competitive environment such as the sea.

Of course, one might ask, what good is a bad lens (such as those which first evolved)? Well, a bad lens is better than no lens at all, and through natural selection the eye would gradually form better and better lenses, until the point at which we see lenses in eyes today. This theory accounts for the different lenses in the eyes of aquatic animals which have more spherical eyes, and terrestrial animals which have a biconcave-shaped eye.




The Advantages and Disadvantages of Octopus and Squid Eyes

There are a couple of advantages and disadvantages of possessing the eyes of squids and octopuses. The first advantage of the eye is that it has no blind spot. This means that squids and octopuses can see everything that is going on in their environment, and are more aware of predators and prey than some vertebrates. Also, have many more photoreceptors than vertebrates, at roughly 20,000-50,000/mm2 which means that their vision is much better than any human (Patton). The disadvantage of the squid and octopus eye is that it can not see in color. The eyes possess no cones, only the vertebrate equivalent of rods. This means that squids and octopuses can only distinguish between light and dark.