Question

how similar are mammal eyes to the invertebrates' cephalopodas?

Answer 1

Mammal and cephalopod eyes are quite different. The eyes of mammals, such as humans, show a number of evolutionary "defects". In mammalian eyes, the light sensing cells (rods and cones) are actually pointed away from the aperture where light enters the eye. There is a layer of pigmented cells that reflect light that enters the eye back onto these cells. In cephalopods, the light sensing cells point directly at the aperture and capture the light on the way into the eye, allowing them to see in much darker conditions.

In mammals, since their rods and cones are pointed the other way, the optic nerve blocks light to some of the cells. This is your "blind spot" where the brain uses information from other light sensing cells to fill in the blank area. Cephalopods have an optic nerve, but it does not come between the light sensing cells and the light source, so no blind spot.

Cephalopods also tend to have a somewhat different overall eye design. Their eyes are more cone shaped to focus light, where ours are round.

They are similar, however, in that they both use rhodopsin. This protein is highly conserved across many animal kingdoms, and although the eye has evolved multiple times, as far as i know all light sensing proteins are close relatives to the rhodopsins that we have in our eyes.

There is lots more to this im sure, but i think i tapped out my knowledge on the subject... anyone else got more?

Answer 2

you have given me a great answer! I just have a few more questions. What is an aperture, and how does the cephalopods see much better when thier light sensing cells capture the light with it?

Answer 3

By aperture, i meant the clear part of the eye (pupil, lol couldn't remember the word for some reason) where light comes in. The cephalopod eye captures more light because the light comes in and hits the cells directly. In mammals, they are pointed away from the pupil and the light bounces off the back of the eye first, so a good deal of the light intensity is lost. Hope that clears it up!

Answer 4

Why does it work like that in mammals? Wouldnt it be better if the cells caught the light instantly

Answer 5

of course it would, but evolution is not perfect. This is one of the reasons why most scientists laugh when people doubt evolution. The eye has evolved multiple times in a convergent fashion. All eyes are somewhat similar (im talking about full eyes, not simple light sensing organs or the like) but differ in many structural details. Our ancestors evolved an eye in this fashion and since evolution cannot go back and correct something. We are stuck with the eyes that our ancestors developed.

Its an interesting subject, if you want to know more read Carl Zimmer (i should get paid by him, im always advertising his books). His book Evolution: The Triumph of an Idea has a lot of good stuff about things like this.

Answer 6

but there must be some advantage over why we bounce before we collect light? Right?

Answer 7

nope, no advantage really. just how things evolved. you could say that being terrestrial we don't need to see in such dark conditions, but thats not really accurate. cephalopods can control the light coming into their eye just as well as mammals can can surface if they wish and see just fine. its just another roll of the Darwinian dice. we got crappy eyes compared to cephalopods, but hey they work fine so don't worry about it.