evolution of the eye

[Richard]

I was reading about giant squids on wikipedia after a show on TV sparked my interest, and was reminded they fall under mollusca. Now this made me think: Why do cephalapods, gastropods etc have eyes, and bivalves etc not have them? (yet they're ALL molluscs)
SO I went and looked at http://en.wikipedia.org/wiki/Bilateria and also noticed that the phyla Chordata (vertebrates) and Anthropoda (insects, arachnids, crustaceans etc) all consist of eyed species too.

Looking at the layout of the phylogeny on that page, how can this be? Does it mean all the 20 or so other phyla (and indeed, non-eyed molluscs) in the Bilataria subregnum have the DNA to...get eyes?

[bdh]

I was reading about giant squids on wikipedia after a show on TV sparked my interest, and was reminded they fall under mollusca. Now this made me think: Why do cephalapods, gastropods etc have eyes, and bivalves etc not have them? (yet they're ALL molluscs)
SO I went and looked at http://en.wikipedia.org/wiki/Bilateria and also noticed that the phyla Chordata (vertebrates) and Anthropoda (insects, arachnids, crustaceans etc) all consist of eyed species too.

Looking at the layout of the phylogeny on that page, how can this be? Does it mean all the 20 or so other phyla (and indeed, non-eyed molluscs) in the Bilataria subregnum have the DNA to...get eyes?

The species preceding those creatures probably had some sort of eye patch/eye but became vestigial and evolved out in some of the species but was developed in other species into a full fledged eye.

[Dr. Dwermer]

I'll check the records, but here are my preliminary thoughts:
-If all molluscs have them, the bivalves likely simply have the inactivated gene.
-Converseley, gastropods and cephalopods may have later evolved eyes following the split from the common ancestor.

[Richard]

But if it was convergant evolution, how then did some molluscs, along with ahtropods and chordata evolve such similar features? The chordata (us) eyes and some mollusc eyes (particularly squid etc) are remarkably similar to have evolved simply from photoreceptor/eyespots which are inactive in most of the other animal phyla (including the ones that molluscs, chordata, and athropods evolved from)

[Dr. Dwermer]

Richard, you are asking great questions. Now, I know the discussion board is a wonderful place to talk and I would never discourage you from asking a question here. However, I’d recommend that you let your inquiry propel you into really seeking the knowledge at a library or on some credible informative websites. I especially recommend going to a Library, because you will usually learn a LOT more than you came to learn just by reading through the passages you find along the way to your answer. In either case, you sound like a true scientist, one who is ready to ask questions and will not be satiated until there’s a convincing answer. The last step of science is in the instance that there is not an answer; then you must start research to find the answer to the problem, and consequently answer everyone else’s answer.

Here’s what I think are the answers to your questions:

“But if it was convergant evolution, how then did some molluscs, along with ahtropods and chordata evolve such similar features?”

http://www.pnas.org/cgi/content/full/94/6/2421
I stumbled upon a research paper that answered a question. It appears that the mollusk eye and vertebrate eye is a consequence of parallel evolution as opposed to convergent evolution. This also likely may be the case with arthropods. Yes, they are similar in function and some of their features; all have photoreceptive cells, examples of an eye “pit”, or examples of a lens can be found in both vertebrates and cephalopods. I don’t think we should discount the differences between the complex examples of the eye that indicate a different evolutionary history. You may ask “why don’t all photosensitive eyespots develop into pinhole eyes, and finally lens eyes?” It appears that for millions of years, animals are successful enough to have survived using pinhole eyes and photoreceptive eyespots. Not only that, with higher levels of taxonomic complexity usually you see a greater facilitation for the development of more complex organs, which may likely buy a strong phenotypic advantage to individual fitness. An Octopus which works in a three dimensional plane, moving at relatively high speeds when hunting, must differentiate prey quickly and efficiently and be able to track it in order to survive.

The differences are also quite poignant, and further the case that the eyes aren’t as “similar” as they seem.
-While both eyes have a “retina” of light sensitive cells, the vertebrate eye’s retina points away from incident light. The cephalopod’s retina points towards incident light. This “everts” the “inverted” picture that our retina decodes in the brain.
-The vertebrate eye also has multiple tissue layers that the light must pass through in order to reach the retina.
-The lens of both eyes, while structurally similar is anatomically different. Cephalopod eyes have a much higher incidence of latticed carbon, and calcium. I am not sure the effect of this.
-Cephalopods lack Ciliary muscles and Zonular fibers.

“The chordata (us) eyes and some mollusc eyes (particularly squid etc) are remarkably
similar to have evolved simply from photoreceptor/eyespots which are inactive in most of the other animal phyla (including the ones that molluscs, chordata, and athropods evolved from)”

Good question, it may originally be though that our eyes evolved from similar precursors, this doesn’t seem to be the case. While it is possible that the eye may have been found in early examples of vertebrates and mollusks, it is also very possible that a future loss in this organ or organ’s function may have no negative effect on fitness and be perpetuated by natural selection (see, blind cave fish).

[Richard]

Wow thanks...it was bothering me a bit that the 3 phyla may have evolved eyes indepentently, however that is a very convincing answer you gave. I thought an eye organ may have been rather unlikely to evolve via parallel evolution in 3 separate cases, but that now explains alot

thanks again!