Squid eyes, human eyes, and evolution

Discussion in 'Physiology and Biology' started by tonmo, Oct 14, 2005.

  1. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    I'll be very direct with my question:

    In the context of evolution, can someone explain to me how a squid's eye (esp. Architeuthis) can so closely resemble human eyes?

    I am definitely not at all intending to spark any debate on "intelligent design" :roll:, but I'm interested in what the ideas are behind how two creatures from completely separate evolutionary paths could share such similarities in something as complex as eyes.

    Am I correct in that Architeuthis has an "eyelid"? Or am I making that up?

    Wondering why some sea creatures with eyes have eyelids and some don't.

    /in a wondering mood...
     
  2. monty

    monty Colossal Squid Staff Member Supporter

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    I have no idea about eyelids, so I'll leave that to someone else.

    The similarity between eyes is frequently referred to as a prime example of "convergent evolution." The general idea is that the survival benefits of having an eye with particular features are so significant that any adaptation that leads to those features will be selected for very strongly, so even when two species have arrived at the basic plan for an eye (or whatever) through independent evolution, there will be a very strong pressure to reach a similar morphology, because that morphology conveys a very definite advantage.

    Although cephalopod eyes' gross morphology is similar to vertebrates, there are a number of salient features that point to their independent origins. Vertebrates have "inside-out" retinas, while cephalapods have more sensible ones. Vertebrates tend to have rods and cones (or similar) packed onto a roughly hexagonal grid arrangement, while cephs have sort of rectangular photoreceptors (rhadomeres or something like that) packed into square-ish grids. There is some difference in the focusing arrangement of the lense as well.

    All this makes it interesting to ask what is "random feature" versus "actually important for eyes"-- it seems likely that the lense, pupil, and basic retina arrangement is so effective that it's been developed independently in cephs and verts, while some of these other aspects are "any way you do it works fine," so they developed independently and were preserved.

    There is another major factor, though-- eyes of some sort were apparently part of the base body plan-- some of the most surprising results of the early HOX gene research was that creatures as different as fruit flies and people had the same genes code for eyes, so eyes of some primative sort developed in the common ancestor of all of the bilaterally symmetrical critters (and it looks like perhaps some of the radiates as well). So, there may be some aspects of the primative eyes of our common ancestors that biased the development of advanced eyes in some particular direction.

    It's particularly interesting to look at the eyes in nautilus, because they seem to be much more primative than most ceph eyes, yet they have some of the essential features of modern eyes, like a pupil of sorts, but not a proper lense or a sealed eyeball. I don't know of any examples in the vertebrates that are "living fossils" that show the "missing link" between primitive eyes and the modern eye structure.

    Come to think of it, I wonder if there is any primitive eye in the sea squirts that are primitive chordates-- since they are often touted as representative of the ancestral vertebrates, and it's known that vertebrates and arthropods (and I assume cephalopods) evolved from a common ancestor that had the same homeobox gene controlling eye creation, it seems like sea squirts should have either eyes or some evidence that they lost their eyes based on their sessile lifestyle or some such.:cyclops:
     
  3. um...

    um... Architeuthis Supporter

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    Here's a little about cephalopod eyelids.

    Here's a little about Pax-6, a gene involved in the induction of eye tissue in everything from arthropods to molluscs to vertebrates. Note the nifty schematic of eye development in cephalopods vs. vertebrates, emphasizing the aspect of convergence in a process induced by a homologous gene (I think).
     
  4. um...

    um... Architeuthis Supporter

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    Monty, I remember reading somewhere that some ascidian larvae (and possibly adults) have ocelli or other photoreceptor organs in or near their brains. That would make sense, although I'm not sure what an adult would do with such a thing.

    What about cephalochordates?
     
  5. OB

    OB Colossal Squid Staff Member Moderator

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    Just my two cents worth on the eyelid query.

    In marine life, the eyelid obviously serves no purpose in keeping a cornea humid and therewith translucent, but it does serve a purpose in protecting the eye itself. If we move into the chondrichtyes, we see that for instance many sharks have eyelids that close, or will roll away the soft part of their eyes, during vulnerable moments such as attacking a prey; the prey might otherwise damage the predator's eyes during the event whilst defending itself against being eaten.

    I would count this amongst major selection pressures to gear evolution towards protective measures. In other words: the greater the chance for a species to get into risky situations with its eyes, the bigger the chance of it to evolve eyelids.

    Plus they add character, evolution is a mating game, remember? :grin:

    On a second note, regarding the development of the cephalopod eye, what do gastropod eyes teach us?
     
  6. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    Interesting stuff here, thanks!

    Still on the matter of eyelids and sea creatures, I have a book on humpback whales (who also have lids (not sure if there are any mammals who don't have lids)), and there is a section devoted to the question of why they breach. Some have suggested they breach in order to survey what's going on in the world above them, and this was supported by the observation that they do not close their eyelids while breaching, and that the eyes do appear to be surveying.
     
  7. um...

    um... Architeuthis Supporter

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    :feet:

    What might they be looking for? Land? Weather (if they even care)? What's the name of the book? I need some cetacean-oriented reading material.
     
  8. DHyslop

    DHyslop Architeuthis Supporter

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    That spaceship from Star Trek IV.

    I really don't know much about cetaceans, either. There's a lot of hype in the media about how intelligent they are. If this is true, perhaps they like to look around above water for the same reason we like to look around below.

    Dan
     
  9. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    It's a great book -- I'll dig it up to get the name and author, don't recall off-hand. It also talks about their technique for swimming under their prey (e.g. krill) , emitting millions of tiny bubbles to stun them, and then they shoot upwards, mouth agape, consuming loads of food (and water along with them). It's probably a bit dated, though. Bought it on our Hawaiian honeymoon in '94.
     
  10. monty

    monty Colossal Squid Staff Member Supporter

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    thanks

    great references, um... -- the PAX-6 paper especially was fascinating!

    :notworth: :read:
     
  11. Fujisawas Sake

    Fujisawas Sake Larger Pacific Striped Octopus Supporter Registered

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    Tony,

    I wrote a paper on the Orientation and Navigation of Mysticete whales when I took mammalogy a few years ago. It is known that Gray Whales use a lot of geographic navigation by "spy-hopping" and checking out the coastline. Humpbacks will do this as well, but the reason they actually breach may serve other purposes as well. Since the use sound as a means of long-distance communication, breaching may help in signaling to members of the pod, as well as to long-distance stragglers. Also, it may serve to observe the surface. And It might just be a lot of fun for them as well. I wish I still had the paper; I would like to read any followup papers on the subject - maybe I still have it somewhere.

    As far as the cephalopod eyes go, there are a few threads already started on the physio/bio section. Great question though; gives convergent evolution a real shot in the arm.

    John
     
  12. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    Yep -- this book mentions those theories as well (i.e., communicating, "having fun", etc.).

    I found my copy. Here's the link to it on Amazon.com -- it is a newer edition than the one I own:

    Hawaii's Humback Whales, by Gregory D. Kaufman and Paul H. Forestell
     
  13. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    Interesting, this brief thread is the 8th most-viewed thread ever on TONMO.com (54779 views), although I can't seem to find a backlink... I imagine someone or something linked into this thread at some point along the way. Anyway, I just read it again, and am giving it a bump because it's a good one!
     
  14. CaptFish

    CaptFish Colossal Squid Staff Member Moderator

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    Its comes up as the first hit if you do a google search containing squid eyes and human eyes
     
  15. tonmo

    tonmo Titanites Staff Member Webmaster Moderator

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    Nice, or just, "squid eyes" will do it. Good for this thread! Gee, with so many views, you'd think one person would have a follow-up question... or maybe they just got completely lost as they read through this :smile:
     
  16. monty

    monty Colossal Squid Staff Member Supporter

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    I just found this while investigating searching for this thread:

    http://genome.cshlp.org/content/14/8/1555.full

    These guys seem to think genetics shows that ceph and human eyes share a lot more gene expression than would be consistent with convergent evolution, so the last common ancestor must have had some sophisticated eye gene expression.
     
  17. Rachael

    Rachael Larval Mass Registered

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    Just as an aside, I have a question realted to cephalopod eyes, does anyone have any views on how fast cephalopods adapt their retina morphology to different environmental conditions, such as culturing eggs in complete darkness and comparing these to eggs cultured in normal conditions, would the development time be long enough for there to be a morphological difference if eggs were exposed to these different conditions from the time at which they were laid until they hatch?
     
  18. monty

    monty Colossal Squid Staff Member Supporter

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    What aspect of retinal morphology did you have in mind? I have a reference somewhere about octopuses adapting to dark conditions by migration of pigment in their photoreceptors, which happens on a timescale of minutes. I'll try to find it when I get home tonight. It sounds like you're more interested in permanent developmental changes in the retinal, though, which I have no idea about, I'm afraid.

    A google scholar search found this paper:

    http://jeb.biologists.org/cgi/reprint/185/1/1.pdf

    which probably has the one I have in its bibliography, and might be good reading is the screening pigment is of interest.

    edit: The paper I had at home is J.Z.Young 1963 Light- and Dark- Adaptation in the Eyes of Some Cephalopods Proceedings of the Zoological Society of London vol 140 part 2 March 1963 p.255
     
  19. Rachael

    Rachael Larval Mass Registered

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    Thanks Monty for the paper by Gleadall, I havent read this before, so will enjoy reading it as I am interested in anything related to vision in cephalopods.

    I was thinking more permanent developmental changes, such as differences in the thickness of the outer and inner segment, also whether the components found within each segment vary for different treatments. I did a project on this for my undergraduate dissertation, my results suggested that there could be a difference between the retina morphology from different conditions, but I could not conclusively say there was due to such a small sample size and the differences in sample orientation. Hopefully I will be able to continue this project for my masters and be able to determine if there is a difference. I was just curious on peoples opinions on the rate of developmental changes, and if it would be possible to adapt in such a short time.
     
  20. DWhatley

    DWhatley Cthulhu Staff Member Moderator

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    While you are thinking about experimenting with light exposure during development of the eggs :gigas: If you had in mind seeing if a nocturnal species could become diurnal or crepuscular if exposed to light during incubation the mercatoris would be an easy animal to raise (relatively speaking). These nocturnals never adjust to daylight and I have always thought that it was because of their eyes (anecdotal hobbiest observations at best). This animal is not in short supply, lives 8-13 months and would make it possible to create a supply of aquarium bred, small octos for the hobbiest if they could be developped to forage during the day.
     

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