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Squid eyes, human eyes, and evolution

tonmo

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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...
 

monty

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tonmo said:
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?

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.

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:
 

um...

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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).
 

um...

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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?
 

OB

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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? :biggrin2:

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

tonmo

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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.
 

um...

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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.
 

DHyslop

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um... said:
What might they be looking for? Land? Weather (if they even care)?

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
 

tonmo

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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.
 

monty

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thanks

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

:notworth: :read:
 

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