Discover the Fascinating One-Eyed Animal: Facts and Myths

[Loren Booda]

What animal normally develops with only one eye?

 

[Another God]

Is this a trick question?


I can't think of any (since at least most animals that have eyes tend to be symetrical), but my mind is tending towards flat fish. Since they are flattened side on, I am wondering if they have lost one of their eyes...actually, no, i think their eye sockets get distorted and it comes around to be on the same side... but maybe one of them has managed to 'remove' its eye in its evolutionary past or something.

 

[Mentat]

Originally posted by Loren Booda
What animal normally develops with only one eye?

Is it a capertillar?

 

[Loren Booda]

Another God,

My guess is that a universal minimum of two eyes provides evolutionary redundancy for all-important vision in case of an accident. Don't euglena (the most primitive example of eyes I could think of) have an eye spot(s)?

Mentat,

have you heard of the Polyphemus moth? Named after the Cyclops that Ulysses killed. It has at least two real eyes, though, as do all caterpillars to my knowledge.

 

[Mentat]

Originally posted by Loren Booda
Mentat,

have you heard of the Polyphemus moth? Named after the Cyclops that Ulysses killed. It has at least two real eyes, though, as do all caterpillars to my knowledge.

Actually, I just heard of that Polyphemus moth, that's why I mentioned caterpillars. But I guess I was wrong on that one...

Do you have an answer, or are you actually asking?

 

[iansmith]

Originally posted by Loren Booda
Don't euglena (the most primitive example of eyes I could think of) have an eye spot(s)?

Yes they have an eye spot but it is an organell and euglena are unicellular organism. It allow the cell to go toward or away from the ligth. The spot actually detect where the minimun ligth is coming from.

As far as i know there is not such thing as one eye animal. 2 is the minimun requiered.

 

[Njorl]

So, did someone ask you this verbally? If so, possible answers are:

pig
chimpanzee
squid
pigeon
lion
tiger
etc.

Njorl

 

[Another God]

I'm not a fan of this "Minimum 2 required' phrase...it makes it sound like things can't live without two eyes...

There is an undoubted evolutionary advantage in having 2 eyes over one, and since we are symetrical it makes sense that everything should have 2 eyes... But I am ready to accept that it was just another evolutionary accident frozen in time on account of its practicality.

Something I learned the other week: Predators tend to have eyes on the front of their head, good for focusing on a single prey animal. Prey animals tend to have their eyes on the side of their head, good for all round vision, preparing them for attack from any side.

 

[FZ+]

Worms? We can consider the light sensitive end to be an "eye"...

 

[BoulderHead]

How could I possibly stay away from a topic about eyes?

I heard tell of a One-Eyed-Flying-Purple-People-Eater, but excluding that, I can't say I've ever heard of a One-eyed critter. Cell division, Pairing off of the sexes,..., things like to come in twos. The Fer-De-Lance often travel in pairs, so you have to on your guard if you bump into one. The "Ayes" have it...


pineal eye?; http://ebiomedia.com/gall/eyes/many.html

 

[Loren Booda]

Mentat,

I'm jest-asking. The Polyphemus moth has a prominant "eye spot" as protective coloration on each wing. Birds spear the spot, not the moth!


Another God et al.,

Are two eyes more an outcome of symmetry or redundancy needs?

Have you heard of the extreme birth defect in humans involving the development of only one eye centered at the forehead? The visual cortex there must maintain some semblance of bilateral symmetry, with one withered optic nerve. Extreme retardation and death accompany this "cyclopia."

 

[BoulderHead]

Originally posted by Loren Booda
Another God et al.,

Are two eyes more an outcome of symmetry or redundancy needs?

I cast my vote for Symmetry.

 

[Another God]

symmetry is almost certainly the cause. Once the two eyes are there though, it makes sense to make the most of them, and so in the case of predators (which our eye configuration appears to point us out to be) it gives great focus. It allows predators to have an almost perfect judge of distance around the 10m mark... Meanwhile, the prey animals have used their two eyes to create a complete view. In rthe prey case in particular, there is nothing redundant about the eyes, each eye has been made somewhat necessary. With the predator case, there is better, more accurate vision with two, but you could live with just one. Redundancy isn't a strong theme.

 

[fish]

The physical environment could influence the organism's development (not just genes)

from the book Evolution as Entropy by the zoologists Brookes and Wiley - certain species of fish, when it grows in fresh water, develops two eyes, but if you place the young fish in salt water, it grows up to have only one eye.

 

[FZ+]

Are two eyes more an outcome of symmetry or redundancy needs?

Hmm... but it seems that multi-eyes were more common with the more primative creatures of the sea, eg. jellyfish etc. At what point did they go back down to two eyes, or were the two-eyed creatures the result of a separate evolutionary development?

 

[iansmith]

Originally posted by FZ+
Hmm... but it seems that multi-eyes were more common with the more primative creatures of the sea, eg. jellyfish etc. At what point did they go back down to two eyes, or were the two-eyed creatures the result of a separate evolutionary development?

For the jelly fish it is still is symmetry. The jelly fish as a radial symmetry and has an eye for each plane, more or less.

Scientist think the eye evolved 40 times. How many times did it evolve to have 2? Also what is interresting in anthropods, arachnids have multple eyes whereas insects and crusteceans have 2 eye.

 

[Mentat]

Originally posted by Another God
I'm not a fan of this "Minimum 2 required' phrase...it makes it sound like things can't live without two eyes...

There is an undoubted evolutionary advantage in having 2 eyes over one, and since we are symetrical it makes sense that everything should have 2 eyes... But I am ready to accept that it was just another evolutionary accident frozen in time on account of its practicality.

That's the point, AG. If the "evolutionary accident" of having two eyes proved infinitely superior to having just one, then it could be considered a "minimum requirement" for surviving species, that they have two eyes (or none, like some species of fleas that live in wells).

 

[Mentat]

Originally posted by Loren Booda
Mentat,

I'm jest-asking. The Polyphemus moth has a prominant "eye spot" as protective coloration on each wing. Birds spear the spot, not the moth!

Well, I was making a joke, since "Polyphemus moth" sounded like some rare breed that nobody usually hears about...so, I say "of course I've heard of that" .

 

[Mentat]

Originally posted by iansmith
Also what is interresting in anthropods, arachnids have multple eyes whereas insects and crusteceans have 2 eye.

That makes me wonder though...some insects have "complex eyes", don't they? Wouldn't one such complex eye be sufficient, without the other?

Also, there are species of lizard (or is it chameleon?) that can see individually through each eye. That is, they can see one set of things in one eye, and another in the other eye. Thus, they can look behind them, and in front of them, at the same time. Couldn't something like this prove that it is alright to live with just one eye?

 

[NateTG]

Originally posted by Loren Booda
Are two eyes more an outcome of symmetry or redundancy needs?

How about parralax? Two eyes make depth perception a whole lot easier.

 

[hypnagogue]

I can't think of the name, but I'm almost certain I looked at something under a microscope in high school biology that was basically not much more than a flagellum attached to an eye spot.

Do even primitive multicellular organisms with eyespots have two of each? If the purpose is for something as basic as "detect light, then approach it," I don't see how two would be that much better than one...

 

[Another God]

I don't think eye spots necessarily come in pairs. I think the symetrical stuff really only starts to become apparent in the Multicellular organisms.

Nonetheless, the advantage of two over one would be the better differentiation of direction.

 

[wasteofo2]

Everyone who said symmetry is responsible for the 2 eye minimum is either overlooking something very basic or working on a level much higher than me.

True: most all animals are symmetrical, but that doesn't mean that you must have 2 of something. Mamals generally only have one tongue, penis/vagina, tail etc. so humans could be just as symmetrical with only one eye.

I didn't read every post, since many seem to repeat each other, but it seems no one mentioned how advantageous two eyes are in depth perception. It would seem, that if way back when you had a bunch of animals with one eye and the rest had 2 or more, that the 1 eyed animals would be more succeptible to being killed by predators or from not being as aware of their surroundings as their two or more eyed counterparts would be.

Aside from more than one eye giving good depth perception, I see no real reason that the number generally is 2 instead of any other number. It would seem to be even more advantageous if organisms could have many more than 2 eyes positioned so that at least 2 eyes that could see everywhere at once so you get full depth perception of your entire surroundings all at once.

 

[Another God]

Good point. Like sexual organs, I guess the eye could be in the center. Der. OK.

as for the depth perception, it turns out that we do have depth perception with only one eye (you can check this for yourself ), but i did mention the more precise nature of our depth perception at a specific distance. This distance is typically right at the perfect distance for a 'pounce'. ie: For predators who need to pounce on their prey, their eyes happen to be perfectly focused at a specific distance, aiding them in judging that distance so they jump exactly the right amount.

As for the prey animals though, having eyes on the sides of their head allows for 360° vision. An obvious advantage for the pray. They of course lack the specific depth ability.

 

[BoulderHead]

True: most all animals are symmetrical, but that doesn't mean that you must have 2 of something. Mamals generally only have one tongue, penis/vagina, tail etc. so humans could be just as symmetrical with only one eye.

Interesting, and I think it might have been nice to have had a single ear where my mouth currently is while having two mouths where my ears are. I could drink and chew a banana all at the same time.

Now I feel cheated.

 

[Loren Booda]

There once was a man from Nantucket...

 

[BoulderHead]

Originally posted by Loren Booda
There once was a man from Nantucket...

How very true!

 

[Another God]

Originally posted by Loren Booda
There once was a man from Nantucket...

... who had... a bucket??

Wha?

 

[Njorl]

I think "the man from Nantucket" would be genetically selected against. I mean, why ever go out?

There are asymmetries. There are singular internal organs that are not in the center, liver, stomach, appendix etc.

I bet external asymmetries might have been selected against. Consider the case where some are natural. An additional asymmetry would be a genetic defect, which would usually be harmful. It would be more rare for a symmetry to be the result of a harmful (or any) defect. So, when choosing a prospective mate, choosing the more symmetric one would have a better chance of producing thriving offspring.

Unless you're a fiddler crab.

Njorl

 

[Loren Booda]

Then there's the asymmetry in some unfortunate humans where almost all of the internal organs and/or systems are reversed. Does anyone know the name of this condition, and what its complications are?

 

[wasteofo2]

Originally posted by Njorl
I think "the man from Nantucket" would be genetically selected against. I mean, why ever go out?

There are asymmetries. There are singular internal organs that are not in the center, liver, stomach, appendix etc.

I bet external asymmetries might have been selected against. Consider the case where some are natural. An additional asymmetry would be a genetic defect, which would usually be harmful. It would be more rare for a symmetry to be the result of a harmful (or any) defect. So, when choosing a prospective mate, choosing the more symmetric one would have a better chance of producing thriving offspring.

Unless you're a fiddler crab.

Njorl

I actually saw some study on television where they tested the symmetry of random people against that of olympians, and the olympians were far more symmetrical than the average person, supporting your statement.

 

[Monique]

Originally posted by Loren Booda
Then there's the asymmetry in some unfortunate humans where almost all of the internal organs and/or systems are reversed. Does anyone know the name of this condition, and what its complications are?

The condition is called Situs Inversus and in principle doesn't bring any complications with it.

 

[Monique]

The prevalence of situs inversus in Scandinavia is about one in 8000, in the USA one in 11000.

 

[Robert Zaleski]

Besides those one-eyed rogues you find in every deck of playing cards, there is Cyclops Bicuspidatus, a one-eyed crustacean beauty.

 

[bobkaka]

I actually found this thread because I was wondering if there were animals with one eye.
The reason I was searching is because I was reading a PDF on my computer, and I had my hand over one eye (non-intentionally). I noticed that the text was moving in and out, having no specific depth. So I instantly discovered that the second eye is there for depth.

For those who say there is depth perception with one eye, I am afraid you are mistaken, open a PDF, read with one eye and see for yourself. Or read a book.

Peace.

 

[Andy Resnick]

My book "The Deep" is at home, but there's a picture of a fish which lives along the transition zone and has one giant eye looking down (where it's dark) and another small eye looking up (where it's bright). So, it has two eyes but does not have binocular vision.

I would think having a single eye is a disadvantage- no redundancy. If it failed, the animal would be blind.

 

[flatmaster]

If I remember correctly, there's a shrimp that lives around deep-sea vents that has a single eye on it's back.

 

[Danger]

I'm curious as to at what evolutionary point a light-sensing organ qualifies as an eye. Does it have to have a lens? A pupil? Focusing muscles?

 

[flatmaster]

I'm curious as to at what evolutionary point a light-sensing organ qualifies as an eye. Does it have to have a lens? A pupil? Focusing muscles?

Exactly, that's why i thought it was weird. I'm pretty sure it was only light-sensing. Perhaps bio-luminescent prey? There was concern as to weather the bright lights on the submarine blinded the critters for life. Also, if they are shrimp, they would have had shallow water ancestors. IE, they wouldn't have evolved on the ocean floor.

 

[Andy Resnick]

I'm curious as to at what evolutionary point a light-sensing organ qualifies as an eye. Does it have to have a lens? A pupil? Focusing muscles?

I would think an eye has more function that a simple sensor- imaging versus detecting. For example, plants exhibit phototropism, but nobody would (seriously) claim plants can see.

so an eye must have some optical element- a curved surface- in addition to at least one detector element.

 

[flatmaster]

I would think an eye has more function that a simple sensor- imaging versus detecting. For example, plants exhibit phototropism, but nobody would (seriously) claim plants can see.

so an eye must have some optical element- a curved surface- in addition to at least one detector element.

The simplest then, would most likely have been a pin-hole eye

 

[Andy Resnick]

Some snakes have 'pit organs', which are similar to pinhole cameras:

http://en.wikipedia.org/wiki/Infrared_sensing_in_snakes

But again, there is a cluster of detector elements within the pit organ- not a single detector element.

The animal I mentioned about is 'Stigmatoteuthis arcturi', the Arcturus jewel squid.

 

[Lok]

Did not read the whole thred .. but some simple cell organisms have only one light sensing detector ... Euglena Viridis if I recall it right.

 

[Moonbear]

I would think having a single eye is a disadvantage- no redundancy. If it failed, the animal would be blind.

Though, if the ancestor of an animal with a single eye had been blind, there's no reason to really think there was any disadvantage to only having one eye, or losing that one eye as long as there was no loss of their other sensory organs. There are cave-dwelling species that are relatives of more surface-dwelling species, and either lack eyes or lack functional eyes.

On the other hand, the eye is an extension of the nervous system and the optic nerves are just one of many paired nerves extending from the brain and spinal cord, so it very well could be that there are not examples of organisms with only one eye, because the eye evolved after a developed brain with two hemispheres and paired nerves had already evolved.

A single, fused eye, might be less likely since the optic cups form so early in embryonic development that any mutation or disruption of development at that stage that might lead to fusion of the eyes may also be embryonically lethal since that's a pretty critical stage for nervous system development in general.

 

[junglebeast]

My guess is that a universal minimum of two eyes provides evolutionary redundancy for all-important vision in case of an accident.

Organisms must be symmetric in order to efficiently store the genetic code, so a one-eyed organism would need it's eye in the center. However, since it does not cost any additional genetic storage space, two eyes are better because they allow

a) wider field of view
b) depth triangulation (eg, stereo vision)
c) placement of eye outside of the symmetry plane
d) sight redundancy in case one eye is damaged

Given that no additional coding is required, and the benefits are substantial, I cannot see any reason why a 1-eyed organism would persist in a competitive ecosystem.

 

[Andy Resnick]

Though, if the ancestor of an animal with a single eye had been blind, there's no reason to really think there was any disadvantage to only having one eye, or losing that one eye as long as there was no loss of their other sensory organs. There are cave-dwelling species that are relatives of more surface-dwelling species, and either lack eyes or lack functional eyes.

On the other hand, the eye is an extension of the nervous system and the optic nerves are just one of many paired nerves extending from the brain and spinal cord, so it very well could be that there are not examples of organisms with only one eye, because the eye evolved after a developed brain with two hemispheres and paired nerves had already evolved.

A single, fused eye, might be less likely since the optic cups form so early in embryonic development that any mutation or disruption of development at that stage that might lead to fusion of the eyes may also be embryonically lethal since that's a pretty critical stage for nervous system development in general.

Good points- I hadn't considered the eye in context of the nervous system. The retina is part of the brain, after all!