Discover the Fascinating One-Eyed Animal: Facts and Myths

[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!