Why No Stalked-Eyed Vertebrates? (Page 1) - General Biology

21st Feb 2009 16:19:50

Question from:Isaac

Often times, a commen theme of sci-fi are wacky creatures with fully-formed eyes attached to thin stalks. I must ask why are there no vertebrates on this planet that have such eyes? One reason I could think of is that nerve signaling would take a little longer. Another reason I might think is that it is too water-costly. Vertebrates like terrestrial amphibians and mudskippers have such protruded eyes, but when there isn't a lot of water, they close their eyes to conserve the water they have.

Of course, we have stalked eyes on Earth in arthropods and mollusks, but why not vertebrates?

Regards, Isaac.

22nd Feb 2009 11:55:07

David Wynick

University of Bristol
Posts: 4,230

All good points but I would guess one other major issue is that eyes on stalks would be very difficult to hold in alignment which in turn would make binocular vision almost impossible. I would expect that the development of 3D vision (giving one depth and the ability to make very fine and precise movements) would be a distinct advantage from an evolutionary perspective so in a trade off of all round vision vs binocular sight the latter would be more advatageous.

23rd Feb 2009 10:30:40

David Hone

QMUL, London
Posts: 1,218

While not in the same league as many invertebrates (especially crabs and some trilobites) chameleons do indeed have eye-stalks to a certain degree. Their eyeballs are certainly not mounted within the orbit as with other vertebrates and can be moved independently. Also there is the four-eyed-fish (Anableps) which has extremely bulged eyes.

23rd Feb 2009 12:08:51

Paolo Viscardi

Horniman Museum, London, UK
Posts: 1,105

I expect the limiting factor for vertebrates is probably support. Stalk eyes are fine if you have a hard tube to keep them upright and protected (like arthropods), or if you have no hard support and the eye can be retracted to move it out of danger (as with molluscs). Vertebrates don't really have a great bodyplan for having stalked eyes, so it may simply be that bulging or turret eyes are the closest that can be achieved with the bodyplan available.

24th Feb 2009 08:53:09

David Hone

QMUL, London
Posts: 1,218

I thought that too, but it occurs to me that there is no real reason I can think of that would prevent a vertebrate simply growing a bony tube with the eye at the apex. It would still be supported largely on most sides and provided there is space for the optic nerve should have no trouble reaching the brain (it's not *too* far from what hammerhead sharks have done). I think David is closer to the truth with the fact that they simply don't need them so they don't have them. At a small scale (especially with compound eyes) many inverts may not have great binocular vision, so by separating the eyes out they can increase the distance between them and improve binocular vision. The generally larger vertebrates simply don't need to do that as their eyes are naturally further apart, so they don't.

24th Feb 2009 10:28:05

Paolo Viscardi

Horniman Museum, London, UK
Posts: 1,105

Good point. Although, herbivorous vertebrates don't go in for binocular vision as much as panoramic vision for predator detection and I think a mechanism that improves vision over cover without presenting a large target (like a stalk-eye) would be quite useful. However, I think it likely that a bony tube would be too fragile to work well - the fact that it would be fixed in place would make it vulnerable to damage and would limit the value of having the stalk-eyes. The value of stalk-eyes could therefore still be offset due to limitations imposed by the vertebrate bodyplan...

24th Feb 2009 18:35:10

Darren Naish

Posts: 80

There ARE stalk-eyed vertebrates. Firstly, scolecomorphid caecilians are able to move the eye out of socket and down the tentacle they possess on the side of the snout. Needless to say, this is a highly specialised set-up (the tentacles are unique to caecilians and nothing else quite like them exists elsewhere). Secondly, the larvae of some weird deep-sea fishes - I'm thinking specifically of the Ribbon sawtail _Idiacanthus fasciola_ - have long cartilaginous eye stalks which extend for as much as a third of body length.

So, 'eyes on stalks' are indeed well within the evolutionary potential of vertebrates. Why aren't they more widespread? Perhaps because they are so vulnerable to damage.

25th Feb 2009 08:16:32