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Author Topic: Why does a giant squid have huge eyes?  (Read 5088 times)

Offline engrByDayPianstByNight

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Why does a giant squid have huge eyes?
« on: 16/03/2012 04:13:18 »
I was listening on the radio this afternoon, and one interesting story was about the giant squid having the world's largest eyes of any animal (the size of a basketball). There have been lots of debates about why their eyes are so big. Researchers have made calculations for what various eye sizes would allow the giant squid to see in the depth of the ocean where it's total darkness, and they assert that a much smaller eye would have served the squid well in its natural habitat. So far, it remains a open problem.

What are some of th competing theories/conjectures about the giant squid's disproportionately huge eyes? Thanks.


 

Offline Airthumbs

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Re: Why does a giant squid have huge eyes?
« Reply #1 on: 16/03/2012 23:06:24 »
I do not know what results have been obtained from specimens about the light spectrum that these creatures can detect.   
If vision is a key sense in such a dark place and to be such a massively successful hunter in those conditions I would not be surprised if these creatures can also detect infra red.  Squids also use bio-luminescence as a way to communicate. The squids with larger eyes will be more successful in their environment and therefore more likely to create successive generations with larger eyes.   Squids also hunt by looking for shadows above them and again it would be advantageous to have larger eyes then just adequate... :)

I have to say that the question did remind me of Little Red Riding Hood.... :P  "all the better to see you with dear"!
« Last Edit: 16/03/2012 23:09:18 by Airthumbs »
 

Offline MikeS

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Re: Why does a giant squid have huge eyes?
« Reply #2 on: 17/03/2012 19:08:21 »
All the better to see you with at great depth where there is little light.
 

Offline Airthumbs

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Re: Why does a giant squid have huge eyes?
« Reply #3 on: 17/03/2012 23:28:20 »
All the better to see you with at great depth where there is little light.

I expect most of the prey it hunts emit their own light. 
 

Offline engrByDayPianstByNight

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Re: Why does a giant squid have huge eyes?
« Reply #4 on: 20/03/2012 23:27:55 »
Yeah, I also thought it might be related to taking in as much light as possible. But apparently that doesn't explain why the eyes have to be so much bigger than necessary. Anyways, thanks for the replies.
 

Offline CliffordK

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Re: Why does a giant squid have huge eyes?
« Reply #5 on: 21/03/2012 02:24:20 »
There are several issues.

The Giant Squid ranges around the world.  The Colossal Squid's range is around Antarctica. 

So, at least for the Colossal Squid, half the year it gets almost no sunlight.  So, not only would it be hunting in the deep where it is dark, but it would be hunting in the depths by starlight/moonlight.  Although they do have bioluminescence to create their own light.

The bigger the lens, the more photons it can capture, and thus the greater the acuity in low light situations.  Adding rods or cones increases acuity, but only with adequate light.

So, then what is the "cost" of the large eyes?  Weight isn't an issue in sea water.  There is presumably less to bump into in the sea, and assuming pressure is equalized, then the sea might even give the eye some support.    Without a skeleton, the eyes can grow with the body.  If they have large rods, or good information integration, then the eyes may not require more brain than other animals would have.

So, I would presume that the increased acuity of vision in the dark would be of great enough of a benefit that the squid would evolve the larger eyes.
 

Offline Don_1

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Re: Why does a giant squid have huge eyes?
« Reply #6 on: 21/03/2012 10:15:22 »
Sunlight does not penetrate the oceans to the depth that these Giant Squid inhabit, so there many creatures which are bioluminescent.

The Giant Squid’s main predator is the Sperm Whale. Imagine a huge Sperm Whale at these depths swimming through a sea of bioluminescent plankton. It would be like a wandering black hole as it displaces the plankton. But light does not travel through water very well, so you need a huge eye to be able to detect this ‘black hole’ in the ocean and to detect it early enough to take evasive action, if you don’t want to be on the menu of an approaching Sperm Whale.

So my reckoning is that the large eye of the squid is to enable it to see differences in the concentration of bioluminescence at a reasonably long distance.
 

Offline engrByDayPianstByNight

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Re: Why does a giant squid have huge eyes?
« Reply #7 on: 27/03/2012 03:43:22 »
Interesting new perspectives, Don_1 and CliffordK. With your theories, I wonder if we could extrapolate that to other deep-sea life forms (those living in the same depth as the giant squid) and claim that they too should have developed disproportionately large eyes (w.r.t. their bodies)?
 
 

Offline Don_1

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Re: Why does a giant squid have huge eyes?
« Reply #8 on: 27/03/2012 09:58:11 »
Logically, you should be right. Other creatures inhabiting these depths would benefit from a large eye, but most of the other creatures at these depths are not big enough to support a large eye and many do not rely on visual ability at all. Species of fish found at great depth, with close relatives which inhabit shallower waters, have lost their visual capability altogether. The inhabitants of the depths tend to rely on ‘seeing’ their world by feeling, either with direct contact using tentacles or by electrical fields. Even in some shallow waters, where visibility is impaired by dissolved soil, sand or whatever, many fish use such methods to ‘see’ their prey. The Catfish, with its whiskers is one good example and eels with their electric field another.

A large eye might not be beneficial to a fish of perhaps 30 cms length, since the eye could only become slightly bigger than that of a similar sized fish in shallow waters and may not be able to become big enough to make a worthwhile difference. In the deep sea fish, it might well be that evolutionary protocol would rule out the development of a large eye in the vertebrates which could not accommodate an eye large enough to be of advantage. Therefore, other means of ‘seeing’ their surroundings were developed or enhanced. So why did the Giant Squid develop a large eye?

There are two possibilities for this. First, that being a cephalopod and invertebrate, the squid’s body was already large enough and adaptable enough to house an oversized eye. The second possibility, and pure conjecture on my part, is that the requirement for a large eye in a deep sea cephalopod may have been the evolutionary driving force behind the Giant Squid’s giantism. The body growing to accommodate the increasing size of the eye. That might suggest that even the Colossal Squid has some growing yet to do.

For the Giant Squid, the problem is that ‘seeing’ by touch or sensing electric fields’ is only effective for close proximity, especially if you are in waters where huge numbers of creatures both large and small, can interrupt any electrical field emitted by others or soak up or deflect slight changes in pressure waves. It would follow, then, that visual contact would be a great advantage and the bigger the eye, the more of what little light is available, can be collected. The Squid’s eye has the ability to see polarised light, which might also be an added bonus when on the lookout for a giant ‘black hole’ created by a Sperm Whale descending from the shallows.

Their main predator, by contrast, has poor vision even in the best of conditions, but the Sperm Whale has developed a far better means of ‘seeing’ in the dark; it uses echo location. All in all, enhanced low light vision would appear to be the Squid’s best defence against attack by Sperm Whales, whether this enhanced vision would be of benefit in hunting, I’m not so sure, but it could well be the case that vision could only be an extra advantage over the other senses. It certainly wouldn't be a disadvantage.
 

Offline Don_1

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Re: Why does a giant squid have huge eyes?
« Reply #9 on: 27/03/2012 10:01:05 »
A quick note to admin:

Perhaps this thread should be moved to Marine.
 

Offline Æthelwulf

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Re: Why does a giant squid have huge eyes?
« Reply #10 on: 02/04/2012 07:04:08 »
 

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Re: Why does a giant squid have huge eyes?
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