• Question: Why are Cephalopod eyes so different from humans?

    Asked by ErmaghedScience to Matthew, Andrew on 20 Nov 2014.
    • Photo: Matthew Camilleri

      Matthew Camilleri answered on 20 Nov 2014:

      Well, the first thing to note is that cephalopods are non vertebrates, and humans are vertebrates, making them quite different in evolutionary terms. This means that the two set of eyes might have developed independently from each other, and that would make it a pretty impressive achievement that they actually developed to be this similar.

      The main difference between the two set of eyes is where the nerve endings route with the retina, with these fibres going thorough the retina for vertebrates and behind the retina for cephalopods, which gives cephalopods an advantage considering that all vertebrates have a blind spot, which is not a problem for squids and octopuses.

    • Photo: Andrew McKinley

      Andrew McKinley answered on 20 Nov 2014:

      Cephalopods are sea creatures, and there simply isn’t as much light available down there. Consequently evolution selected those with bigger eyes as they can see better in low-light conditions and hence avoid predators.
      We have plenty of light available to us, so there was not an evolutionary advantage to those with bigger eyes.

      Fascinatingly, our eyes have nerves in front of the retina – this means the back of the eye, where the nerves pass through the retina, has a blind spot. To see this in action do the following:

      1) Take a sheet of A4 paper and a thick marker pen
      2) Draw two dots side by side (about 1cm in diameter) about 20cm apart.
      3) Close your right eye, and focus your left eye on the right spot
      3B) Or: close your left eye, and focus your right eye on the left spot
      4) Move the paper to and from your face. You should find that when the paper is around 15-20 cm from your face, the spot your are *not* focussing on suddenly disappears. This is your blind spot where the nerves pass through your retina.

      Cephalopods have the nerves behind their retina, so do not have a blind spot. A blind spot is not a problem for us as we have two eyes pointing in the same direction – cephalopods however need to have full 360º vision, so a blind spot would give an evolutionary disadvantage.

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