The Extraordinary Secret of Cephalopod Vision


The other day I was getting my eyes tested, and in spite of the multiple adjustments they
made to my lenses, I was still seeing very blurry, and frustrated, I complained about
this to the opticians. He speculated that perhaps it was because
I had started some new medication recently, but the thing is, he didn’t actually know
this about me, but he was absolutely right, so perplexed as to whether he had psychic
abilities, I asked him how come he knew. Turns out my pupils were dilated, aaaand of
course it all made sense then! You see, sometimes when you start taking a new medicine, it can
actually cause the muscles in your iris to contract, which means that the pupil widens
and more light than normally will enter the eye, scattering at the back and forming a
blurry image. You can get a really similar effect on your DSLR camera if you open the
aperture as wide as it will go, this creates a very shallow depth of field and allows you
to get those very creamy, bokeh artsy pictures that everyone seems to love. Anyway, it was at this moment that it struck
me, we humans are really focused on seeing clearly, but for other animals it really just
isn’t a big deal, and in this video I’m going to tell you all about the cephalopods, such
as the octopus and the cuttlefish, because they actually blur their eyes on purpose to
do something very special. Cephalopods have always been a bit of a mystery
to scientists, you see them swimming across the ocean floor and changing their skin to
perfectly match the colour of their surroundings, or putting on the most elaborate and colourful
mating displays to their potential partners, but then when you look at the back of their
eye, they only have a single photoreceptor type. You see, in order to be able to detect
colour using photochemistry you need at least two different types of photoreceptors. We
have three of them for detecting colour, our cones, which detect three different wavelengths
of light and they send an integrated signal to our brain which it then interprets as the
colours that we see. Now there are some advantages to having a
single photoreceptor type, for instance, you are going to be a lot more sensitive in low
light environments, such as the bottom of the ocean, but it doesn’t really tell us how
cephalopods are seemingly detecting colour, and it turns out that the answer lies in the
shape of their pupil! We have a circular pupil through which light
passes and focuses at the back of the eye forming a sharp, crisp image. On the other
hand, cephalopods have a really characteristic U or W-shaped pupil which means that the light
enters the eye off-centre and the curvature of their eyeball actually splits the light
into its constituent wavelengths, and by altering the shape of their lens they can control where
each of these wavelengths fall in the back of their eye. Essentially, they’re able to
just focus across the different colours in their immediate surroundings. Anyway, if you ask me, I think cephalopods
have killed three birds with one stone: they’ve gotten away without evolving fancy photoreceptors,
they’re able to see very well in the dark and they can detect colours anyway by simply
focusing in and out of different colours. So remember, next time it seems like something
has psychic abilities, just look towards the laws of physics, because there is always more
than meets the eye! I hope you enjoyed this and learnt something
new today, let me know what you thought and if you want to find out even more about the
fascinating vision of cephalopods check out the full blogpost I wrote at drawcuriosity.com
because it goes into a lot more detail if you want to find out more. Thank you so much
for watching, and I will see you in the next one! CREDITS:
ART&ANIMATION: Caro Waro & Cristina de Manuel MUSIC: Thastor & CryoSleepKitten
SCRIPT, FILMING, EDITING: Inés Dawson

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