Cuttlefish

Sea Shambles Advent Calendar - December 1st

Day One

We’re starting our advent calendar with a cuttlefish. 

We tend to assume that life looks like us, more or less, which is very arrogant. But a cuttlefish is the perfect example of what evolution can do when given an entirely different set of constraints on exactly the same planet.  I took this photo several years ago, and the cuttlefish had just been temporarily borrowed from the ocean by a cuttlefish biologist.   It was quite small, just 20 cm long, but it packs a lot of evolutionary ingenuity into that mottled orange skin. 

I could talk about cuttlefish for hours – they’re molluscs with an internal shell (that’s the cuttlebone that seemed to be the constant companion of all captive canaries in the 1980s).   The brown ink that they produce is known to us as sepia, and it’s this pigment that Leonardo da Vinci wrote in his notebooks with.  They’ve got a W-shaped pupil in their eyes and they can distinguish the polarisation of light. They can also rapidly change colour to present an astonishing variety of personas to the outside world.

But none of that is why I spent half an hour interrogating that same cuttlefish biologist on a car journey that evening. I’m sure his major memory of me is that I wouldn’t give up on questioning him during that trip.  All he had done to deserve this was to tell me that even though cuttlefish are masters of colour, changing on a whim to match their surroundings perfectly and to signal to each other, they seem to be colour-blind.  I couldn’t let that go. What? He explained that their eyes only have one type of colour-detecting pigment.  We have three, allowing us to detect red, green and blue and all their wonderful combinations, which is what allows us to bask in the richness of our visual world. But a cuttlefish only has one, the equivalent of on or off.  It sees in black and white.

As our cuttlefish was re-released to the ocean at dusk, I had watched it settle on a piece of coral and disappear. The colour and texture matching seemed perfect. I quizzed the biologist on how any colour-blind animal could do this. When I asked about visual testing, he explained that it is hard to put a blindfold on a cuttlefish, and even harder to attach a pirate-style eyepatch.  Yes, he said, they can clearly distinguish colours. But yes, they are also colour-blind. No, he said, we vision biologists don’t understand this either. 

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In the intervening years, science has provided an update.  It’s possible that the eyes of a cuttlefish can distinguish colours, but they don’t do it using different detection cells like we do. A 2016 paper pointed out that there’s another way to do it.  When you shine light through a lens, different colours of light will be bent by different amounts. This is called chromatic aberration. If you have a single lens, only one colour will be perfectly in focus at once, and all the others will be slightly blurred.  The lens in the cuttlefish eye produces significant chromatic aberration. So it’s possible that what the cuttlefish is doing is rapidly shifting between focusing at different distances, and looking at how various parts of the image go in and out of focus. The things that are all perfectly in focus at the same time must be the same colour. You can absolutely detect colour like this, although it seems very inefficient to those of us used to mammalian eyes. And it seems that this would work for the cuttlefish. Science is continuing to march along, testing this hypothesis. And maybe someone has worked out how to put a blindfold on a cuttlefish by now…

This picture was from Dr Helen Czerski, posted on Twitter back when we first asked people for the sea pictures.

Sea Shambles is a one night only live extravaganza celebrating the oceans. Hosted by Robin Ince and Helen Czerski with Steve Backshall, British Sea Power, Josie Long, Lemn Sissay and more it’s a night of science, comedy, music, lasers and more in which we’ll be turning the Royal Albert Hall into an underwater playground the likes of which you’ve never seen! May 17 2020. Tickets start at just £10! Book here.

Dr Helen Czerski is a physicist, first and foremost, but she’s acquired a few other labels along the way: oceanographer, presenter, author and bubble enthusiast. A regular on The Cosmic Shambles Network, she has also presented a number of acclaimed documentaries for the BBC and Fully Charged.  Recently she was awarded the prestigious William Thomson, Lord Kelvin Medal and Prize from the Institute of Physics.

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