A Tale Of Two Polychromats

 

A brightly coloured mantis shrimp facing towards the camera.

This gorgeous but rather disturbing creature is a mantis shrimp. It looks like an alien being, and indeed it has superpowers!

But it’s not an alien. Nor a mantis. Nor a shrimp. But it looks a little bit like a mantis. And it’s related to shrimps.

Mantis shrimps are remarkable crustaceans. They are ferocious marine predators, up to about 30 cm in length. There are many species which come in two kinds: ‘clubbers’ and ‘spearers’. Something both kinds share is lightening speed. The clubbers give the prey (such as other crustaceans, or fishes) a knock-out punch, the spearers impale it. To give an idea of how fast these animals can strike, the claws of punchers can achieve speeds of about 17 m per second, or 40 mph.

Anyone who keeps mantis shrimps in an aquarium may find a clubber can break the glass, and a spearer can give a nasty hand injury if handled.

Why do I write about mantis shrimps? It’s because as well as being super-boxers, they have another super power. Extraordinary colour vision.

Humans are typically trichromats, with three types of colour receptors (cones) in our retinas. One type of cone is most sensitive to red, one to green and one to blue. Each type of cone has a peak response at a particular wavelengths of light.

Some people (mostly men) are colour blind, and have more restricted colour vision. It’s also thought some women may have a fourth kind of cone and are tetrachromats. If so they may be able to see more different shades of colour than the rest of us. However the existence of human tetrachromacy is not proved. There are web sites that claim to test for it but I think the results are unlikely to be be reliable.

So three types of colour receptor is the commonest situation for humans. The mantis shrimp can knock coloured spots off this. Some species have as many as 12 dfferent sorts of colour receptor (and that’s leaving out kinds attuned to ultraviolet, and to different polarization of light). Four times the number of an average human being.

Intuitively we’d think from this that mantis shrimps can see far more many different shades of colour than we can. However very recent research has challenged this idea. By rewarding mantis shrimps with snacks researchers were able to train them to respond to 10 particular wavelengths of light. (I was amazed that this degree of training was possible.) The researchers then tested whether the animals could discriminate between the wavelengths they had learned and other wavelengths nearby. And they couldn’t tell the colours apart.

How can this be? If the mantis shrimp has many different types of colour receptors then surely it’s a no-brainer that they must be able recognise a huge palette of colours?

Brainer is the cue. Things are more complicated. Think of a daffodil flower. It’s yellow. But we have no special photoreceptors for yellow, only for red and green. But both types of cones are stimulated to a certain extent by yellow light, and the brain takes this information and combines it. So we perceive the daffodil as yellow (and have a strong sensation of yellowness, a qualia for a colour we can’t see except as a mixture of others).

The research suggests the mantis shrimp cannot (it turns out) see more colours than human beings, in spite of having more different forms of colour receptor. What it seems it  can do is recognize colours from a limited range, but do so blindingly quickly.

Humans have evolved a strategy of being trichromats, but able to enormously expand the variety of colours they can perceive by sending the signals from the different sorts of cone to the brain and combining them there. This requires a lot of brain capacity and effort, but it lets us distinguish between about 10 million colours.

The mantis shrimp has a smaller brain, and seems to follow a simpler strategy. Essentially it has a more complex retina, one that generates enough colour discrimination ‘up front’, without its brain having to process the information in the way ours would. The mantis shrimp can recognize the colours it needs to ‘at a glance’, without investing effort in combining signals from different inputs. It’s reasonable to theorize that this works faster – less to transmit to the brain, less for the brain to process. The mantis shrimp must be swift in club and claw and so speed matters.

I promise a tale of two polychromats, so what is the second sort?

Dragonflies. These are savage predators of air (as flying insects) and water (as larvae). Like mantis shrimps they are brightly coloured, almost jeweled, and they have large and conspicuous eyes.

A dragonfly perched on a leaf.

Other recent research has investigated how many different kinds of colour receptor dragonflies might have. This is more complicated because (as I understand it) there is no direct evidence of what different types of cones the insects actually have, only or what genes they have for opsins – pigments used by visual receptors. The count is 15-33, a bit a ahead of mantis shrimps. It’s been suggested that they can therefore see many more different colours than humans. But there is no direct evidence that dragonflies actually have that many different kinds of cone, and if they do, they probably can’t recognize any more different colours than we can, for the same reason a mantis shrimp can’t – they don’t have the neurological equipment (a dragonfly is too small) to carry out complicated processing of visual signals, but just rely on instant recognition of a small colour range.

It cannot be a coincidence though that predators that hunt by daylight have large eyes and good colour vision. If only we could examine the vision of a velociraptor!

While writing this post I saw a post on one of the blogs I follow (Adventures in Low Wision), which rather magically was also about the number of cones we have, and also mentioned the beloved mantis shrimp. It’s well worth reading. You can find it here.

Credits

Mantis shrimp image from http://commons.wikimedia.org/wiki/File:Mantis_shrimp_%28Odontodactylus_scyllarus%29.jpg

Dragonfly from http://commons.wikimedia.org/wiki/File:Dragonfly_9187.JPG

 

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About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
This entry was posted in 101 experiments in seeing, Animal Intelligence, Sensory perception, The brain and visual perception, Uncategorized and tagged , , , . Bookmark the permalink.

4 Responses to A Tale Of Two Polychromats

  1. As usual, enjoyed your post.

  2. There’s a good article about colour vision in the New Scientist 18 April 2015, pp. 40-43.

  3. Squitchtweak says:

    I learned about this from Radiolab – you may enjoy it http://www.radiolab.org/story/211119-colors/

  4. Thanks very much, it does look interesting!

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