Deep Sea Sight
Chris - It's The Naked Scientists with Chris Smith and with Ben Valsler. We're talking about the science of eyes and the visual system this week and in a second, we'll be finding out what actually is an optical illusion, how does it work, and do bees fall for the same trick. That's on the way but first, we welcome Professor Ron Douglas. He's from City University in London. Hello, Ron.
Ron - Hello.
Chris - Now, you've made an amazing discovery about one way in which some fish see things which will come to in just a second. But first of all, tell us first very, very simply, how do eyes work?
Ron - Okay. Well as we've heard already, it's important to understand that vision involves both the eye and the brain. And really, in some ways, the eye is like a camera in that it produces an optical image of the world and it then converts that optical image into something that the brain can understand which is a series of electrical signals. So the cornea on the lens of the front of the eye make the image and then the retina as we just heard converts that image into electrical signals which the brain then interpret.
Chris - Now one of the things that you're very interested in is animals that live beneath the sea. The sea is blue for a reason because it soaks up all the red light. That's why blood doesn't look red underwater so the people who made Jaws were kind of misleading us a little bit weren't they because bloods looks a sort of black colour because there's no red light to illuminate it. So what do animals deep underwater where the spectrum of light is very different, what do they do to accommodate or adapt to that.
Ron - Well because of that and in the deep sea, there's only two sources of light. There's light from the sun but that's all gone by about a thousand meters and the ocean is up to 10,000 meters deep and most of the animals that live there have large and fully functional eyes so they clearly must be looking at something. And they are looking at light produced by other animals, that is their looking at bioluminescence. Now this bioluminescence like the sunlight tends to be blue because blue light is just transmitted best by seawater so deep sea animals generally have eyes which only see blue and they're not at all sensitive to red because as you said, all the red light is gone.
Chris - And things like a giant squid have enormous plate-sized eyes presumably because the bigger you make your eyes like the telescopes we use, the bigger the telescope, the more light it can gather. Therefore, the more you can see.
Ron - Absolutely! You want a big aperture, so you want a big pupil, so that means you need a big eye. So, you just have a big eye to catch as much light as you can.
|Figure 1. Surface Morphology of Dolichopteryx longipes © Julian C. Partridge et al.; Current Biology; 27 January 2009.|
Chris - So if you look at the retina in these undersea creatures, are they largely, then because most of these animals that they're looking at are emitting lights that are bluish, are they mainly picking out blue light? They haven't got the ability to detect light at the other end of the visual spectrum, like reds and things.
Ron - Well I would say 99.9% no. They can't see reds, but we have found one group of fish, the so-called dragon fish which produce red light. So, they're producing red light that really nobody can see except they themselves, we've shown that they are very sensitive to red light. So, they basically have a secret wavelength. They have a red searchlight stuck on the top of their head and they can use that for instance to illuminate potential prey and the prey just don't know they're being looked at. It's kind of like a sniper scope on the end of a rifle.
Chris - It was like an infrared camera I suppose, isn't it? We can't see infrared but the person operating the infrared camera can see us because they're looking in a wavelength regime that we're not sensitive to.
Ron - It's exactly like that. And of course, they can also use it to talk to each other because down there, all the animals have very big teeth, basically because the density of animals is quite low so you rarely meet your lunch so when you do, you want to make very sure that you can eat it. So, it's a rough tough place down in the deep sea, but if you have a light that you can flash on and flash off and talk to your friends, you know, for sex or whatever you have on your mind, then nobody else can see that you're there.
Chris - Do they use that to communicate so they can hunt together or is this purely just to attract a mate or to ward people off, "this is my territory, stay away."
Ron - We don't know because it's very difficult to make observations on live animals that deep. My guess is it's mainly for communication. There's no evidence that they're territorial and there's certainly no evidence that they hunt as a pack.
Chris - What about these fish that you've recently described? Just this one species that don't have traditional eyes. They actually use mirrors, a bit like some of the funky telescopes that we're making to look at the heavens these days, use mirrors rather than lenses. How do these fish do that?
Ron - Well, they've got very interesting eyes because the eyes are made up of two parts. Now, the two sources of light in the oceans we've said are the sunlight which of course comes from above. So, a lot of deep sea animals have tubular eyes which point out towards the sky to make the most of the residual sunlight but of course, the bioluminescence which the animals make happens all around you. So if you've got these tubular eyes looking upwards, then you're not going to see all the bioluminescence it's happening to the side and it's happening underneath. So, we found these fish that have an eye that are made of two parts. It has a tubular eye looking upwards but it also has a second eye or part of the same eye which actually looks downwards and the interesting thing was while the tubular eye that looks upwards focuses light using a lens, just like anybody else, the eye that looks downwards doesn't have a lens in it. Instead, it has a mirror and it focuses light using a mirror.
Chris - When you say mirror, what tissue is doing that? How have they evolved to be able to reflect light rather just absorb it or focus it?
Ron - I think most of the scene, when we look at nocturnal animals, something like a cat or maybe a deer, when you catch it in the headlights of your car, they're eyes suddenly light up and that's because animals that live in low light levels have a reflective layer behind their light sensitive cells called the tapetum, so that the light that goes through the retina and isn't absorbed by it, gets bounced back, so the retina has a second chance of absorbing it. So, most of animals including deep sea fish have these reflective tapeta and this mirror is really just a modification of this reflective tapetum.
Chris - Is it evolutionarily, the same origin then as the dog and the cat and the deer and the cow and the horse or is it evolved independently?
Ron - No. It's absolutely the same. It's made of the same chemicals. It's made up of plates of a crystal called guanine. It's identical in every way.
Chris - Fantastic! Thank you, Ron. That's Ron Douglas, he's from City University.