Creatures of The Deep Sea

The Naked Scientists spoke to Dr Ron Douglas, City University, London
23 April 2006

Interview with 

Dr Ron Douglas, City University, London


Chris - Tell us about your research.

Ron - I'm interested in what animals are down there and what it is they do. The problem is that it's very difficult to make observations on living animals in the deep ocean. You have two ways of catching animals. Mainly I'm interested in fish. You can send down nets but it's very difficult to fish with nets because they're the size of a football goal. If you say the average depth of the ocean is about 4000 metres and you want to fish there, you have to let out fifteen kilometres of cable. That takes maybe twelve hours to get down and get up and what you end up with is a bucketful of organisms, most of which are dead. The alternative is to go down in a submersible, but a submersible also has problems because it's noisy, it has lights and you're going to scare things away. It's rather akin to deciding to look at lion behaviour and going out in a Land Rover at night with the lights flashing and the stereo on full blast. You're not going to see meaningful behaviour. So really all we see in the deep sea in submersibles are the dumb, the deaf, the stupid and the dead.

Chris - Can we define first what deep sea actually is, because Jason said that he's been plumbing the depths at one kilometre. How deep is deep in your book?

Ron - The average depth of the ocean is 4000 metres, but the deepest point is around 11000 metres. Deep sea is usually defined as that area below where photosynthesis can occur, which is usually 200 metres.

Chris - And beyond that point there's not enough light for anything to be meaningful.

Ron - There's not enough light for photosynthesis. Humans can perceive light if you go down in a submersible and switch off all the lights. You can see some sunlight down to about 700 or 800 metres. The animals that live there are a little bit more sensitive and can see sunlight down to about 1000 metres. What we must remember is that 80 to 90% of animals in the deep sea produce their own light; that is they are bioluminescent. If you go down into the deep ocean and switch off the lights inside a submersible, as the sunlight fades, it'sreplaced with a bewildering array of flashes. It's rather like being in a firework display as all the animals are talking to each other and illuminating each other with their bioluminescence.

Chris - Anybody who's been underwater will notice how blue - dominated the underwater environment is because, I think I'm right in saying, water scatters and gets rid of red light and only blue light comes down. So by the time you get very deep you're in a blue world, and therefore the animal that are knocking around down there are optimised to see blue and not reds.

Ron - Absolutely right. If you go diving even off the coast of Britain, if you cut yourself you don't bleed red but you bleed this alarming green gooey stuff. This is because all the long wavelengths or the red light has been absorbed by the water. So as you go down the water column, after about 200 or 300 metres, all you're left with is blue light. Almost all the animals down there have eyes that are only sensitive to blue because that's the stuff that's transmitted furthest.

Chris - But what about the animals that have exploited this evolutionary niche and are now producing their own light which is red, which means that they can essentially focus a search light on all their prey because their prey aren't sensitive to the red light? The fish that makes the light can see them beautifully.

Ron - That's right. Most bioluminescence, as I've said, is blue and almost all fish are therefore sensitive to blue. But there are three species of deep sea fish which have the lovely name of dragon fish because of their large teeth and jaws, that are able to produce light and their eyes are sensitive to that red light. They can use this for a number of purposes. For instance, they can illuminate prey and the prey won't know that they're being looked at because they're not sensitive to the red light. It's rather like a sniper scope in the army. The other thing is that these animals can talk to each other without their potential predators knowing that they're there. So if you like it's a teenagers dream: as much sex as you like but nobody knows that you're at it because you can flash your red lights at each other but you're quite immune from detection by anyone else.

Chris - The statistic I said earlier was that three quarters of the Earth's surface is covered with water and we know more about the surface of the Moon and Mars than we do about the deep sea. What about the deep sea and the trenches? They're about seven kilometres down some of them and when was the last time that someone went there? How do we know what's down there?

Ron - The statistic you say that three quarters of the world is covered in ocean is absolutely right, but what you have to realise is that life on land is really two dimensional. Everything happens within a few metres of the ground. But as I said, the average depth of the ocean is 4000 metres, so the volume of habitat available is absolutely enormous, and in fact 99.9% of all available living space on the planet is deep ocean. We know incredibly little about it. To answer your question, the only two people that have ever been to the bottom of the ocean at 11000 metres are Don Walsh and Picard in 1960, and nobody has ever been back since.

Chris - How do such animals withstand such tremendous pressure?

Ron - They can withstand this pressure because they do not contain air spaces. They won't implode like Jason would if he went down one kilometre below the ocean.

Chris - How much weight of water have they got sitting on them and when you bring them to the surface, why don't they do the converse and explode?

Ron - Again, they don't explode because they don't have any air spaces in them. They don't have these swim bladders that most shallow water species have. So if you bring them up they don't explode.


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