Exploring fish eyes in the deep
Professor Ron Douglas talks about fish eyes, their evolution and how they could one day help treat human eye diseases
“This is one of the few professions in the world where you can actually still claim to do exploration,” says Professor Ron Douglas as we sit down in a communal meeting room on the ground floor of the Gloucester Building at City. “Really you and I are just irrelevant in terms of the total number, biomass and importance on earth. Everything is deep sea. The rest is a little decoration on the surface.”
Simply put, we still know very little about our own oceans. It is estimated that 95 per cent of this underwater world remains unexplored. If you take a globe and study it for a while you’ll see that large swathes of the planet are covered with water. In fact, 71 per cent of the surface area of the earth is water, but in terms of living space, that number is much greater.
“If you think about it life on land is largely two dimensional as we live within the first couple of meters of the surface,” explains Ron, “but the average depth of the ocean is 4000m, so in terms of volume it’s huge – 99.9 per cent of living space on the planet is deep sea.”
Searching the deep
As the Chair of Visual Science at City University London, Professor Ron Douglas gets to investigate the optical systems of many weird and wonderful things.
“To me deep sea fish are fascinating because the deep sea is such an extreme environment. So what these fish are looking at and how they see it is a really interesting question.”
The deep sea can be a very dark and gloomy place, as even in clear water sunlight doesn’t reach further than 1000 metres below the surface. To compensate for this, deep sea fish have evolved some interesting solutions.
“Almost all have very sophisticated visual systems with big and fully functional eyes. Most animals in the deep sea bioluminescence – they make their own light – so it’s the relationship between the bioluminescence and the visual system that is important.”
However studying these fish – and working out what they see and how they see it – is no simple task, with the ships needed for such expeditions extremely expensive. “The techniques are also quite crude so it’s not always easy to catch what you’re after,” he adds.
Typically Ron uses two methods when trying to observe and catch the fish he is looking to study. The first involves using a net with a mouth the size of a football goal and the second uses submersibles. In order to fish at 4000m the net requires 15 kilometres of wire and is then towed behind the boat at a speed of 2 miles an hour. The problem is that with a small net and a low speed the catch isn’t always representative of what’s present that far below.
The issues are also comparable when using submersibles. Once locked inside with bolts from the outside, the subs – which are normally either a round sphere or long tube – are lowered into the sea and released enabling Ron to explore the depths for a day or so. While the sights can be incredible there are also major issues when it comes to exploring the ocean this way.
“The truth is that our ability to study the deep sea is severely limited as we can fish and use submersibles, but many of the things that are there we never see as the lights and engines scare most animals away,” says Ron.
Flying from London via Johannesburg to Walvis Bay, a small plane containing Ron and his colleagues landed on a tiny strip of desert in Namibia’s skeleton coast. From there Ron boarded an ex-German fisheries vessel named FS Sonne before leaving port quickly to avoid an impending storm. Heading 400 miles westwards, they sailed towards a series of seamounts around the Walvis Ridge to go fishing.
Having spent four days fishing on the ridge, the ship then headed south, passing the Cape of Good Hope to get to Durban where the trip finished.
“While these ships can carry around 20 scientists most projects don’t need that many so you normally invite your friends along. Of course you don’t want people who do exactly what you do; you want people who study different things. In this case I have a colleague at the University of Tübingen in Germany and he got a place on this trip and was able to take three other people, one of whom was myself.”
Following on from previous trips, the main aim of this expedition was to investigate two different fish, both of which display remarkable traits which have yet to be fully explained. The first concerned a family of fish known as dragon fish and the second another group of species known as spookfish, both of which have fascinating visual systems.
Finding fish at the bottom of the ocean
If you were to dive down 100 meters and be unfortunate enough to cut yourself while swimming, you would bleed green instead of red as red light cannot travel that far in water. Light has many peculiar properties, one of which is that it is comprised of many different wavelengths. The longer wavelengths of light, such as red, are absorbed by the sea leaving the shorter wavelengths – such as blue and green – to travel much more easily through the ocean. This is the reason why we bleed green instead of red at such depths and also why the oceans appear blue, but this phenomenon also has repercussions for deep sea vision.
“In the deep sea you only really see blue light, so all the animals down there have eyes that only see blue except for one family of dragonfish which have special organs called photophores producing light. Some emit blue light but others strangely produce red light, so they are making light that nobody can see. So we looked at the eyes and found that actually they are incredibly sensitive to red light, so they have a private wavelength at the bottom of the sea. This enables them to illuminate prey without the prey knowing that they’re being looked at.”
But Ron suspects that having their own ‘private wavelength’ may also have further benefits.
“When it comes to mating, a lot of fish flash light at each other to attract mates using bioluminescence. By doing that with blue light you may find a mate but you may also get eaten as other fish down there can see the light. But if you can make and see red light, only your mates can see you so you can reproduce as you like while remaining invisible to predators.”
But how do they detect the red light? The answer was something that no one was expecting, as Ron and his colleagues found that the fish see red light by using chlorophyll, a compound normally used by plants and bacteria to make energy from light.
“Nobody believed me when I first suggested this,” said Ron. “In their eyes, the chlorophyll absorbs the red light, activating the other visual pigments – which normally detect light – indirectly. We actually think that they may get the chlorophyll from colonies of bacteria living in their eyes, and we were trying to get more evidence for it on this trip. If it’s true, this would be a very big deal.”
Since Ron first published his research in Nature in 1998, in the intervening years people have taken chlorophyll and inserted it into the eyes of cows and other animals, subsequently improving their sensitively to red light. When people looked further into the reasons for this it was found that in fact chlorophyll stabilises one of the visual pigments – known as rhodopsin – in the eye. With the degradation and destabilisation of rhodopsin the basis of a lot of retinal diseases, there is hope that chlorophyll could be used to stabilise rhodopsin in these human conditions, potentially providing a therapy for some forms of blindness.
The spookfish also have an unusual visual system. Instead of possessing one type of eye which uses a lens to focus light onto photosensitive receptor cells, the spookfish augment this visual system with another which is almost completely unheard-of in vertebrates.
Separated from the main eye is a visual system which works not unlike a radio telescope, using reflective guanine crystals – one of the constituent bases of DNA – to reflect light towards another set of photoreceptor cells independent to the main eye. This split structure allows the fish to see up and down at the same time. But still the mystery of how such an incredible structure could evolve remains obscure.
Once back on land, Ron will continue to explore the visual systems of these creatures, using molecular biology, microscopy and computer modelling to try and further work out how these wonderful eyes work. By sequencing their genes he will also gain an insight into the evolutionary history of these incredible eyes.
But while the implications are wide ranging, as one day these weird and wonderful fish eyes could perhaps provide the key to solving common eye diseases, maybe even providing the basis for a cure for some sight threatening diseases, there is still much work to be done. This is in many ways fitting as the deep sea continues to cling on to many of its secrets due to the difficulties associated with exploring such depths.
As a result, it will take Ron and other deep sea explorers many more years to uncover all the mysteries, as we still know very little about our deepest oceans. As Ron says, “every time you go in a submersible you see something that hardly anyone has ever seen before, and every third dive you see something nobody has seen before.” There is still much to discover.
source: City, University of London