How does a fish see where there’s no light? To find out, you have to join them in the gloomy nether reaches of the ocean. At a recent Packed Lunch, Professor Ron Douglas described his life as a visual science researcher, and Lydia Harriss was there to hear tales of the deep sea…
Out of the watery gloom, there comes a small, dark shape. It looks rather like a small fish. A tasty snack, perhaps? It’s definitely worth a closer look… Before you know it, something clamps on to you with sucker-like lips, digs in with razor sharp teeth, and twists to cut out a circular plug of flesh!
It sounds like the stuff of nightmares, but it’s not. It’s the cookie-cutter shark, and, as I discovered at a recent Wellcome Collection event, it’s real.
Speaking to Wellcome Trust’s Dr Daniel Glaser at a Packed Lunch session entitled ‘The Deep’, Professor Ron Douglas opened a window onto the secretive world of deep sea biology. He is Professor of Visual Science at City University and an expert on the visual systems of deep sea creatures.
The ‘deep sea’ describes the region from 200 metres below the surface down to the seabed, which can be 11 000 metres deep in some places. This means that his lab is the ocean and he does many of his experiments on board boats in warm and exotic locations. Costa Rica, Nicaragua, Samoa, New Zealand, Hawaii… his gleeful list of research destinations had me contemplating a dramatic change in career direction.
Although the locations sound idyllic, the research itself can be pretty tough. Catching the deep sea creatures that Professor Douglas studies is a “lucky dip” exercise. Despite dragging nets the size of a football goal behind the boat for up to ten hours at a time, a catch will often barely cover the bottom of a domestic-sized bucket.
As boats cost £25 000 per day to hire and run, scientists work as close to non-stop as they can manage, on trips that last for four to six weeks. In vision research, it’s important to minimise the amount of light that the animals come into contact with, so they usually fish at night and try to persuade the captain to switch off the deck-lights (captains are normally reluctant to oblige, as “people tend to fall off the back”). Weather conditions can also be very rough, so good sea legs are a must. And then there are the other scientists…
To spread the research costs, a single boat may carry 20 scientists from, say, 10 different labs. Living and working at such close quarters with collaborators, and perhaps potential rivals, is bound to be difficult at times. Although cruises are carefully planned to avoid having multiple scientists with the same research focus, there’s likely to be competition over who gets first dibs on what’s in the bucket.
This is where Professor Douglas has the advantage, as his experiments require the animals to be kept in the dark. Thus, the first port of call for the bucket and its contents is his dark room. “I get to see what’s there first, and then I hand the bucket out into the light and I say ‘no, there’s nothing of interest in there’”, he jokes. His particular expertise is finding out what colours animals can see, by extracting and analysing the chemicals found within their eyes.
There’s an intriguing and rather enchanting alternative to fishing for “a few mangled creatures in the bottom of a bucket” (Professor Douglas’ choice of words). Namely, going down to observe these animals in their natural environment.
The conversation between Professor Douglas and Dr Glaser carried us into a submersible and down through clear blue Caribbean water. The deeper we went, the darker and bluer it became, as though we were descending through a cathedral of blue light. This is because water more readily absorbs light of longer wavelengths, such as red and yellow, than light of shorter wavelengths, such as blue. The blue component of sunlight therefore penetrates further into the ocean than other colours of light and is the last to fade out.
By 700 metres, we were beyond the reach of sunlight, but it was not completely dark. Many of the creatures living in the deep sea make their own light through a chemical process called bioluminescence. This light is almost always blue, probably because it can travel further through water than light of longer wavelengths. The chemical reactions that produce it are similar to those found in fireflies or glow sticks (the sort that you activate by bending, which breaks an internal glass separator and allows different chemicals to mix and react).
Bioluminescence occurs in tiny pits known as ‘light organs’, which may be covered with filters that are used to expose or hide the light. Often located under an animal’s eyes or on their forehead, light organs can help to illuminate the way ahead. They are also distributed across the bodies of some fish, in characteristic patterns that may help the fish to identify each other.
In the case of the cookie-cutter shark, which can migrate between the surface and depths of as much as 3700 metres on a daily basis, light organs act as camouflage. They produce a glow that helps the cookie-cutter to blend in with sunlight from the surface, rather than appearing as an ominous silhouette likely to scare away its prey. Light organs are absent from a dark patch around the shark’s neck, which is shaped roughly like a small fish. It’s thought that this may lure the shark’s prey, who are themselves hunting for food. Many fish, whales and dolphins have been found with circular ‘crater wounds’ characteristic of cookie-cutter bites. Although these sharks have been known to attack humans, they are not usually considered a serious threat.
The majority of the deep sea creatures that we know about only see blue light, enabling them to detect most bioluminescence and any residual sunlight from the surface. Single-colour vision is also more sensitive than multicolour vision, which is a real advantage where light levels are so low.
In this mostly monochrome world, there is at least one animal exploiting the evolutionary niche of multicolour vision. Dragon fish, named for their monstrous teeth, are able to bioluminesce and see both red and blue light. A large light organ beneath their eyes produces red light, effectively giving these fish their own private wavelength. The potential advantages are huge. Imagine being able to flash your lights as a signal to potential mates without drawing unwelcome attention from your predators, or hunting with a bright searchlight that you can see but your prey cannot.
Unlike dragon fish, the humans investigating the deep sea are far less stealthy. Professor Douglas likens going into the deep sea with a submersible to going into the savannah with a Land Rover to see lions. At night. With the headlights on, the stereo blasting, and a blue flashing light on the roof. “All you really see are the deaf, the blind, the stupid and the old. In other words: the things that can’t get out the way.” Despite this, researchers reckon that roughly one in three dives find an entirely new animal that no one has ever seen before. “Deep sea biology is one of the few fields where you really can just be an explorer.”
At present, submersibles can go down as far as about 4000 metres. As we develop technology that will take us further into the depths of the ocean, allow us to stay down there for longer and enable us to switch off the metaphorical stereo, we are likely to discover more incredible creatures. Creatures that are already lurking out there in the deep, just waiting to be discovered…
Lydia Harriss is a graduate trainee at the Wellcome Trust.