Imagine what it might be like at the bottom of the world’s deepest ocean. As well as being enveloped in total darkness, you would need to contend with the bitter cold, not to mention crushing pressures of up to one tonne per square centimetre.
Surely no creature could possibly survive that far down. Or could it?
Surprising though it may seem, life does indeed exist at the bottom of the west Pacific’s Mariana Trench, a staggering 11 kilometres down, marine scientists report.
“Special adaptations are needed for living things to survive the pressure, cold and low food availability of the deep sea,” says Monash University marine biologist Richard Reina. “Despite this, there is a surprisingly large diversity of life – although at very low abundance.”
Conditions there are similar to those in other areas of the deep sea, although more extreme, Dr Reina says. “Fish at such great depths tend to be fairly simple, with large mouths and stomachs and flabby muscles because they don’t swim a great deal.”
Many use bioluminescent lures to attract prey, he adds. “They have a sit-and-wait strategy, waiting for food to come along so they don’t expend energy searching for it.”
Most other animals living on or near the bottom of such deep trenches will also be small and simple, he says, although some invertebrates become very large. For example, 30-centimetre long prawns, called penaeids, have been found in several deep trenches.
“We don’t really know why these invertebrates get so big,” Dr Reina says. “Overall, life is simple, [it] moves slowly and is scarce because of low food availability.”
Scientists are fishing for answers as part of the first systematic study of life in ocean trenches, which range in depth from 6000 metres to more than 11,000 metres. The Hadal Ecosystem Studies Program, or HADES, is global in scope and uses state-of-the-art submersibles and imaging equipment to scrutinise the composition, diversity and adaptations of life that plumb the ocean depths.
The phenomenal technical hurdles involved in reaching the deepest part of the ocean, known as the Hadal zone, explains why so little is known about trench life, despite advances in understanding organisms frequenting deep-sea hydrothermal vents and seamounts.
Although the ocean deep forms the largest habitat on Earth, it is, as yet, the least understood.
“The Hadal trenches are the least-explored part of the ocean, and there have been more trips to the moon than to this part of Earth,” Dr Reina says.
Scientists know more about the potential for life in other parts of the solar system than about life in our own marine trenches.
“The difficulties of surveying and sampling mean there are many undiscovered species waiting to be found,” he says. “This project will greatly advance our knowledge of what lives in the deepest seas, as well as how they manage to survive in such an inhospitable environment.”
Work in progress
Pioneering research is already under way at the Kermadec Trench, more than 10 kilometres deep, off the north-eastern tip of New Zealand. More expeditions are also planned to the Mariana Trench, parts of which have been explored by Hollywood filmmaker James Cameron.
Scientists are enthralled by what has been found. They have catalogued several species of snailfish, each in its own trench. One species was found in the Japan Trench, one in the Kermadec Trench, and one in the Peru-Chile Trench. The deepest that fish, including a giant amphipod, have been found is 7700 metres.
Hamburg University marine biologist Torben Riehl, meanwhile, is studying a cosmopolitan isopod crustacean group, the Macrostylidae. These mysterious creatures have seven pairs of legs, typically adapted for crawling, and an oddly flattened body. They include several aquatic parasites of crabs and shrimps and a smattering of swimming or bottom-dwelling species.
“Isopods are the only crustacean group – next to insects – that have colonised the land due to brooding. They occur from mountain ranges down to almost 11,000 metres in depth,” Mr Riehl says. “This means they are one of the largest vertically distributed of all animal groups.”
The deepest isopod ever recorded was collected by Russian biologists, Mr Riehl explains. “Macrostylis mariana is its name and it was collected at a depth of 10,700 metres.”
These animals, he adds, are little different from those living higher up in a marine zone, consisting mainly of a vast, rolling sea floor, known as the abyssal plains.
More to discover
The three-year HADES project, supported by a $1.4 million grant from the US National Science Foundation, has more in store.
“The sea floor is extremely muddy as trenches often accumulate sediment,” Mr Riehl says. “There is a very gradual transition between the water column and the bottom. Consequently, life there needs to be almost neutrally buoyant in order not to sink into the mud. Because of the low nutrient availability, abundances would generally be very low.”
Beyond 8000 metres, deep-sea experts expect to find small crustaceans with laterally compressed bodies and no carapaces. They also anticipate encountering stationary organisms such as picturesque sea lilies.
“Sea cucumbers might also occur in obscure shapes down there, along with a few mud-dwelling sea anemones,” Mr Riehl adds. “Fish and cephalopods might also show up, but should be rather uncommon.”
He also expects to uncover new varieties of crustaceans, worms and molluscs, including shells, clams and snails. Most of them would survive by digging in the mud and feeding on detritus that sinks to the bottom, Mr Riehl says.
“We might also find weird critters such as gnathostomulids, or jaw worms,” he says. “Then there are bacteria and archaea, of course, some of which may live in extreme environments – for example, deep within the sediment or even inside the Earth’s crust.”
Most of the trailblazing exploration is carried out by remote-controlled submersibles. “These are connected to the mother ship with a tether,” Mr Riehl says. “Usually one or more pilots control the vehicle in real time from the ship.”
For shallower waters, engineers are testing autonomous underwater vehicles. But at present they are limited in what they can do by themselves.
“They are mere torpedoes with cameras and sensors,” Mr Riehl says. “And I know of no autonomous underwater vehicle that is capable of reaching full abyssal or hadal depths.”
One limitation is computer power, he says. “The human brain is largely superior in making decisions. And as we never know what to expect in the deep sea, human responses are still essential to conducting most research. This is important when conducting experiments or manipulating devices.”
The fleet of special submersibles is assisted by recent advances in imaging technology, and by the sampling and exploration capabilities of Nereus, a deep-diving remotely operated vehicle. Conceived in 2000, the vessel took nearly a decade to design and build.
Another special system, built at Aberdeen University’s Oceanlab facility, is a submersible vehicle with programmed time-lapse cameras, along with environmental sensors to measure temperature and pressure and two “acoustic-release” devices that hold three steel weights to the underside.
Above the lander is a mooring line, with several flotation modules. When the line is lowered into the water and released, control from the surface ceases. The device takes pictures and video footage as it free-falls to the sea floor and crash lands. Bait in front of the camera lures animals towards it.
At the end of the experiment, which can take up to 24 hours, scientists send an acoustic signal to the lander from the ship. Each signal has a unique command. Controllers aboard the ship instruct the submersible to release the weights and it drifts back to the surface using the flotation modules.
“There might be hundreds of thousands of species in the deep sea which we have not yet discovered,” Mr Riehl says. “Even more fascinating is that, next to unknown species, we can expect to find creatures that often are unlike anything humans have encountered before.”
These, he believes, could be called “living fossils”. As such, “they often allow dramatic new insights into the evolution of life in the ocean and on land”.
It’s important to connect the information gathered by different disciplines such as oceanography, geophysics, evolutionary biology and ecology, he says. “This will help us understand how things work now – and how they have worked before – to enable us to predict how things might work in the future.”
Explore the HADES world at: www.whoi.edu/hades
Discover the three depth zones – bathyal, abyssal and hadal at: www.whoi.edu/hades
Watch the IMAX Melbourne Museum movie Under the Sea 3D: imaxmelbourne.com.au/movie/under-the-sea-3d/education
AusVELS Science: Biological sciences: ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10