Lying off the eastern coast of the Philippines is an underwater canyon so deep that you could hide Mount Everest in it with more than 3,000 meters (9,800 feet) to spare. In perpetual darkness, and faced with incredible pressure, it’s easy to imagine that the Mariana Trench is one of the most inhospitable places on Earth. And yet, somehow life still manages to not only cling on, but flourish, forming its very own unique ecosystem.
From the cold to the never-ending darkness and the unimaginable pressure, life in the deep is by no means easy. Some creatures, such as the dragonfish, produce their own light in order to attract prey, mates, or both. Others like the hatchet fish have evolved enormous eyes in order to try and catch as much of the scarce light that makes it that deep. Some creatures simply try and be avoided, which normally means either becoming translucent or red, because this absorbs any blue light that has managed to make its way down to the depths.
The Mariana Trench contains the deepest point known on Earth. Susan Merle/NOAA
Then they also have to deal with the pressure and the cold, which in effect “sets” the fat that forms the membranes of the body’s cells. If left unchecked, it would cause the membranes to crack and break, so in order to get around this, deep sea creatures have lots of unsaturated fat in their membranes, which help to keep them fluid. But is this enough to survive the deepest place known on the planet?
The Mariana Trench stretches in a horseshoe-like shape around 2,550 kilometers (1,580 miles) in the eastern Pacific, with an average width of around 69 kilometers (43 miles) wide. The deepest point of the trench was first discovered during the expedition of the Challenger in 1875, which recorded using drag lines a maximum depth at the time of around 8,184 meters (26,850 feet), towards the southern end of the canyon. Since then, a more accurate measurement using sonar has revised this to an impressive 10,994 meters (36,069 feet) at the point now known as Challenger Deep, named after the ship.
But it would be almost 100 years before humans would descend to the depths, when in 1960 Jacques Piccard and Don Walsh got into the submersible Trieste and started their descent. Using gasoline for buoyancy and iron shot for ballast, the Trieste took 4 hours and 47 minutes to reach 10,916 meters (35,814 feet) deep, and confirmed unequivocally for the first time that life survives at the bottom. Piccard reports having seen a “flatfish,” although the general consensus is that this was in actual fact a sea cucumber.
Image in text: The bioluminescent dargonfish uses its ability to produce light to attract prey. Jason Bradley
Small balls of sediment, which have tentatively been idenfied as a species of amoeba known as Gromia sphaerica. NOAA Office of Ocean Exploration and Research/2016 Deepwater Exploration of the Marianas
But the sea cucumbers are not alone on the seabed. They are joined by large, single-celled organisms known as foraminifera, which are a bit like giant amoebas, reaching up to 10 centimeters (4 inches) long. Normally, these organisms produce calcium carbonate shells, but at the bottom of the Mariana Trench, where the pressure is around 1,000 times greater than at the surface, calcium carbonate dissolves. This means that the organisms instead have to use proteins, organic polymers, and sand with which to craft a shell.
A "supergiant" amphipod, a type of crustacean, 7,000 meters (23,000 feet) below the surface just north of New Zealand. University of Aberdeen
Also sharing the muddy depths are shrimps and other crustaceans known as amphipods, the largest of which look like massive, albino woodlice, and can be found at the very bottom of Challenger Deep.
With no light getting anywhere near the sea floor, the next question turns to what these organisms eat. Bacteria are able to survive at these depths, feeding off methane and sulfur emitted from the crust, and some organisms will feed on these. But many will rely on what is termed “marine snow,” or little bits of detritus that float down from the surface. The most extreme example of this, and a massive boon for all the creatures living at depth, is a whale fall.
The deepest ever fish, recorded over 8,000 meters (26,000 feet) down, which is still unidentified. Schmidt Ocean Institute
But what about fish? The deepest living fish in the Mariana Trench were found only in 2014, swimming at 8,143 meters (26,715 feet) below the surface. Ghostly white and with broad wing-like fins and an eel-like tail, the unknown species of snailfish was recorded multiple times by cameras sent down to the depths. But scientists think that this might be the limit at which fish can survive, meaning that the absolute depths of the Trench probably can’t support fish simply due to the constraints of the physiology of vertebrates.
So the deepest depths of the oceans, while supporting some large organisms such as sea cucumbers and shrimp, are really dominated by the ubiquity of bacteria. Able to survive in the boiling hot pools of Yellowstone and the sulfur rich springs of the Danakil Depression, it might therefore come as no surprise to find them thriving close to 11,000 meters (36,000 feet) below the ocean's surface.