Emily Wardrop

The deep sea is an almost alien landscape. In fact, we know less about it than we do the entire surface of the moon. The deep oceanic seafloor is pitted by even deeper trenches, reaching a whopping 11,000 m (36,000 ft) beneath the surface. The immense depth leads to enormous pressure, so humans cannot freely explore this region without the use of submersible vehicles. Despite thousands of people having climbed to the highest peak of Mount Everest, only 3 have ever ventured into the depths of the Mariana’s Trench. Minimal exploration means we know very little about these great depths and the enigmatic species that inhabit it. 

Just How Deep is Deep?

As we move further from the shallow coast and away from the continental shelves of land, the seafloor rapidly begins to increase in depth. From the sunlit epipelagic, the seafloor passes through the meso-, bathy- and finally abyssopelagic, growing darker and colder each meter. The Abyssal plain is the name given to the vast expanse of deep seafloor that extends from the Arctic to the Antarctic. It can range between 3,000 and 6,000 metres deep and only organisms that are adapted to the extreme pressure and temperature can survive here.

The Abyssal plain is occasionally interrupted by massive ocean trenches that descend even deeper into the ocean. This zone of the ocean is also known as the Hadopelagic, named after the Greek underworld “Hades”. Mariana’s Trench is by far the deepest, reaching a massive 11,000 metres (36,000 ft) deep, and the organisms found here are like none that you’ve ever seen on land.

Life in The Deep

The immense depth causes an extremely high pressure and a low average temperature along the Abyssal Plain, so not only are the organisms here well adapted to life in the shadows, but also to these other extreme conditions. Species diversity and abundance is significantly lower in comparison to shallower zones, yet they do find a way to survive.

Scientists predict that life on earth actually originated from these great depths and around hydrothermal vents. The vents spew plumes of hot water, reaching 400oC into the surrounding environment. Using Remotely Operated Vehicles (ROVs), researchers discovered a myriad of microorganisms that can actually live in these extreme environments. Larger animals such as tube worms, grow on the side of the vent and survive by working in symbiosis with bacteria. The bacteria utilise hydrogen sulphide, produced by plumes, converting it into sufficient energy for both itself and the tube worm host. This unique ability of the bacteria to thrive in these conditions, may have pioneered the evolution of species as we know them today.

Yeti crabs are another deep-sea species that survive around hydrothermal vents. Named after their large, hairy claws, the yeti crabs are perfectly adapted to life here. It is theorised that, due to their blindness, the crabs use their hairs as sensors to detect prey and mates in an otherwise pitch-black world.

Blindness is common in many deep-sea species, because below 200 metres the ocean becomes a vast expanse of darkness. Sunlight cannot penetrate beneath these depths, so some animals have evolved without eyes. However, for migratory species that travel up and down the water column, eyesight is essential, so many can produce their own light. Bioluminescent species are in abundance throughout the ocean, ranging from the smallest microorganisms to large fish including the gulper eel. In the dark depths of the ocean, bioluminescence is used by predatory species, like the gulper eel, to attract prey. Their tails glow in the dark depths of the ocean and the predator waits until curious fish approach. The mouth of the eel is very large and, by loosening its hinges, can open wide enough to consume fish much bigger than itself. Another predatory species that uses bioluminescence to catch prey is the angler fish. Infamous for the scene in Finding Nemo, the angler fish uses a bioluminescent barbel on the end of a lure on its head to attract prey close to its mouth, where it then snatches the fish with its large, pointed teeth.

Benthic deep-sea animals (those that live on the seafloor) don’t have the ability to explore the open ocean looking for food, so they rely on debris from dead organisms that sink through the water column (AKA marine snow). Many benthic organisms also use whale falls as a food source. When a whale dies, its carcass will sink to the depths of the ocean, settling on the seafloor. The presence of the carcass attracts organisms from vast distances, including some species of shark, hagfish, crab, and octopus. Once the flesh of the whale is consumed, zombie worms bore into the bones, to feed on fat stores. After a few months, any evidence of this whale’s existence has been destroyed. 

Our Impact on the Deep Sea

There have been so few research expeditions to the deep that new species are discovered on every mission. Only three people have ever explored the depths of Mariana’s trench, but the impact of humans can be seen there already. A shocking discovery was made in 2020, when researchers found a new amphipod species that was already polluted with microplastics. Aptly named Eurythenes plasticus, a sample of this organism was taken to classify the new species. Upon further inspection, a microplastic fibre was found within its tissues. This research highlights the significant impact that global plastic pollution is having on our ocean environment.  It’s a sad thought that species we don’t even know exist, may already be experiencing the incredible detriment that we’re inflicting on our environment. 

The deep sea environment is as enigmatic as the species that inhabit it, so it is exciting to think about the future discoveries that will be made. New discoveries could contribute to fighting diseases, or understanding more about the origin of life on Earth. The vast expanse of the deep ocean is ready to be explored, so what are you waiting for?

In Spotlight on Sealife we celebrate the creatures that inspire our love for the oceans and encourage us to fight for their homes. Follow this series to uncover lesser-known facts about well-known marine life, and discover weird and wonderful marine animals that you may have never come across before.

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