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Wonders of the Deep
The deep blue. The abyss. The glaring nothingness. How often have you looked at the sea and thought, ‘I wonder what’s down there?’ We all know of the ocean: stormy waters, white sand beaches and reefs – but how much do we really know about it?
With an average depth of 4000m, much of the deep blue was thought to be barren and void of life – but a remarkable community of lifeforms exist down under, from giant glowing squid to sharks older than our great great great grandparents, and many species that look better placed in a science fiction movie rather than on earth.
Amazingly, only 26% of the ocean floor has been explored – and that’s even lower for the thousands of metres between the surface and seabed. That’s like you reaching a quarter of a tub of cookie dough ice cream. Imagine what’s left to discover?! Crazy, really. One thing we can say for certain is: there’s a whole world down in the deep sea you might not have expected.
Going Down
Now, let’s imagine a journey down – one very few have made. (Twice as many people have walked on the moon – 12 – than have gone to the deepest part of the ocean.) The top layer, the sunlight or euphotic zone, is where the Sun’s touch nourishes absolute snacks like phytoplankton, algae, and seagrass, which gobble these rays up and get nice and strong. But what happens when you start to go below that?
As we descend, the Sun’s presence starts to fade away. Temperatures start to cool. And the pressure starts to build. Here’s where things start to get a little fruity. You may expect that you’d enter a world that is vast and lifeless. What could survive freezing cold waters and a pressure 1,100 times greater than at the surface? Quite a lot, as it turns out.
Buckle up, and let’s go further into the depths to meet some incredible species and discover why they’re under threat.
The Twilight Zone - Between 200m and up to 1000m below the surface
A mysterious section of the sea where sunlight starts to disappear, and oxygen levels reach all-time lows. But far from lacking in life, the Twilight Zone is home to the largest animal migration on earth and, potentially, a greater collection of fish than the rest of the ocean combined.
What initially confused World War II U.S. Navy sonar operators, who appeared to see an ever-changing seafloor while looking for enemy submarines, is now known as the diel vertical migration. When darkness falls, a high concentration of fish, plankton, squid, and more species swim hundreds of metres up to shallower waters to feed safely under cover of night before returning to the depths by the morning.
As you look around the dimly lit waters, you see an exciting range of animals. Some of the largest in the ocean, such as the giant oarfish, the longest species of bony fish, to tiny zooplankton. You’re also likely to say hello to a bristlemouth fish or two, a lil’ nipper with a mouth full of jaws, which is the most abundant vertebrate on Earth.
Let’s be frank: it’s tough living here, but these species have adapted with features like large eyes (Telescope fish, stand up) and gaping mouths to make survival here possible. One viral adaptation is that despite the low amounts of sunlight that makes it this far down, the waters are still alive with light, the flashing lights of bioluminescence.
Bioluminescence is like having your own neon sign, little light bulbs on your skin powered by natural energy stores. A familiar example is the firefly, but even that pales compared to the glorious displays of the deep. Bloodbelly comb jelly to stage. This spectacular glowing red orb has a series of flashing combs that look more like something you would see at a full moon festival than a living creature.
These jellies don't just look the part, but they’ve adapted an ingenious way to avoid being eaten. In the deep sea, red light is rapidly absorbed and scattered by water molecules. So, organisms with red lights – surprisingly – blend into their surroundings. What may seem to us like a glowing beacon is a masterful disguise. But why is it needed? BCJs are predators (shock). Unfortunately for them, they’re see-through. When they snack on smaller, glowing creatures, these will illuminate their belly, so the disguise helps them snack in silent mode.
There are other ways to avoid predators with light; for example, many fish species have adapted a strategy called counterillumination. Since light in the Twilight Zone comes from above, many predators will hunt from below, looking up to spot their prey's silhouettes. Therefore, several small prey fish species have rows of organs called photophores along their bellies that emit light similar to the light from above, so their silhouettes blend in. On the flip side, many predators use bioluminescence to their advantage, for example, by luring unsuspecting prey fish their way.
The Midnight Zone – 1000m below the surface to 4000m below the surface
We’re really plunging now, almost as deep as Mount Blanc – the tallest mountain in Europe. Also known less fantastically as the Bathypelagic Zone, here we say goodbye to any last remains of sunlight and hello to a habitat Dracula would be proud of. A realm in a permanent state of darkness. Oh, and did you pack a puffer coat? It’s a steady 4°C, pretty much forever.
If that wasn't hostile enough, the pressure ranges from around 100-400 times stronger than at the surface. Sounds like a pretty unpleasant place to live, right? This 'unpleasant place’ is the largest single habitat on the planet, accounting for 70% of all seawater. Perhaps unexpectedly, life here has found a way to thrive.
Bioluminescence is once again a major adaptation many midnight zone dwellers have. That and excellent hearing allow them to navigate a pitch-black world. Finding Nemo fans step up because the next species we’ll look at is the deep-sea anglerfish – these peculiar fish use bioluminescence for more sinister means. Anglerfish, as the name suggests, are master anglers, sporting a ‘fishing pole’ on top of their bulbous bodies with a glowing orb at its tip. This orb is actually filled with glowing bacteria, and each species of anglerfish has its own unique rod and lures. Anglerfish use these glowing bulbs to lure in targets who expect a small glowing snack. But instead, they are snapped up!
On a more romantic note: what do today’s Greenland sharks have in common with Shakespeare? They may well have been alive at the same time. One of the largest shark species at over 6m long, midnight resident Greenland sharks live to at least 400 years old, so some may have been slowly swimming through their murky habitats, completely unaware Romeo & Juliet was being written way above their heads.
These gentle giants (the sharks, that is) love a cold plunge. They’ve adapted to freezing cold temperatures by producing their own anti-freeze-like chemical in the blood and tissues, which stops ice crystals from forming in their body!
The Abyssal Zone – up to 6,500m below the surface
We plunge downwards, sailing narrowly past the anglerfish’s gaping jaws. Phew. Although most oceans bottom at around 4,000 metres, the Abyssal Zone occurs where underwater trenches can form, and we get much deeper!
Again, the category is: how does life survive down here? Pitch black, cold, and the pressure gets 600 times stronger than on the surface. Humans certainly wouldn’t want a part of this. But it doesn’t stop some species.
Say hello to the dumbo octopus. Named after Disney's Dumbo, the elephant, this little cephalopod is the deepest-living octopus and, despite their calm appearance, are voracious predators. They have a mouth different from other octopi and a large gaping hole, which they use to slurp up a tasty snack, such as a bristle worm. Never judge a book by its cover.
The Hadal Zone – over 11,000m below sea level in some places
Named after the Greek god of the underworld, Hades, this incredibly hostile environment stretches from the Abyssal zone to the very bottom of the deepest part of our oceans.
Despite most ocean areas not getting anywhere near this deep, the Hadal zone still makes up an area about the size of Australia. Shock horror – it’s pitch black and freezing. But here something different occurs. Pressure begins to reach unimaginable levels, as much as 1100 times stronger than at the surface! That's like having 100 adult African elephants standing on your head. The pressure is so great that the very water molecules themselves are distorted and change shape. Yet again, life prevails in this remarkable habitat despite all the challenges.
One particularly fascinating Hadal resident is the hadal snailfish, the deepest living vertebrate on record. They are perfectly adapted for the pressure with soft bones, an incomplete skull to stop their brain from getting crushed, and, most remarkably, high concentrations of an organic molecule called piezolytes. These highly adapted molecules counteract the weight of the water column by increasing the space that proteins take up inside the cells of the snailish, stopping their cellular components from being crushed under vast pressure.
And as your feet finally touch down at the deepest part of the ocean, Challenger Deep in the Marianas trench, we reach 11,000m below sea level. Good job. Looking up at the pitch-black water, we can now understand that although it may seem bleak and barren, it’s teeming with life and harbours a very important and fragile ecosystem of amazing and awe-inspiring creatures.
However, despite how remote each layer of the ocean may appear, they may very soon be under threat, and the eerie silence and blackness may be replaced by the screams of drills and human-made floodlights.
Ocean mining threatens the deep sea
Close your eyes for a second. You’re in your room, which is insanely cold (just how you like it), and pitch black, but that’s fine because you love it that way. You’re just existing, as you normally do, floating around thinking about snacks and life. BAM. BOOM. ZZZZZZZZZZ. People have burst in and are drilling all around your sacred space. What’s that all about?!
While most of us think about habitat destruction only affecting rainforests, over recent years, there has been increasing interest in the potential treasures of the deep. Norway recently became the first country to approve deep-sea mining, and whilst other countries have been resistant for now, there are concerns that this may change. A mineral-rich area of the Pacific Ocean called the Clarion-Clipperton zone is currently targeted by prospective miners due to its rich nodule deposits of minerals such as cobalt and nickel. Uh oh.
These minerals would be valuable in building batteries for electric cars, but there is a major catch. As we know now, the deep sea is not a lifeless area that is out of sight and out of mind but a rich and expansive habitat filled with life. And that is just the life we know about! About 90% of species found in the Clarion-Clipperton zone have not yet been identified, and they may already be in danger. What would happen without them in our ecosystems? Nobody knows.
It is still not too late; major-scale mining has not yet started, and we have the opportunity to save these species and ecosystems. It all starts with awareness, as many people will not know about what lies below the waves, as you can see when we interviewed people on the streets of London about deep-sea life. There are people trying to get this message heard to protect our oceans, such as the team behind Deep Rising. As you return to the surface, think about all the species you pass on the way up, from the pressure-resistant snailfish to the colour bloodbelly comb jelly and think about what more could be out there yet to be discovered.