We humans like to think of ourselves as the undisputed masters of planet earth.
We have ardously evolved, over hundreds of thousands of years,
the advanced adaptations required to survive in a slightly warm living room. With the aid
of advanced technology our civilisation has settled in every petrified pocket
and withered wasteland on this wicked world. Except, that’s not entirely true - as of today,
humans can be found in significantly less than 1% of all the liveable locations on earth.
At first that might sound hard to believe... aside from the tallest
mountains, the hottest deserts and that Brazilian island full of fucking snakes,
we've built permanent settlements pretty much everywhere… haven’t we?
Actually, not even close, for the simple reason that well over 99% of the planet’s habitable
space is found in the world's oceans. And as of today, we've colonised precisely none of it.
From our 'terra firma-first' perspective,
it's easy to underestimate the sheer scale of the underwater world. There are over a
billion cubic kilometres of water in earth's combined oceans, and the Mariana Trench,
the deepest point on the ocean floor, is a staggering 6.8 miles beneath the surface.
Just saying the number - 6.8 miles - it doesn’t really give you a sense
of just how far down we’re talking here, so let me make a comparison.
The world's tallest building, the Burj Khalifa in Dubai, is almost twice the height of the
Empire State Building. And yet, you could stack *13* Burj Khalifa's one on top of the other in
the Mariana Trench and you *still* wouldn't break the surface of the Pacific ocean above.
That, dear Thoughty2 viewer, is absolutely bloody mental.
Most of the ocean is significantly shallower than the Mariana Trench, but the global average
depth is still a healthy 2.3 miles, and pretty much all of that space is habitable.
Considering its sheer size it should come as no surprise that the ocean is home to some
78% of earth's total animal biomass. Near the surface, that biomass comes in the form
of animals that are fairly familiar - whales, sharks, brightly coloured fish, sexy mermaids,
that sort of thing. But dive a little deeper, and a deluge of disturbing creatures await.
At 400 metres you’ll meet the monstrous Goblin Shark whose extendable jaws can be
catapulted forwards to engulf unsuspecting prey.
Descend to around 1000 metres and a humorous respite awaits you in the form of the blobfish,
multiple winner of the ugliest animal alive award (that’s actually a thing by the way).
Sink further still into the shadowy depths, to 2000 metres below and you’ll encounter
the ghastly gulper eel, a creature that brings new meaning to the phrase 'all mouth and no trousers.'
But it isn’t just weird fish you’ll find in this watery abyss - nature’s wettest workshop
also breeds giants. From the Kraken to Jörmungand, we've been telling stories
of sea monsters for thousands of years. And perhaps there’s good reason for that. Because,
thanks to a phenomenon known as deep sea gigantism, many creatures that have evolved to
live in the darkest ocean depths are significantly larger than similar species found in the shallows.
Probably the most famous example of Deep Sea Gigantism is the giant squid. Reaching
a maximum length of 13 metres and weighing in at over 250 kilograms, this substantial
cephalopod is literally hundreds of times larger than many of its shallow-water cousins.
The giant squid is a genuine monster of the deep,
but it turns out this great giant has a great big brother - a much, much bigger brother,
the aptly named colossal squid. Weighing as much as *three times* more than the already enormous
giant squid, the colossal squid is the largest invertebrate on planet earth, with eyes the
size of basketballs and hooked suckers capable of leaving sperm whales with permanent scars.
But if that doesn’t freak you out, here’s a subsurface specimen that’ll give you mental
scars. This “little” critter is a giant isopod. If it looks strangely familiar
that's probably because it's closely related to the humble woodlouse. A well-fed woodlouse
is about a centimetre long and weighs less than a gram soaking wet. The giant isopod,
on the other hand, can grow close to half a metre and weighs almost 2 kilograms.
Everywhere we look in the deepest, darkest depths of the ocean,
we see a similar story - familiar-looking animals have been mysteriously dino-sized.
Like the Japanese Spider Crab with a leg span of up to 4 metres, and the giant oarfish, sometimes
known as the King of Herrings, which can reach a maximum length of around 11 metres. 11 metres!
It’s like God turned up to work drunk on the fifth day and made a bunch of rounding errors.
So, just what the hell is going on here? What’s
causing deep sea species to grow so freakishly large?
The truth is, we actually aren't 100% sure. As you're probably already aware, we know
surprisingly little about the ocean floor. In fact, up until 2019 more people had walked on the
surface of the moon than had visited the deepest part of the Mariana Trench, Challenger Deep.
The sheer remoteness of deep ocean environments makes them - and the creatures that live
there - incredibly difficult to study. Take the colossal squid as a prime example. As long as
a London bus and weighing more than a fully grown polar bear you'd think it’d be pretty
bloody hard to miss. And yet, with all our fancy technology and remote operated submersibles,
not a single colossal squid has *ever* been observed in its natural habitat. Not one!
Since we never get to bag ourselves one of these beasties, research into the causes of deep-sea
gigantism has proved rather difficult - but we do have some surprising theories.
The first is that food scarcity is the primary cause. The idea that a
*lack* of food could somehow stimulate the evolution of larger animals might sound
like nonsense - surely the opposite would happen, right? But dig a little
deeper into the science and this apparent contradiction starts to make a lot of sense.
Unlike almost every other ecosystem on the planet, the ocean depths have
close to zero food production of their own - no light means no photosynthesis,
and no photosynthesis means no phytoplankton, which is the main producer near the surface. As
a result, the primary food source in the deep sea is so-called marine snow. A deep ocean delicacy,
marine snow is basically a mix of dead animals, decaying phytoplankton, and a whole load of
fish shit that slowly drifts down into the abyss from the shallow waters above. Sounds delicious!
Marine snow - supplemented by the carcasses of larger animals - is consumed as it falls,
meaning the deeper you get the less there is to go around. That makes food on the ocean
bottom both scarce and unpredictable - there isn't a great deal of it,
and you never know where the juiciest morsels will land.
For a bottom-dwelling scavenger like the giant isopod, being bigger means
it can cover longer distances more quickly, increasing its foraging
radius and improving its chances of being the first on the scene when food arrives.
Larger animals are also able to store more food than smaller ones,
helping them to survive long periods without sustenance.
Many deep sea creatures have evolved extendable stomachs that allow them to take in the maximum
amount of food on the rare occasions a solid meal presents itself. The results can be surprising - a
giant isopod with the catchy nickname 'Giant Isopod #1' once went 5 years without eating
a single thing, presumably in protest at its imprisonment in the Toba aquarium in Japan.
5 years without food is undeniably supermodel-tier dieting, but these
long fasts are made possible by another benefit of increased size - a more efficient metabolism.
According to Kleiber's law, an animal's metabolic rate scales to the 3⁄4 power of its mass.
To translate that into normal person English,
larger animals need less energy per unit of mass than smaller ones.
A mouse for example needs to eat up to 20% of its body weight every single day to survive,
whereas an elephant only needs to consume 4 or 5% of its body weight. In other words, larger animals
use energy more efficiently. And that is a massive bonus in an environment where food is scarce.
The other main theory as to why creatures of the deep grow so big is related to temperature.
Because cold water is denser and therefore heavier than warm water it sinks, and that
means deep-sea environments are extremely cold. Frigid temperatures inhibit the speed
of metabolic processes, decreasing the amount of food and oxygen required to
sustain life. A slower metabolism is also thought to be part of the reason many deep
sea creatures grow slower and live longer than similar species in shallower waters.
As an extreme example, recent research suggests that the bottom-dwelling Greenland Shark may be
able to live to well over 500 years old, making it the longest-living vertebrate ever discovered.
Just think about that for a second - there may be a Greenland Shark swimming around today that
was alive when Leonardo Da Vinci painted the Mona Lisa in the early 16th century.
The idea that lower temperatures play a role in deep-sea gigantism is backed up by the fact
we *also* see something called *polar* gigantism - a tendency for species that live in the frigid
waters of the Arctic and Southern oceans to exhibit gigantism, even in shallower waters.
Sea spiders are a good example - in most parts of the world, sea spiders are relatively tiny,
from a few millimetres to a couple of centimetres across. The Antarctic sea spider, on the other
hand, has a nightmare-inducing 70-centimetre leg span - easily big enough to latch onto your face
and inject thousands of its babies directly down your throat. Similar massive increases in body
size can be seen in polar sponges, worms, starfish, and even single-celled organisms.
OK, so we've got the food scarcity theory and the temperature theory, but there's also a far simpler
reason deep-sea species have evolved to get so big - there are significantly fewer predators
down there. That's allowed Mother Nature to be a little bolder with the body sizes of species
that might traditionally stay small to attract less attention from things that want to eat them.
The truth is, there's probably no single unifying explanation for deep sea gigantism,
and different variables will play a bigger role in some species than others. It's also worth
pointing out that plenty of abyssal creatures exhibit the exact opposite effect - deep sea
dwarfism - which is a clear indicator that this little puzzle is unlikely to have a simple answer.
Ultimately, the deep ocean is a chance to see how life somehow finds a way in
one of the most inhospitable environments on planet earth.
Thanks for watching.