The deep sea is a hidden world of wonder. As we have explored so far in this series,
it is home to countless ecosystems of abundant biodiversity, though veiled beneath the inky
waters of the deep. But there is one location in the deep sea that poses an even greater
mystery. Hidden away beneath the sea ice of the Arctic and Antarctic, lies a lost world
of giants. Creatures that have adapted in remarkable ways to a life in these icy, sunless
waters, forming vast communities that we are only just beginning to explore. Let’s take
a closer look at what’s been found lurking in the dark beneath the ice.
Life at the poles can be incredibly difficult. Long, cold winters bring bitter winds that
whip across the landscape. Temperatures plummet deep into the negatives, as the winter night
falls, bringing darkness that can last for months. Sea ice reaches far beyond the shores
of any frozen land-mass. A constant, though seasonal feature of polar marine ecosystems,
the importance of which cannot be understated. Sea ice maintains the low temperatures of
polar regions and helps to moderate the global climate; its bright surface reflects the sunlight
that strikes it back into space. Without this vast expanse of white, the dark ocean would
absorb this light and heat up, throwing ecosystems around the world into chaos.
Sea ice is important also for many of the surface-dwelling creatures of polar regions, for
these seemingly barren landscapes host a rich diversity of wildlife, both above and below
the ice. In the Arctic, ice is an essential hunting ground for polar bears. These lone
hunters are totally dependent on it, prowling the icy plain in search of breathing holes
where their seal prey come up to the surface. In the Antarctic, the ice platform is equally
important for seals and penguins, relying on sturdy ground where they can breed and
raise their young. However, sea ice is also home to a diverse community of micro-organisms
and invertebrates. Creatures whose populations support the entirety of the polar food webs.
The underside of sea ice is not as smooth as the surface. Here, it is riddled with channels
and cavities that create an uneven surface. Formed when pockets of salty brine trapped
between the ice crystals drain away and form channels, this is the perfect habitat for
micro-organisms. Photosynthesising algae and bacteria, waiting patiently to bloom in spring
in a mighty oceanic feast. Some channels are large enough to provide a refuge for larger
creatures. Amphipods, krill, and tiny worms, clinging to the safety of these ice formations.
They utilise them as a sort of upside down seabed. In the Southern Ocean, krill feed
on the phytoplankton and graze on the sea-ice algae. Because of the plentiful food found
under the ice, krill are known as a keystone species, underlying the entire food web of
the Southern Ocean.
In the Arctic Ocean, a community of creatures thrives in a world without sunlight. From
carnivorous sea stars to floating ctenophores, life persists in perpetual conflict. There
is no marine snow directly beneath the ice sheets, for all that lies above is a thick
layer blocking out the sun. As a result, most animals are predatory. Even the most innocent-looking,
the graceful sea angel, feasts solely on its relative the sea butterfly.
Around Antarctica, there are a number of hidden ocean cavities that play important roles in
the circulation of currents, nutrients and creatures. At 480,000 square kilometres, the
Ross Ice Shelf is the largest floating slab of ice on Earth, and it conceals an ocean
cavity that extends 700km south of Antarctica’s coast, and remains largely unexplored. But
recently, the Antarctic Deep has become the focus of a number of expeditions, in search
of what life could possibly survive down there. It was believed by many that in the depths
beneath the ice, you would find nothing more than a biological dead zone.
The deep ocean around Antarctica is a unique place. The weight of the ice sheets has pushed
down the submerged continental shelf to be deeper than most places. Usually, the continental
shelf is found between 100 and 200 metres down. But here, we must sink to 600 metres
before the edge looms out of the darkness, and a gaping void opens up beneath.
This is a gateway to the deep, not only for researchers but for marine life. The water
temperature near the surface is not too different from the cold depths, meaning deep sea creatures
are able to come into much shallower depths than usual. As such, a world of wonders lies
in wait. Communities of creatures that are often out of reach of submersibles in other
parts of the world. Oxygen-rich waters support a plethora of life. This is made possible
by the freezing of Antarctic seawater in winter, which leaves behind extremely cold, salty
water. This water sinks and flows around the world, providing the life-giving oxygen to
deep waters all around the world, preventing the deep sea from becoming a dead zone due
to hypoxia. But here is it most concentrated. The most oxygen-rich waters on the planet,
concealed beneath a barren world of ice.
Creatures make the most of this phenomenon. Gigantic sea-spiders tread across the sea
floor, with leg-spans of up to 40 cm. The lack of a respiratory system means that, in
most parts of the world, sea spiders grow no larger than a few millimetres across. Here
in the oxygen-rich conditions, they can grow far larger, exhibiting the phenomenon of polar
gigantism. Marine invertebrates that occur at the poles tend to have much larger body
sizes than their relatives that live elsewhere. It can be seen in many of the creatures here.
Two metre tall giant sponges cling to dropstones - large boulders that fall from passing icebergs,
providing islands of rocky habitat on the soft mud for filter-feeders to cling to. Thanks
to them, feather stars and cold-water corals have colonised the region, feeding on the
abundant marine snow that accumulates here. Well adapted to a stable and unchanging environment,
these plant-like animals grow very slowly, but to enormous sizes.
Very few fish are found here. Unlike in most marine ecosystems where they dominate the
food web, they struggle to survive the freezing temperatures here. Instead, the Antarctic
deep is a prehistoric world dominated by giant invertebrates, like the Antarctic sunstar
which captures passing krill with tiny pincers. The few fish that do live here must find creative
ways to adapt. One, known as the ice dragonfish, has antifreeze proteins in its blood to stop
it from freezing into ice.
Creatures here belong to a community that has been isolated from the rest of the planet
for millions of years. The powerful Antarctic Circumpolar Current swirls around the frozen
continent, creating a steep temperature gradient that stops marine animals from spreading.
Their isolation has led to the diversity we see today - creatures unique to the region
that have evolved to live on the sea floor.
Overall, the deep sea found beneath the poles is a world of hidden wonders - organisms unlike
any that are found elsewhere, owing to their isolation in an oxygen-rich pocket of life.
A stable and undisturbed cradle of life, yet one that is threatened by human activity.
The warming of the climate could disturb these ancient ecosystems, allowing invasive species
to wander further south or north than they would normally be able to. The equilibrium
of life in the polar deep faces an uncertain future. We risk losing one of the ocean’s
best-kept secrets, before we even understand its importance.
To find out more about the deep sea, and to visualise just how deep some of these creatures
live, visit the ‘exploring the zones’ page over on our Deep Sea Hub, where you can
scroll down through a scale model of our oceans.