The expanse of the deep sea is slowly becoming more understood, with new discoveries constantly
challenging our understanding of the ocean’s complex processes and diversity. But one deep
sea ecosystem remains very much a mystery to us. We are only now beginning to explore
deep sea canyons, and in doing so, we have uncovered diverse and abundant coral reefs
that we did not know existed until recently. Their scale is immense, with colourful sponge
gardens and corals lining the chasms and ridges of expansive canyons at 6,000 metres or 20,000
feet deep - some rivalling the Grand Canyon in their enormity. But how do they survive
down here in the cold, dark abyss? Let’s dive in, and take a look.
The last place you might expect to find corals is in the deep sea. But down in the darkness,
there are lush gardens thriving in the icy waters. An expanse of colourful structures
blooming out of the sea floor. These communities are hot spots of biodiversity, providing important
habitats for deep-dwelling life. And yet, they are one of the most poorly understood
of all deep sea ecosystems. Much like in shallow reefs, deep-sea corals are made up of colonies
of coral polyps, working together to survive. They form when a polyp attaches itself to
a rock, and divides into thousands of clones through the process of budding. The polyps
then secrete a protective limestone skeleton called a calicle, which creates the coral
structure.
What’s unique about the corals of the deep, however, is their resilience to the extremities.
As they are not dependent on warm water, they can thrive in waters that are as cold as -1
degrees Celsius, and in all the ocean’s basins, forming havens of life amidst vast
canyons, on continental shelves and slopes, and on towering ocean seamounts. Unlike tropical
reefs, they live from 150 feet to more than 10,000 feet below sea level, where sunlight
is dim to nonexistent. So, instead of obtaining energy or nutrients from sunlight, they live
off the dead, and gather all they need by catching organic material called marine snow
that drifts past on the currents. But leftovers from the shallows are not enough on their
own. Here, the corals also eat microorganisms, such as krill and plankton, ensnaring them
with their branching, feathered, fan-shaped structures.
Deep sea reefs are sometimes referred to as mounds. This is because as the reef grows,
corals below die-off and leave behind a large calcium carbonate skeleton. Over time, these
build up and provide the rocky surface needed for new polyps to attach to and grow from.
This coral species is known as Lophelia pertusa, and it is one of the most widely distributed
deep sea coral. This branching stony coral began its life as a tiny larva, drifting through
the deep. Once it settled on hard rock, it began to grow into a colony. Now, as it continues
to grow, the outer branches prevent water from flowing to the inner branches. As a result,
food and oxygen cannot reach the polyps within. The inner branches die, weaken, and break
apart, while the living branches overgrow the dead skeleton.
New life at the reefs depends on this cycle of death, leading to the expansion of the
ecosystem over time. The constant sequence of growth, death, collapse, and overgrowth
goes on for thousands of years, and creates reefs that grow to be hundreds of feet tall.
Gigantic, complex structures that provide unique and vital habitats for marine life.
Other coral types, known as tree corals, form dense coral gardens, providing important spawning
and nursery grounds.
There is an astounding diversity of life found at these deep sea gardens. This is because
they are well-established communities. It is estimated that one particular gold coral
colony found near Hawaii is over 2,700 years old. Another black coral colony is thought
to be 4,200 years old, making these colonies the oldest marine organisms ever discovered.
Due to the ability of polyps to continuously regenerate, some deep reefs have been thriving
and expanding for over 40,000 years. Thus, their stable and unchanging nature has provided
these habitats for marine life, and allowed organisms here to diversify and evolve into
some truly unique creatures.
Worms, anemones, starfish and lobsters are just a few of the abundant invertebrates that
depend on these deep corals. Many fish live here too, relying on the corals as a source
of food, a place to hide from predators, and to nurse their young.
Animals found here are demersal. This means that they live on or near the sea floor, like
this red coffin fish, a species of deep sea anglerfish that walks through the reef using
specially adapted fins. In some cases, the corals become involved in symbiotic relationships
with the demersal life at the reefs. This unidentified crab was found carrying a colony
of Zoanthid corals on its back. The crab benefits from obtaining a degree of protection from
the coral, while the coral gets better access to food as the crab moves around, and it no
longer has to compete for space with other corals.
Despite the remote isolation of deep sea reefs, they are not out of reach of human destruction.
They face the threat of bottom-trawl fishing; an indiscriminate, destructive method that
sweeps up unwanted animals, including the corals, as bycatch. The destruction of these
corals devastates deep sea life. Unlike shallow water corals, they do not depend on a symbiotic
algae (zooxanthellae) for growth and survival, so they grow 10 times slower. Trawling reduces
them to rubble, so with a growth rate of a few millimetres per year, damaged coral groves
won’t regenerate for hundreds of years, if at all.
Overall, deep sea coral gardens are vital to the delicate deep sea ecosystem. In providing
food and protection for marine life, many of which is unique to these reefs, they support
the ocean’s biodiversity. Their beautiful colours and structures, unique adaptations
to a cold, dark sea, and their supporting role for the larger ecosystem represent a
distinctive habitat that we have only just begun to explore. They must be protected and
preserved, or we risk losing one more wonder of the deep sea.