The wonders of the deep sea are shadowed by a veil of rolling waves. For centuries,
the nature of the sea floor remained a mystery, while humanity could only ponder on its secrets.
How deep did it go? What landforms, if any, could be found there? Whether there
was anything more than a featureless stretch of mud, they did not know.
But when early explorers cast their weighted lines over the sterns of boats to measure the depth,
they uncovered clues to a far more complex landscape.
A world of sprawling ocean plains, yawning chasms eclipsed by the dark of the abyss,
and underwater mountains that rose hundreds of thousands of feet from the seafloor, reaching
for the surface. The seamounts. In subsequent years, exploration of these profound monuments
unveiled a dominion of deep sea dwellers. A hotspot of life where currents convened,
bringing nutrients and creatures from the marine expanse together in one place.
Let’s dive in, and explore the life of the ocean’s staggering seamounts.
Breaking the flat monotony of the abyssal plains, the underwater mountains emerge from the mud.
Those that rise less than 1,000 m (3,000 feet) above the plains
are called abyssal hills, while anything taller is a seamount. And there are at least
100,000 of them, with over half found in the Pacific Ocean near the tectonic plate boundaries.
An unsurprising statistic, for Seamounts are formed from
volcanism. The same process responsible for the formation of many island chains.
At mid-ocean ridges, the converging tectonic plates collide at boundaries called subduction
zones. One plate is subducted, or forced down towards the Earth’s molten interior.
As it is subducted, the crust melts and becomes magma,
which rises through the rock and erupts to form volcanoes and seamounts.
In the Pacific, this has led to the formation of the Ring of Fire. More than 450 volcanoes line
this path where the Pacific plate collides with or pulls away from neighbouring plates. Many of the
seamounts that line the ring of fire are located within the boundaries of Monterey Bay National
Marine Sanctuary, a wild reserve encompassing 444 kilometers (276 miles) of California’s coastline
and over 15,500 square kilometers (6,000 square miles) of open ocean.
This Serengeti of the Sea is home to seamounts that host a breathtaking variety of life.
While the scale of these underwater monuments is impressive in its own right,
the true grandeur of seamounts is defined by their ecological importance. With structures
for creatures to cling to and settle on, and currents carrying food and nutrients from all
throughout the oceans, the abundance, variety and complexity of life at seamounts is breathtaking.
Many deep-sea animals are sessile, meaning they attach themselves permanently to rocks
to live out their lives. So for the corals and sponges, seamounts provide the sanctuary they need
to survive. For this reason, creatures that are sparse and scattered out on the abyssal plain
gather on seamounts in vibrant communities.
Currents play an important part in creating this ideal haven, washing away sediment to
expose the rocks that corals require, while also supplying a copious feast.
As they cannot move around to find food, corals and sponges are dependent upon ocean currents to
bring this food to them. And seamounts boast the perfect conditions for this.
The currents produce a localised upwelling of water around the mounts.
Planktonic prey and marine snow are lifted from the cavernous deep
towards the sunlit surface. Nitrates and phosphates are carried along too -
nutrients that are essential for the growth of phytoplankton in the sunlight zone.
Such a veritable feast as this creates an explosion of activity.
Larger animals, from whales and sharks to swarming seabirds,
gather at the seamounts to feed, migrating for miles to convene here.
Tiny larvae of marine creatures are carried for hundreds of miles,
with the currents forming transport routes between geographically isolated locations.
As such, animals not normally seen together can be found living side by side.
And nowhere is the extent of this bewildering biodiversity more visible
than at the Davidson Seamount, off the coast of Central California.
An inactive volcano, and the first undersea island to be called a seamount.
At 42 km (26 mi) long, and 13 km (8 mi) wide, it is among the largest ever discovered.
From its base to its crest, it rises to 2,280 metres (7,480 ft) tall,
yet its summit is still over a kilometre beneath the waves.
It was here at the Davidson Seamount, where one of the most important deep sea discoveries was made.
For thriving in this cold, dark and mysterious habitat, are large and ancient coral gardens.
Prior to their discovery, it was believed that corals could only exist in high abundance
in warm water, and required the light of the sun to survive. But on the ridges of the seamount,
far below the reach of the sun, dense and colourful communities cling to the rocks.
This is one of the oldest coral forests,
with some individuals rising more than 3 metres high and having lived for over 100 years.
Unlike tropical reefs, their survival is due to their resilience to the extremities.
As they’re not dependent on warm water, they can thrive in waters as cold as -1 degrees Celsius,
and as deep as 3,050 metres (10,000 feet) below sea level. And instead of obtaining energy from
sunlight, they have become scavengers and predators, catching dead organic
material or ensnaring plankton with their branching, feathered, fan-shaped structures.
Coral communities are not the only gardens found at the depths of Davidson Seamount.
Aggregations of thousands of mother octopuses are found nursing their eggs at a location now known
as the Octopus Garden. Tucked into rocks near shimmering hydrothermal fluid seeps,
they make use of the heated water to incubate their long, white eggs, while protecting them with
their eight arms facing outwards. This sprawling community was discovered as recently as 2018
by the E/V Nautilus team when they blindly descended 3,000 meters (9,800 feet) down the side
of the seamount and captured the footage you’re seeing now. This goes to show that with less than
0.1% of Pacific seamounts having been explored, there is much that remains to be uncovered.
Not all seamounts are formed at the ocean’s plate boundaries.
In some locations, known as hotspots, plumes of magma rise through the crust at the centre of
plates and erupt through the sea floor to create chains of seamounts, such as the Hawaiian Islands.
The hotspot itself is fixed, while the plate is moving. And as it drifts, successive mountains
are formed one by one, creating the string of islands that make up Hawaii’s island chain.
These isolated islands are home to familiar creatures
that have transformed themselves in bizarre ways, letting evolution run wild.
This process is prominent too in the Galapagos islands, demonstrating the
effects of isolation on communities of life that find themselves secluded from the mainland.
Join me in the next video as we explore the life of remote islands,
and the fascinating ecosystems that form when seamounts rise above the waves.