Deep sea ecology: sea floor | WWF
Like land above the sea, the sea floor is a varied landscape of mountains, canyons, plains, and volcanoes.
Deep-sea monkfish (Lophius piscatorus) capturing a cod species at a depth of 400m, North East Atlantic Ocean. © WWF / Ian HUDSON

Following on from the continental shelf, the continental slope continues gently down for thousands of kilometres towards the ocean floor - the vast, flat abyssal plain, reaching depths of 2,200-5,500m and covering over half the Earth’s surface.

Varied landscapes
It's not all flat, however. In some places the seabed drops down into trenches. The deepest is the Mariana Trench in the western North Pacific Ocean, whose deepest point is an incredible 10,911m below the surface.

The seabed also rises to form mountains. Running for over 56,000km through the middle of all oceans, the Mid-Oceanic Ridge is the world’s longest mountain range. It’s created by lava erupting from the Earth's crust - eruptions that account for 95% of all volcanic activity on our planet!

There are also a number of deep-sea geysers called hydrothermal vents and isolated underwater mountains called seamounts.

99% unknown
Scientists have only explored 1% of the sea floor. Indeed, we know less about the sea floor than the surface of the moon. Unsurprisingly, very little is known of its ecosystems.

Relying on manna from above
From what we do know, life is mostly scarce: the density of animals on the continental slope is much lower than on the continental shelf, and on the abyssal plain it is 1,000 times lower still. But the enormous area of this region makes the dominate form of life here, echinoderms (starfish, sea urchins, sea lilies, brittle stars, sea cucumbers, and sea daisies), amongst the most numerous animals on the planet.

With a few notable exceptions, the food chain depends on detritus - dead animals and faeces - falling from shallow waters. Rocky outcrops provide anchoring points for sea lilies, brittle stars, sponges, anemones, and cold-water corals, which filter this detritus from the water. Detritus reaching the muddy abyssal plain is eaten by sea urchins, starfish, sea slugs, crabs, shrimp, worms, bottom-feeding fish, and bacteria.

This life extends into the trenches: when a US Navy expedition made it to the bottom of the Mariana Trench in 1960 (the first and only manned venture this deep), fish and shrimp were observed.

More on seamounts and hydrothermal vents...