The ocean covers 71% of the Earth’s surface, yet humans have only mapped a small percentage of the
ocean floor. From shallow coastlines to deep sea trenches, the ocean floor offers diverse,
often extreme ecosystems, where many of the world’s strangest creatures survive and flourish.
What lives in the ocean’s most extraordinary oases? What mysterious
phenomena are hiding at the bottom of the ocean? When you stand on the beach and look across
the ocean’s surface, you may see nothing but shimmering water, stretching beyond the horizon;
but under the surface, the ocean is teeming with spectacular organisms, thriving inside a wide
array of complex and delicate ecosystems. Along coastlines, estuaries, and islands,
marine plants and animals flourish in sunny, shallow waters. The bright sunlight nurtures
many vibrant ecosystems, but none as biodiverse as tropical coral reefs, the rainforests of the sea.
Coral reefs are living ecosystems, built by hard, colorful coral. Often mistaken as flora,
corals are marine invertebrates that grow in tight clusters along the ocean floor.
Warm-water corals maintain a symbiotic relationship with microscopic algae
called zooxanthellae. Zooxanthellae live safely inside coral tissue,
providing their hosts with oxygen and energy. With the help of zooxanthellae, corals build
rigid exoskeletons in dynamic formations, which create a coral reef’s three-dimensional structure.
Tropical coral reefs support 25% of all marine life, from the largest rays to the smallest
microorganisms; but they aren’t the only plentiful ecosystems found in the ocean’s sunny shallows.
In temperate and polar climates, up to 40 meters below the surface, dense populations
of giant kelp, the largest brown algae, grip the rocky seafloor with branching roots called
holdfasts. Colonies of giant kelp, anchored to the ocean floor, form three-dimensional ecosystems,
which eerily resemble underwater forests. The canopy of a giant kelp forest is rich
in nutrients. The canopy feeds and shelters a myriad of organisms,
like playful sea otters, colorful rockfish, and barking sea lions. But when the water becomes
too warm or populations of herbivores grow too large, this prolific ecosystem can change rapidly.
Unlike terrestrial forests, which stand tall for hundreds of years, giant kelp forests
can disappear and reappear. Warm spells, tropical storms, and stress from predators,
like sea urchins, endanger these unique ecosystems. When in danger, giant kelp
detaches from the seafloor and drifts along the current, searching for colder and calmer waters.
Up to 50 meters below the surface, tropical and temperate waters house another unusual
environment: a meadow grown entirely underwater. Marine meadows are made of seagrasses,
a group of angiosperms, or flowering plants, that are completely aquatic. Seagrass meadows
support a diverse food chain, including manatees, sea turtles, and many species of crustaceans.
Beyond the sunny shallows, and their lush ecosystems,
the deep sea is a freezing environment, engulfed in darkness.
Steep, V-shaped valleys, called submarine canyons, sink over 1,000 meters below the surface.
Most submarine canyons are found along continental margins, often at the mouth of a
large river. These hyper-productive, nutrient-rich ecosystems support large populations of megafauna,
like sharks, swordfish, and giant tuna. While submarine canyons plummet into the deep sea,
rocky formations, called seamounts, rise from the darkness. Seamounts are the remnants of
dormant volcanoes, some of which climb from the deep sea all the way to the ocean’s surface.
Seamounts create huge, three-dimensional ecosystems, providing habitats for a variety
of marine organisms. Species like sea stars and deep-sea corals latch on its rocky cliffs, while
fish and migratory animals, like whales, feed on numerous species of shellfish and plankton.
Beyond submarine canyons and undersea mountains, deep sea trenches, like the Mariana Trench in the
Pacific Ocean, plummet into a world of darkness and desolation. The Mariana Trench is the deepest
place on Earth. This remarkable formation plunges roughly 11 kilometers below the ocean’s surface,
where the pressure is 1000 times greater than atmospheric pressure at sea level.
For decades, scientists have debated whether organisms live at the bottom
of the Mariana Trench. In 1960, the first manned submarine dove to the Challenger Deep,
the lowest point in the Mariana Trench. To their surprise, they discovered living fauna
thriving in the deepest place on Earth. To survive in dark, barren trenches,
organisms have taken bizarre evolutionary turns. Several creatures,
like sea cucumbers, navigate the deep sea with specialized, bioluminescent cells. Others,
like angler fish, use colonies of bioluminescent bacteria to hunt and lure their prey.
With limited resources in the deep sea, many species feed on animal waste, called marine snow,
that rains down from the shallows. Marine snow can take weeks to reach the ocean floor,
creating a storm of white waste, which looks like an underwater blizzard.
Occasionally, the bodies of megafauna, like whales,
sink to the ocean floor. Whale falls are big events in the deep sea,
attracting thousands of scavengers, like octopi, hagfish, and deep-sea sharks.
Some deep-sea creatures flourish around undersea geysers or hydrothermal vents, which erupt in
streams of mineral-rich seawater, warmed by magma deep in the Earth’s crust. Hydrothermal
vents nourish millions of microscopic organisms, called chemoautotrophic bacteria, which create
energy in the absence of sunlight. Hydrothermal vents also support large colonies of mussels,
worms, and scaly gastropods, creating a diverse oasis thousands of meters below the surface.
From the sunlit shallows to the darkest trenches, the bottom of
the ocean is an ecological wonderland, home to extraordinary creatures. To the untrained eye,
the ocean appears simple and flat, but underneath, it’s a strange and mysterious world, full of color
and life. Though we’ve unraveled some of the ocean’s mysteries, and discovered its
remarkable diversity, our investigation of the ocean floor has only just begun.
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