The Deep Seafloor: A Desert Devoid of Life?

Once thought to be a flat and lifeless desert, the deep seafloor is now known to have more topographical relief than the Himalayas and a diversity of animal life that may exceed that of the Amazon Rain Forest and the Great Barrier Reef combined. If all of the dry land on Earth were pushed into the sea with bulldozers, it would fill only about one twenty-third of the ocean's volume. The Himalayas, the Alps, and the Rocky Mountains are all dwarfed by the Mid-Atlantic Ridge, an enormous range of mountains that divides the deep Atlantic from north to south, with only a few scattered islands, like Iceland and the Azores, peeking above the waves. The sheer vastness of the deep seafloor boggles the mind.

Apart from bioluminescence produced by the animals themselves and perhaps a faint glow emanating from hydrothermal vents, the deep sea is dark—much too dark for plants to grow—so all life in the deep sea consists of either microbes or animals. Deep-sea forests have animals instead of trees: sea anemones, corals, and tubeworms. Deep-sea plains have no grasses or shrubs, but they do have wandering herds of animals. Instead of wildebeests and antelopes, deep-sea herds of sea urchins and sea cucumbers graze across these plains, their food consisting not of leaves but of mud. Despite continuous darkness, high pressure, near-freezing temperatures, and scarce food, animals occupy virtually all of the deep seafloor, from Arctic to Antarctic and from the margins of continents to the deepest ocean trenches.

Nearly three-fourths of the deep ocean floor is very flat. Vast abyssal plains lying between 4,000 and 6,000 meters in the deep ocean basins are blanketed with the accumulated skeletons of small plants, protists, and animals that live and die in the overlying waters. In relatively shallow seas (less than 3,000–5,000 meters in most places), the calcium skeletons of tiny protists (foraminiferans), algae (coccolithophores), and snails (pteropods) sink to form soft chalky sediment known as calcareous ooze. At greater depths, calcium dissolves away, leaving sediments composed mostly of silica (glass) skeletons of radiolarian protists and diatoms. Below the very transparent oligotrophic waters of the Sargasso Sea and other midocean unproductive areas, the sediment has very few skeletons, consisting instead of fine volcanic ash and desert dust that settles from the air and sinks below the surface of the sea.

The flatness and apparent monotony of the abyssal plains belie their biological diversity. When fine seafloor sediments are carefully sieved, one finds that most of the action is below the surface; large numbers of tiny worms, clams, snails, brittle stars, and bizarre crustaceans burrow in the sediments or move slowly through the mud, playing out age-old dramas of feeding, breeding, and survival. A careful look at the muddy seafloor from a submersible window reveals traces of these dramas in the form of tracks, trails, mounds, pits, and grooves. Long worms extend their mucus-covered bodies from deep burrows to make star-shaped patterns as they sweep up mud from as far as they can reach. Shrimp, lobsters, and clams shoot sediment from their holes to make tiny volcanoes with caldera-like burrows. On top of the sediment, sea cucumbers by the millions mop the uppermost layers, licking first one tentacle then another, while tiny tube-feet propel them in an endless quest to harvest the bacteria-laden mud. When disturbed, some of these lumbering animals leap off the seabed and dance away with surprising grace. Deep-sea urchins with soft, balloon-like bodies sport whimsical gelatinous bags that hide painfully venomous protective spines. Delicate sponges with roots and skeletons of woven glass reach into the water, filtering bacteria and providing off-bottom resting places for brittle stars, crinoids, and crabs. Small fish stand quietly on tripod-like fins, capturing whatever the currents deliver. Others hover with heads pointed down, awaiting the emergence of unsuspecting animals from the sediments. Graceful eels, rattail fish, and sharks prowl slowly above the bottom, stalking prey and following the odors of whatever carrion they can find.

Most conditions on the deep seafloor are quite stable, with pressure, temperature, and salinity being virtually unchanged during the course of years, decades, and even millennia. It came as a great surprise, therefore, when scientists discovered not long ago that a small proportion of deep-sea animals reproduce at particular times of the year. How do sea urchins and brittle stars on the deep ocean floor know when it is spring or fall at the surface? Food supply appears to be the answer. Except at hot water vents where chemical energy drives the ecosystem, all animal life on the deep seafloor depends ultimately on the plants and animals that live and die in the upper water column. Dead material from above eventually sinks to the bottom, and because plankton production in the upper water column follows seasonal cycles of light and nutrients, the arrival of dead plankton on the bottom also has a seasonal cycle. Thus, animals gorging themselves on newly arrived plankton corpses (detritus) can invest more energy in egg and sperm production at some times of year than at other times. Once gonads are produced, animals must find mates in complete darkness. Many sea urchins and sea cucumbers wander about by themselves for most of the year, then pair with others of their species for the short breeding season. Nobody knows if they find each other entirely by chance or if they communicate with chemical scents.

The deep seafloor is not entirely flat and muddy. The midocean ridges are made of volcanic rock, as are the active and inactive underwater volcanoes called seamounts. The relatively steep margins of continents are also rocky, and melting icebergs drop stones or boulders onto the ocean floor. Wherever rocks are found, there are specialized animals that live permanently attached to them. Vast reefs of ghostly white corals may extend several meters high off the ocean bottom. Stalked sea lilies (crinoids) bend in the current like tall wind-blown umbrellas, collecting tiny particles from the water that passes them by. Specialized starfish reach upward with pincer-covered arms that grab the legs of tiny shrimp. Carnivorous sea squirts with wide-open mouths wait for hapless baby fish to land, then grab them like Venus flytraps. Sponges, crinoids, and sea fans extending upward from the bottom are colonized by a diverse assemblage of other animals. In the clear and food-limited waters of the deep sea, high spots with maximum exposure to water currents are prime real estate. The most colorful and diverse assemblages of animals are often found, therefore, on the tops of boulders and the peaks of seamounts.

Craig M. Young is professor of biology at the University of Oregon and director of the Oregon Institute of Marine Biology.

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