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ompared to what we know about the terrestrial Earth -- or even about other planets -- our knowledge of the ocean depths is embarrassingly limited. It's an issue, ultimately, of accessibility. The deep oceans, however beautiful and inviting, are a notoriously dangerous place for a land-lubbing species such as ourselves, with the risks increasing the farther we descend. What we can't easily get to, we also can't study.

The most obvious problem is oxygen. Fish and other water-dwelling organisms are equipped to access the oxygen dissolved in water. We are obviously not fish, and we can't breathe -- naturally, that is -- underwater. Divers holding their breath (free divers), many of whom have developed extraordinary control over their heart rate and other body functions, can stay underwater for minutes at a time. (The most accomplished free-divers, like Umberto Pelizzari -- profiled in the SAVAGE SEAS episode "The Deep" -- can stay underwater for five minutes or more, plunging to depths up to 200 feet.) Buying more time underwater is easy: get an external source of air -- say, a tank. Want to dive deep? Well, that's trickier. You've then got to deal with water pressure.

Free diving
Free diver Umberto Pelizzari gets acquainted with underwater life.
But using compressed air also has its share of hazards. Compressed air is denser than air at normal atmospheric pressure, and so a diver breathing it will inhale proportionately more molecules of its component gases -- predominantly nitrogen and oxygen. The excess oxygen gets metabolized (although oxygen can be toxic at very high concentrations, that is not usually a problem for divers). Nitrogen, however, is inert, and is not used by the body. Instead, the excess nitrogen is dissolved in the blood and tissues. When the diver rises to the surface and the pressure on the body is reduced, the nitrogen comes back out of solution and is eventually exhaled.

Should the diver ascend too quickly, however, the nitrogen forms into bubbles. If the bubbles are large enough, they can get lodged in the body's capillaries and block blood flow. Depending on where the bubbles get stuck, they can lead to skin mottling, tingling, headache, pain in the joints, paralysis, stroke, and, sometimes, death. This is decompression sickness (DCS), also known as "the bends" (which actually refers to just the joint pain). The only effective treatment for DCS is recompression in a hyperbaric chamber, which forces the bubbles to dissolve again, followed by a slow, controlled decompression so that the nitrogen leaves the body without forming bubbles.

Decompression sickness can also be caused by bubbles of other inert gases, like helium or hydrogen. The gas mixtures of professional divers descending to depths of thousands of feet, where even small concentrations of oxygen can be toxic, can be as much as 98 percent helium. These divers can develop DCS even when following the most rigorous ascent procedures. To prevent them from getting DCS, Navy researchers are currently looking at some rather novel techniques -- including microbes that munch on hydrogen, removing it from the bloodstream. The illness can be avoided entirely, of course, if divers can breathe air at surface pressure (one atmosphere). There's no compression of the lungs -- hence no decompression. The key: a pressure-resistant hull, to fight back against the crushing weight of water. With such protection, divers in untethered one-man, one-atmosphere armored suits (like the so-called "Jim suit"), can work for hours at depths of 2000 feet or more; military subs can cruise at 3000 feet; small, manned submersibles (like the research ship Alvin and the Japanese 3-man sub Shinkai) can dive beyond 10,000 feet. In 1960, two divers in the pressure-resistant, one-atmosphere bathyscaphe "Trieste" made the ultimate voyage to the bottom of the deepest part of the ocean, touching the sea floor nearly seven miles below the surface. (See sidebar.)

Such protection has allowed us to make tantalyzing forays into the depths. But it also limits what we can actually do and see down there. The divers in the Trieste, for example, were squeezed inside a six-foot diameter steel sphere. Only a small porthole gave them a peek at the alien world they had descended into -- and through it, one diver spied a fish swimming idly by. That fish needed no shield against the crushing pressures, nor do any of the organisms that occupy the ocean depths. The reason is simple: their internal pressure is the same as the pressure of the waters surrounding them. But there are other difficult environmental conditions to be dealt with -- no light, low oxygen, scarce food, etc. Just as our bodies have adapted, over the eons, to a life on land, so too have these creatures adjusted to their unusual world.

To appreciate the challenges marine organisms confront, it helps to first have an understanding of the general organization of the ocean environment. The marine realm is essentially divided into two realms: the pelagic region, comprising the open waters, and the benthic region, or the ocean bottom. Moving outward from the continents, the seafloor drops in elevation -- from the relatively shallow continental shelf all the way down to the deepest ocean trenches -- and so the waters get progressively deeper.

In the sunlit, upper surface waters, life is most abundant. Here, microscopic phytoplankton and other plants photosynthesize like mad, forming the base of a complex food web consisting of zooplankton, copepod crustaceans, shrimp-like krill, jellyfish, whales, squids, snails, sharks, fish, and a multitude of other organisms. The waste and dead bodies of these organisms rain down to the sea floor below, feeding the sponges, crustaceans, mollusks, and other benthic organisms there.

Landing on the sea floor
A simulation of the Trieste arriving to the deepest point of the ocean.
For the pelagic organisms of the surface waters -- as well as the free-swimming creatures inhabiting the depths below -- just staying afloat is a major concern. These animals have evolved all sorts of special tricks over the millennia to maintain buoyancy. Fish have gas-filled swim bladders or store lighter-than-water substances (like oil), while jellyfish and other creatures absorb huge amounts of water in their tissue; other creatures -- snails, for example, and jellyfish -- sock away air in air sacs or special chambers.

Sunlight cannot penetrate below a depth of about 660 feet, around the start of what's known as the bathyal zone (it ends where the water temperature drops to 4 degrees Celsius -- at about 6600 feet). Some fish and crustaceans at these depths are blind; other animals -- as many as half of the creatures in the deep oceans -- have become bioluminescent, producing their own light in specialized organs called photophores.

Without sunlight, there is no photosynthesis, and without phytoplankton to kick start the food web, animal life is sparse. Because of the scarcity of food in the deep sea, many fish have evolved bizarre adaptations to help them get what they can. Some fish -- gulpers and swallowers, for example -- have huge mouths or enormous guts, so they can gobble up as much food as they can when it is available; other fish, like anglerfish, have modified part of their dorsal fin to take the shape of a dangling lure, to entice potential prey.

Below the bathyal zone, and extending to depths of about 20,000 feet -- where the region of the still-mysterious ocean trenches begins -- is the abyssal zone. The abyssal is the world's largest environment, comprising some 115 million square miles, or 60 percent of the surface of the Earth. Pressures here range from 200 to 600 atmospheres; the waters are cold, dark, and -- far removed from surface storms and currents -- serenely still. (The exception can be found at the spreading centers where new crust is formed, and where hot, chemical-rich hydrothermal vents support a diverse and unusual community. See sidebar.)

In those calm waters, animals often have delicately-structured, unstreamlined bodies. (In part, this occurs because the water is deficient in calcium, which is necessary for spines and other appendages. It also lacks sunlight-produced vitamin D, which is crucial for proper bone growth. This may explain why deep-sea fishes have such grotesque forms.) Some bottom-dwelling crustaceans have long, spiny legs to prop themselves above, and probe into, the soft ooze; other benthic animals are attached to the substrate with long stalks that carry their bodies up above the bottom water, which is often depleted in oxygen.

-- By Kathy Svitil

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