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Deep-Sea Benthos

Introduction

Scientists call the open ocean the Earth's last frontiera distant and mysterious realm that resists our efforts to explore and understand its resources. And yet, it covers more than two-thirds of the world. The deep seafloorthe part of the bottom lying off the edges of the continental shelvesmakes up nearly 60 percent of the planets surface.

At the bottom of the sea, the benthosthe scientific name for all the organisms living on and in the seafloormust cope with extreme physical conditions. Although the seafloor is the worlds most widespread habitat, it is also one of the least hospitablecold and dark, nutrient-poor, and bearing the tremendous weight of the ocean.

Absence of Light

The deep-sea begins as sunlight ends. Depending on latitude and water clarity, the ocean falls into perpetual darkness between a few hundred and one thousand meters below the surface.

To cope with darkness, many animals create their own light, a colorful, chemical glow they can turn on and off. Called bioluminescence, living lights like this serve several purposes. Many species flash colors and patterns to communicate with their own kind. Hunters bioluminesce to search out prey. And the hunted use light to warn off or confuse predators.

Other animals, unable to create their own light, "borrow" it from those who can. These creatures are fluorescentthey absorb the light from other organisms and re-emit it in a different color.

Because bioluminescence and fluorescence are so common in the deep-sea, many animals have large and extremely light-sensitive eyes. Other creatures embrace the darkness. Blind and poorly sighted species simply feel their way through life with tentacles, antennae, or other appendages. Many others use an excellent sense of smell to find food, and tell friend from foe.

Water Pressure

As depth increases, so does the total weight of the water pressing on the bottom and the benthos. Over most of the deep seafloor, organisms must withstand thousands of pounds of water pressure on every square inch of their bodies.

Marine scientists often use styrofoam cups to show what water pressure can do. Styrofoam is light because its internal structure is full of air-filled spaces. When styrofoam cups are lowered down into the deep-sea, high water pressure collapses the air spaces inside their walls and compresses the cups to a fraction of their normal size.

Bottom-living animals avoid a similar internal collapse by saturating their tissues and internal cavities with water instead of gas, so they can't be compressed by water pressure.

Extreme Cold

Cut off from not only the light but also the heat of the sun, the temperature of the deep-sea hovers close to freezing, averaging an icy 4º-Celsius. Such extremely cold water slows biochemical processes.

As a result, deep-sea animals, like the elephant sponge, tend to live in a sort of slow motion. They have lower rates of respiration, reproduction, and metabolism than their shallow water relatives. And many live much longer. Deep-water sponges, as an example, grow very slowly and their large size indicates that they can live for centuries. Some marine scientists have even proposed that the oldest animal on earth is a giant sponge, found beneath the Antarctic ice shelf and possibly as much as 10,000 years old.

Limited Food Supply

Darkness, cold, and high pressure do indeed present problems for the benthos, but the biggest challenge faced by deep-sea bottom dwellers is a severely limited food supply. Many deep-sea food chains start with marine snow bits of plants and animals that slowly drift down from the seas surface. Most of the organic matter is eaten before it hits the bottom. The small percentage of food that does reach the benthos is generally old and of poor quality.

Different species have taken different paths to compensate for the limited food supply. Some conserve energy by reducing muscle mass and employing a sit and wait hunting style. Other deep-sea species, like the spider crab, have become unusually large, allowing them to cover more ground in a ceaseless search for food.

Despite these varied survival strategies, the shortage of nutrients means that much of the sea bottom is nearly lifeless, home mainly to mud and silence. Over vast regions, the deep-sea benthos is little more than widely spaced invertebrates and microbes.

Hydrothermal Vents and Deepwater Coral Reefs

In some areas, however, the deep-sea benthos turns unusual geologic circumstances into complex communities brimming with life. Two settingshydrothermal vents and deepwater coral reefssupport abundant, high diversity ecosystems. But these two habitats operate in very different ways.

Corals must cement onto a hard surface in order to grow, but most of the seafloor is too muddy for them to gain a foothold. Where a rare patch of bare rock is exposed, a reef can begin to grow. Once established, deep-sea coral reefs provide food and shelter for many species. But coral communities still depend on distant photosynthetic activity. They contain only consumers, subsisting on marine snow, and on each other. Because the food supply is scanty but steady, the corals grow very slowly but can live for hundreds of years.

Unlike deep coral reefs, hydrothermal vent faunas run on chemical energy. The vent faunas grow around plumes of chemicals rising from the seafloor. Bacteria are able to turn the chemicals into food, and act as primary producers. They support both themselves and a web of symbionts, predators, and scavengers, and make the vent faunas self-sufficient.

Food is more abundant around vents than anywhere else on the deep seafloor, fueling rapid and abundant animal growth. But the vents are short-lived, rarely lasting more than twenty years before they, and their faunas, are extinguished.

Global Impact

The same physical extremes that challenge the deep-sea benthos long defeated scientists who wished to study the seafloor. Recently, though, advances in sensing and deep-diving equipment have allowed researchers to see, sample, and even visit the ocean bottom. They have discovered that life there is more complex than they ever suspected. And they have learned that although the benthos is remote, its influence is far-reaching.

For example, bottom dwellers are crucial to the health of ocean fisheries. Many fish, crab, shrimp, and other animals, valuable as economic and food resources, spend part of their lives on the deep coral reefs. Eggs are laid and juveniles hide within the coral framework, while adults find prey more abundant there than in the open sea. Researchers also hope that the survival strategies used by the deep-sea benthos to cope with environmental extremes may one day be adapted to improve human health and life.

Bioluminescent and fluorescent chemicals are already being used as genetic markers in medical research. Other enzymes, genes, and compounds that allow animals to function despite crushing pressure and freezing cold are under investigation for use in new medicines, industrial processes, and toxic waste clean-up.

At the same time, scientists have found that human activities are invading and disrupting the deep-sea habitat. Fishing equipment is tearing up the bottom and pollution is impacting animal development. Many fear that we may destroy this remarkable ecosystem long before we understand it.