Keepers of the deep.

What keys to survival do creatures hold in the cold, dark depths of the sea?

First in a series about Earth's oceans

In pitch blackness, the monstrous jaws open wide. An unsuspecting victim draws near ... Thwop! The jaws slam shut. Knifelike teeth sink into the still-squirming catch....

A horror flick? Our creature feature's for real, appearing daily in the deep sea, 1,000 meters or more below the surface.

If you're surprised, you're not alone. Back in the mid-1800s, scientists thought nothing could exist in the ocean's depths. They even named the deep sea the "lifeless" zone.

Their reasoning? For one thing, a supposed shortage of food. The plants that fuel the ocean food web bloom near the surface. Through photosynthesis, they use sunlight to make food for themselves and the animals that eat them. But no sunlight reaches the water below 200m. So much for finding plants in the deep, or any of the living things that depend on them, right? We'll see.

But even if there were creatures that had solved the food problem, scientists thought, they'd still have to face the big chill. Temperatures can plunge to 1 [degree] C (34 [degrees] F) in the deep sea. And how could any creature stand up to the bone-crushing water pressure? The deeper you go, the worse it gets as more water--and weight--pushes down. At a depth of 600m, for example, a critter would have to withstand pressure of some 1,000 pounds per square inch. That's like having a horse stand on every square inch of your body!

THE WORMS CRAWL IN . . .

Yet a life-size surprise surfaced in 1860, when a damaged telegraph cable was raised from the deep-sea floor for repairs. Scientists were shocked to learn that the cable was encrusted with living corals, mollusks, and worms.

Since then, the search for creatures of the deep has taken off. But there is still so much to learn. Although the deep sea covers nearly two-thirds of the globe, less than one percent of it has been explored.

It's too rough for human exploration, explains oceanographer Fred Grassle. So to collect samples he sends down large metal traps. Recently, he collected nearly 800 never-before-seen species from an area the size of a classroom. Imagine all those creatures swimming around your room.

Chances are, there are many more unknown critters down there. "We can't even estimate the total number," Grassle says. Obviously, they've evolved some way to survive the harsh conditions of Earth's basement. How do they do it?

ADAPT A SEA CREATURE

Check out the deep-sea dwellers on these pages. You'll see that the answer lies in their adaptations, traits that help organisms survive in their environment.

To stand up to the pressure, for instance, many critters of the deep evolved as fluid-filled jellyfish-like blobs. Because they are filled with the same stuff as their surroundings, the pressure inside their bodies increases and decreases with that of the water around them. Since they are never out of balance, they never feel the pressure.

What about the cold? It's no problem if you're cold-blooded, as all deep-sea animals are. Their body temperature always matches that of their surroundings, so they don't have to use up energy trying to stay warm.

Cool! But how do these critters find food in the dark? Line up for the deep-sea smorgasbord. The main course? Detritus. This constant "rain" of decayed matter from animals and plants above may not appeal to you, but many sea creatures thrive on the stuff.

And those that prefer live lunch? They've evolved ways to lure and capture prey. Many flash built-in lights by inducing a glowing chemical reaction in their cells. Others check out the scene with unusually large eyes that pick up even the faintest glimmer of a passing meal.

What other wonders lurk at the bottom of the sea? We'll tell you in future features where we'll explore more of what's known--and what's not--about Earth's oceans.