Aliens of the deep: meet the most peculiar creatures that call Earth home.

Imagine a place where the temperature hovers at 4[degrees]C (39[degrees]F). There is no sunlight. Tremendous pressure squeezes you on all sides. Residents are some of the most fearsome-looking creatures ever seen, with ghastly bodies and mouths packed with wickedly sharp teeth.

Where in the world would you find such a formidable place? On some distant planet? No, this place is much closer to home--in an area of the deep ocean called the bathypelagic zone (see Nuts & Bolts, p. 9). And while it is very much of this world, its inhabitants are so unusual they might as well be aliens.

The bathypelagic zone is an area of ocean located 1,000 to 4,000 meters (3,280 to 13,123 feet) below the water's surface. It is also called the midnight zone, due to the complete lack of sunlight. Although many species would find the region unlivable, it comprises Earth's largest available habitat. The animals that do live there have adapted to thrive in the cold, dark, and otherwise harsh conditions.

ENERGY SAVERS

Sunlight is unable to penetrate the ocean's deep waters. So, the bathypelagic zone has no living plants, which require energy from the sun to grow. That means deep-sea fish living there have a hard time finding the nutrients necessary to fuel themselves. As a result, they have become extremely energy-efficient. Their bodies are designed so that common activities like moving and eating--which normally require a lot of effort--use very little energy. "Many of these animals don't move much and have slow digestive rates, so they don't burn up energy," says Michael Vecchione, a deep-sea biologist with the National Oceanic and Atmospheric Administration (NOAA) and the Smithsonian's National Museum of Natural History in Washington, D.C.

This energy-efficiency is reflected in their anatomy: weak muscles and soft skin. Since the deep sea is so dark, "animals [there] aren't threatened by predators that can see and chase them, so their bodies are constructed without powerful muscles," explains Bruce Robison, senior scientist at the Monterey Bay Aquarium Research Institute in California. "Their skin doesn't have to be tough and protective because there are few threats from those kinds of predators, so the skin is soft and squishy."

FEELING THE PRESSURE

The animals have also developed ways to cope with the deep ocean's intense water pressure. This force, which is the result of the weight of a column of water pushing down, is so strong in the bathypelagic zone that it would crush anything with air in it. That would be a problem for most fish, since many of them have a gas-filled organ called a swim bladder that helps them stabilize their bodies at different water depths. But many deep-sea fish have adapted to life without a swim bladder. Without the bladders, "[deep-sea fish] don't have gas spaces in them that would cause them to get crushed," says Vecchione.

A notable exception: bathypelagic rattails. These bottom-dwelling fish have a swim bladder filled with liquid rather than air, so it compresses little under pressure. By using this unique air bladder to adjust their buoyancy, or ability to float, rattails can root around the seafloor in search of food.

FISHING IN THE DARK

Unlike the rattail, which actively seeks out a meal, some deep-sea fish use tricks to attract prey. The scaly dragonfish and anglerfish rely on a bioluminescent lure, a fleshy appendage illuminated by millions of light-producing bacteria living inside, to "fish" for food. The lure dangles near the predator's mouth. When prey goes for the shiny "bait," the fish strikes.

Regardless of how they acquire a meal, food is scarce at these depths, so most creatures of the deep can't afford to be picky eaters. "The animals have to be able to get by without eating very often," says Vecchione, "and they have to eat whatever they can when they find it." As a result, anglerfish have expandable jaws and a stomach that can stretch to accommodate food nearly twice their size. Additionally, many species have needle-sharp teeth that curve toward their throats to ensure that prey can't slip away.

SEEING THE LIGHT

Scientists also theorize that some deep-sea species use bioluminescence for reproduction or to escape predators. Some researchers believe that the dumbo octopus--named for its swim fins that resemble large ears--might use the glowing suckers on its tentacles to attract potential mates. And deep-sea shrimp temporarily blind attackers by spewing a luminous substance in their face, allowing the shrimp to make a getaway.

NEW DISCOVERIES

The bathypelagic zone and its inhabitants are still very mysterious. But technological advances are increasing access to this remote region. "There are lots of different kinds of animals that we're just beginning to discover," says Robison. And if current known species are any indication, the unknown ones are sure to be just as peculiar.

nuts & bolts

Ninety-seven percent of the water on Earth's surface is contained in its oceans. On average, the ocean is 3.8 kilometers (2.4 miles) deep. That makes this habitat quite challenging to study. But by using technology like deep-sea submersibles, scientists have been able to glimpse parts of this watery world. They've found that conditions change as one descends from the ocean's surface to the seafloor. Dive in to see what each zone is like.

(1) EPIPELAGIC ZONE: Sunlight easily penetrates this zone, making living plants plentiful. Commonly known animals, like seals, manta rays, and this porpoise, live here.

(2) MESOPELAGIC ZONE: Also called the twilight zone, only dim light penetrates this region of the ocean. Here, no plants grow. Residents include this crown jellyfish.

(3) BATHYPELAGIC ZONE: Known as the midnight zone, this area of the deep sea is pitch-black. As a result, some species living here lack eyes. But not this viperfish.

(4) ABYSSOPELAGIC ZONE: Abyss comes from the Greek word meaning "bottomless." Due to crushing water pressure, most residents--like this sea spider--lack a backbone.

(5) HADAL ZONE: Such extreme depths are mostly found in deep-water trenches and canyons. Temperatures are barely above freezing. Still, life exists here.

PRE-READING PROMPTS

Jump-start your lesson with these pre-reading questions:

* At 150 meters (492 feet) beneath the ocean's surface, there is a region of the sea nicknamed the twilight zone. In this region, sunlight just barely shines through. At 1,000 m to 4,000 m (3,281 ft to 13,123 ft) beneath the ocean's surface is the bathypelagic zone. What nickname might this zone have? Why?

* Deep-sea creatures often have very frail anatomies. That's because the zone that they live in is also surprisingly stable. At ocean depths below 100 m (328 ft), surface currents no longer influence the water movement of the open ocean. Stagnant bathypelagic waters are safe for frail creatures that would normally be torn apart by surface waves or bad weather. What are some other physical characteristics that bathypelagic creatures feature?

* Many fish in the bathypelagic zone possess small eyes--or no eyes at all. Eyes aren't of much use if there's virtually no light for the eyes to detect. Instead of looking for food, the fish capture prey by "touch." Well-developed nerves under their skin help deep-sea fish detect motion-reduced vibrations created by passing fish. What is another tactic that's employed by some fish of the bathypelagic zone, such as the anglerfish, to capture prey?

CRITICAL THINKING:

* It is often said that people know less about the deep ocean than they do about the moon. As a class, discuss what this statement means and why this may be the case.

CROSS-CURRICULAR CONNECTIONS:

ART: First, have each student do research to find information about a creature that lives in the bathypelagic zone. Instruct him or her to pay special attention to the creature's unique physical characteristics. Then have each student use papier mache to make a three-dimensional replica of the selected creature. Slip a piece of string through the top of the creature and hang it in the classroom.

RESDURCES

* To learn more about the extreme life-forms that live in the deep ocean, visit this Web site from PBS: www.pbs.org/wgbh/nova/abyss/life

* Want to learn more about some of the animals featured in "Aliens of the Deep," plus hundreds of others? Click on the common names of the fish featured on this site: www.amonline.net.au/fishes/fishfacts/specomm.htm

* Read about Alvin and other deep-sea submersibles at: www.whoi.edu/marops/vehicles/index.html

DIRECTIONS: Rewrite the false statements below to make them true.

1. The bathypelagic zone is an area of ocean located 10,000 to 40,000 miles below the water's surface.

2. The bathypelagic zone is also called the twilight zone because it receives partial sunlight.

3. Deep-sea fish are extremely energy-efficient. They move quickly and have fast digestive rates to help them burn up food for energy. They also have sleek, armored bodies.

4. Many deep-sea fish have a swim bladder to help them stabilize their bodies at the bottom of the ocean. The bathypelagic rattail fish has a swim bladder filled with air.

5. The anglerfish dangles a bioluminescent lure near its tail to lure predators. The lure is illuminated by millions of neurons firing inside.

1. The bathypelagic zone is an area of ocean located 1,000 to 4,000 meters below the water's surface.

2. The bathypelagic zone is also called the midnight zone because it receives no sunlight.

3. Deep-sea fish are extremely energy-efficient. They don't move much and have slow digestive rates so they don't burn up a lot of energy. This energy-efficiency is also reflected in their anatomy: weak muscles and soft skin.

4. Many deep-sea fish have adapted to life without a swim bladder. One notable exception is the bathypelagic rattail. It has a swim bladder filled with liquids rather than air.

5. The anglerfish dangles a bioluminescent lure near its mouth to lure prey. The lure is illuminated by millions of light-producing bacteria living inside.