 | Coronate Jelly. R. Mickens / AMNH |
Like astronauts drifting in space, deep-sea creatures are surrounded on all sides by a vast, dark emptiness. Their world is a black, featureless void that seems to go on forever in all directions.
How do they find food?
How do they find a mate?
Each night, countless deep-sea animals rise to feed in the richer surface waters. When dawn comes, they return to the deep, where the darkness conceals them from predators.
Billions of tons of sea creatures join in this round-trip journey, known as a vertical migration—the largest daily mass movement of life on Earth.
Many vertical migrators come up because that is where the food is. Almost all ocean plants live in the top 200 meters (650 feet), where the sunlight for photosynthesis is brightest. Here phytoplankton—tiny ocean algae—are eaten by small animals called zooplankton, which in turn are eaten by larger animals.
When vertical migrators go back down, they bring surface nutrients to the deep-sea predators that eat them. Dead plankton, animal carcasses and fecal matter drift down as sea snow, also providing food for the animals below.
 | The enormous teeth of the fangtooth are a useful adaptation in the deep, where food is scarce and animals must snatch whatever they can.
©2002 Norbert Wu/www.norbertwu.com |
In this vast, dark realm where food is hard to find, predators must seize whatever passes by, regardless of size or shape. A variety of adaptations help them hunt the limited food available in the depths.
All deep-sea fishes are carnivores, and long, sharp teeth are the main weapons of the aptly named fangtooth, daggertooth, dragonfish and bristlemouth.
Large, expandable mouths and stomachs let gulper and swallower eels swallow creatures bigger than themselves. Some can stretch their mouths to ten times the size of their bodies and
Some jellyfish kill passing animals with stinging tentacles, while tunicates collect sea snow with sticky mucus nets.
Salps pump water through their tubelike bodies to move by jet propulsion, filtering out food as they go.
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The hinged jaws of this deep-sea swallower (Saccopharynx lavenbergi) swing wide open, enabling it to swallow animals much larger than itself. Its body is a thin, stretchable tube that expands to hold large meals. © 2002 Norbert Wu / www.norbertwu.com |
Many animals in the twilight zone use bioluminescent light to attract mates. Deeper down, where there are fewer animals spaced much farther apart, other unusual mating strategies have developed.
In many deep-sea species, females are larger than males and produce huge numbers of eggs, but they have less muscle mass, which would weigh them down. Males are small but are strong swimmers with a well-developed sense of smell for tracking down egg-laden females.
In some species, individual fish can change sex. These hermaphrodites begin life as small, mobile males, then change to females when they grow bigger. In other species, each individual fish has both male and female organs at the same time. These fish can mate with any individual of the same species, increasing their odds for reproduction.
BY THE NUMBERS: Deep Sea
The oceans contain 170 times more space for living things than all the land, air and freshwater on Earth combined. The deep-sea rattail fish Coryphaenoides armatus easily outnumbers the total human population on Earth, with an estimated 20 billion individuals. The legendary sea serpents pictured on many early ocean charts may have been based on sightings of the oarfish, or king of herrings, a deep-sea giant that grows up to 15 meters (50 feet) long. The coelacanth, an ancient fish that lived before the first four-legged animals walked on dry land, was thought to have gone extinct 70 million years ago until a living coelacanth was caught by fishermen in 1938.
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