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Learn about six of the twilight zone's residents and how they've adapted to life in the dark. A jellyfish that flashes rainbows as it moves.
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• Steinhart Aquarium
• Space jellyfish - Wikipedia
• Why we must protect the ocean's 'twilight zone'
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The familiar body plan that looks like an upside down bell with tentacles hanging down from the inside is called the medusa. The polyp, the other cnidarian body plan, is the opposite, with the mouth and tentacles above, like a sea anemone. Jellyfish also have a stinging adaptation that is unique to them and their close relatives including sea anemones and hydras : nematocysts, or stinging cells.

Jellyfish and comb jellies vary greatly in size depending on the species. Most jellies range from less than half an inch 1 cm wide to about 16 inches 40 cm , though the smallest are just one millimeter wide! Larger individuals have been seen, but they are not typical. Jellies don't have brains as we typically think of them: rather, they have a network of neurons "nerve net" that allows jellies to sense their environments, such as changes in water chemistry indicating food or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli , which can sense the presence and absence of light.

Additionally, some jellyfish have sensory structures called rhopalia, which contain receptors to detect light, chemicals and movement. Their nerve ring, a ring-shaped concentration of nerves found in jellyfish, seems to be involved, however. A study of the upside-down jellyfish, Cassiopea , found that a brain is not required to experience sleep. At night Cassiopea enters a sleep-like state where it pulses less frequently than during the day and is slow to respond to disturbances.

When kept awake throughout the night, the next day the jellyfish appear to be tired—their pulsing was noticeably slower than if they had a solid night of sleep. It is the first time an animal without a brain was observed sleeping. The discovery suggests sleep among all animals is an ancient characteristic with a shared evolutionary beginning, considering the neural network of jellyfish evolved before centralized nervous systems like a brain. All jellyfish are Cnidaria, an animal phylum that contains jellies, sea anemones, and corals , among others.

There are more than 10, species of Cnidaria, and less than 4, of these are Medusazoa—those animals we think of as jellyfish. Those 4, jellyfish can be divided into four different groups.

SCYPHOZOA are the most familiar jellyfish, including most of the bigger and more colorful jellies that interact with humans, and are sometimes called "true jellyfish" for this reason. Scyphozoa spend most of their lives in the medusa body form, and there are at least species. In the water column, the colonial siphonophores may be quite spectacular. These include the notorious Portuguese Man-o-Wars and many deep-sea forms, some of which stretch out up to 50 meters in length like giant fishing nets.

Colonial siphonophores are composed of many specialized individuals called zooids that are genetically identical because they all come from a single fertilized egg. In , researchers discovered what they believe to be a new hydrozoan species of Crossota , 12, feet 3, meters deep within the Mariana Trench. Floating in the water column like a glowing spaceship, this Crossota jellyfish is an exception to most hydrozoans and will spend the majority of its life as a large medusa. There are around 3, species of Hydrozoa.

Some cubozoans, such as the sea wasp Chironex fleckeri , produce some of the most potent venom known.

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Some even engage in elaborate for a jellyfish courtship behavior! There are at least 36 species. In , Allen Collins, a jellyfish expert at the Smithsonian, discovered a new species, which was named Tamoya ohboya in a public naming contest. Listen to a podcast about box jellies. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage.

Jellies are found in oceans worldwide, in shallow and deep water, and a few can even be found living in freshwater. Compared to jellyfish, there are far fewer species of ctenophores: only species have been found, but quite a few are out there yet to be discovered and fully documented. The best-known comb jellies are those found close to shore because, there, they are most likely to run into people.

Those can be roughly divided into three groups. This means that their tentacles are fringed with smaller tentacles. These tentacles can be withdrawn into the jelly's body into special sheaths or pouches on either side of their mouths. They also have short tentacles and tend to grow larger than cydippids. They tend to be very fragile because they don't have to endure rough coastal waves; many of them are so fragile that they cannot be collected by submersibles and are known only by photographs.

They come in a great diversity of forms. These are known as benthic ctenophores. Jellyfish and comb jellies are in different phyla, but scientists have long argued over whether they have an especially close relationship apart from the rest of the animal kingdom. To distinguish them, all Cnidaria and Ctenophora were once described as Coelenterata—but that term is no longer commonly used. To this day, some researchers believe they are sister groups, while others think they are not closely related. Either way, there are still plenty of other questions to argue about, such as how long ago the two groups diverged, and even whether ctenophores might be the most ancient group of animals, diverging even earlier than sponges in the animal tree of life.

These arguments continue because, as some of the simplest animals alive today, understanding their place in the tree of life helps people understand how all other animals—including people—evolved. Whichever came first, comb jellies and jellyfish and other Cnidarians made an important step in evolutionary history: they are the earliest known animals to have organized tissues—their epidermis and gastrodermis—and a nervous system.

They're also the first animals known to swim using muscles instead of drifting with the whims of the waves. The oldest ancestors of modern day jellies lived at least million years ago, and maybe as long as million years ago. That makes jellyfish three-times as old as the first dinosaurs! Because jellies have no bones or other hard parts, finding jellyfish fossils is rare.

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Fossils described in from the Quingjiang site in southern China are some of the best preserved Cambrian fossils ever found, with the cilia and plates easily visible. Scientists are optimistic this discovery will help tease out the relationship between jellyfish and comb jellies. Jellyfish and ctenophores are carnivorous, and will eat just about anything they run into! Most jellies primarily eat plankton, tiny organisms that drift along in the water, although larger ones may also eat crustaceans, fish and even other jellyfish and comb jellies.

Some jellyfish sit upside down on the bottom and have symbiotic algae zooxanthellae in their tissues, which photosynthesize, and so get much of their energy the way plants do. While their nematocysts and colloblasts do help them defend themselves, plenty of animals manage to catch and eat jellies: more than animal species are known to eat jellies, including fish, sea turtles, crustaceans, and even other jellyfish. Jellies are the favorite food of the ocean sunfish Mola mola and endangered leatherback turtle Dermochelys coriacea , which will migrate thousands of miles for the gelatinous delicacy.

Young jellyfish are small enough to be part of the general zooplankton population and are eaten by many animals.

Why we must protect the ocean's 'twilight zone'

Humans also eat jellyfish: people have fished for jellies for at least years off the coast of China. Some , tons more than million pounds of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia. Eating jellyfish may become more common around the world as we overfish more preferable fish species. Jellyfish and ctenophores both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively. Jellyfishes' nematocysts are organelles within special cells cnidocytes that contain venom-bearing harpoons.

The cell is activated upon touch or chemical cue, causing the harpoon to shoot out of the cell and spear the prey or enemy, releasing toxin—a process that takes only nanoseconds. A small number of jellyfish are very toxic to humans, such as the box jellyfish Chironex fleckeri and Irukandji jellyfish Carukia barnesi , which can cause severe reactions and even death in some people.

Many comb jellies have colloblasts lining their tentacles, which work like nematocysts but release glue instead of venom. Upon touch, a spiral filament automatically bursts out of colloblast cells that releases the sticky glue. Once an item is stuck, the comb jelly reels in its tentacle and brings the food into its mouth. One species of ctenophore Haeckelia rubra recycles nematocysts from hydrozoan jellyfish it consumes and uses these to stun and kill prey.

Comb jellies come in many shapes and sizes , and so within the group there are many ways to feed. The rounded and tentacled cydippids have branched tentacles lined with colloblasts that they use, in the traditional jelly style, like a fishing line to trap food and bring it to their mouths. The lobate ctenophores have two flattened lobes that reach below their mouths.

Special cilia waving between the lobes generate a current to pull planktonic food between the lobes and into the jelly's mouth, allowing them to feed on plankton continuously.

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They also use colloblast-lined tentacles to catch food. The tentacle-less beroids depend on their large mouths. Instead of catching food with colloblasts, they swallow their prey often other ctenophores! Inside their mouths they have small cilia that act as teeth, pulling food apart, which also direct the food into the comb jelly's gut.