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Jellyfish and Other Coelenterates

If you live near the sea, you have probably seen some of the oldest forms of life on Earth. Their traces have been found in rocks nearly 7 million years old. These soft-bodied creatures called coelenterates, or cnidarians, lived in the ancient seas long before the first fish. They include jellyfish, hydras, corals, sea wasps, and sea anemones.

There are about 9,500 species, or kinds, of living coelenterates. All of them are found in water, and most of them are found in salt water environments of oceans and bays and brackish water (a mixture of fresh and salt water) environments of estuaries. Many coelenterates of today resemble those early relatives that existed millions of years ago.

Characteristics of Coelenterates
Jellyfish and their relatives are not true fish. All fish are vertebrates, or animals with backbones. Coelenterates are invertebrates. Unlike most fish and other water animals, they do not have fins, legs, or tails. Nor do they have scales or shells to protect them.

Body Structure
The coelenterate's body is little more than a hollow jellylike bag. One layer of cells forms the outside of the bag. Another layer lines the inside. Soft, watery jelly separates the two layers of cells. The body has one opening, or mouth. The coelenterate takes in food, such as the larvae of crustaceans, mollusks, and worms, through its mouth. Some species even take in small fish. Once inside the body cavity, the food is digested by the inside layer of cells. Any undigested food is thrown out through the same opening.

Tentacles, or feelers, that look like hanging threads surround the mouth. The tentacles of some species have stinging cells called nematocysts. Each stinging cell contains a capsule in which a long, hollow thread is coiled. The barbed tip of the thread sticks out of the cell and acts like a trigger. The stinging cells can be stimulated by chemicals given off by potential prey and predators, by physical touch, or by increased water pressure from the slightest of movements. When the cells are stimulated, their coiled threads spring out. The barbed tips pierce the body of the victim, injecting it with poison fluid. (Coelenterates' bodies produce new stinging cells to replace the used ones.) The tentacles capture the food and draw it into the opening.

In some coelenterates, the poison is so powerful that it can paralyze large creatures, even human beings. Anyone who has ever brushed against a jellyfish knows that the sting can be quite painful. Even when the animal is washed up on shore, the stinging cells keep their power for some time.

A coelenterate that does not have stinging cells can grasp small animals that swim into its tentacles and carry them in through its mouth.

Along with the stinging cells and the body plan, coelenterates have other features in common. Coelenterates have no blood or central nervous system. However, they do have nerve cells scattered in the jelly between the cell layers. They also have cells that respond to light, touch, and the pull of gravity. These sensory and nerve cells send information to the muscle cells that make the animals move.

Basic Body Forms
Coelenterates are an interesting group of animals because they are found in two basic body forms: the polyp and the medusa. Some kinds of coelenterates have both forms at different stages of their lives. Others exist in only one form all their lives. Though they appear different, both body forms have the same basic structures.

A polyp has a body shaped like a hollow cylinder. One end typically remains fixed to rocks and shells or to the ocean floor by a footlike disk. At the other end, the tentacle-ringed mouth faces upward. Sea anemones, hydras, and adult corals grow as polyps.

Although they are attached to something, these animals can make motions. They can move their tentacles and stretch or shrink their bodies. For example, when a sea anemone is feeding, its body and petal-like tentacles extend upward, ready for a tiny animal to swim or drift against one of them. But when danger threatens, such as low tide, the animal draws in its tentacles and looks like a brown, lumpy knob.

Also, a polyp does not always remain fixed to one spot. It may glide along slowly on its sticky base. It may also use its tentacles as arms, somersaulting base over tentacles. But some part of the body is always touching another surface. A few hydras can produce a gas bubble in their base. Then they float up to the surface of the water.

Some polyps — corals, for example— usually live in groups, or colonies. The individual polyps may be close together or joined in various ways. Most of these colonies remain in one place, their tentacles capturing prey that swim within their reach.

A medusa has a body shaped like an umbrella or a bell. The mouth points down in this form, and the tentacles float outward. Medusae (plural of medusa) are able to swim about freely. However, they are relatively poor swimmers and usually go wherever currents and winds push them. They do have some movement control. By contracting their umbrella-shaped bodies, they are able to change their direction and depth in the water. Adult jellyfish and sea wasps are medusae.

Almost every kind of coelenterate can produce offspring, or make copies of themselves, by means of both sexual reproduction and asexual reproduction.

During sexual reproduction, male reproductive cells (sperm) and female reproductive cells (eggs) are produced. Fertilization takes place when the sperm and egg unite. From the fertilized egg, a new creature is formed.

During the process of budding, an asexual form of reproduction, a budlike lump forms on the coelenterate's body. In a few days the bud has tentacles and a mouth. Eventually it drops off from the parent and takes up life on its own. In time it produces its own buds.

Kinds of Coelenterates
Different kinds of coelenterates vary in size, color, and even shape. Coelenterates are separated into four classes, or large groups. They are Hydrozoa, Scyphozoa, Anthozoa, and Cubozoa.

Hydrozoans include freshwater hydras and hydroids, or hydralike animals, which usually live in saltwater environments.

Freshwater Hydras
Freshwater hydras are small solitary polyps that live in ponds, lakes, and streams. They are usually found on the underside of the leaves of water plants and lily pads. They are some of the few coelenterates found in fresh water. Many kinds are no bigger than a pencil point. Others may grow 1/2 inch (1 centimeter) tall. Hydras can be green, pink, yellowish, or brownish in color. They look like frayed bits of string.

Unlike most hydrozoans, freshwater hydras have no medusa stage. Along with the typical methods of reproduction, the tiny freshwater hydra can regrow lost parts through regeneration. If a hydra is cut into many pieces, almost every piece will grow into a new hydra. It is its ability to regenerate that has given the hydra its name: Hydra was a many-headed monster of Greek mythology that could grow two heads for each one cut off.

Hydroid Colonies
The hydroids, or hydralike animals, usually live in colonies in saltwater environments. There are two basic kinds of colonies: fixed and floating.

Obelia is an example of a fixed colony. It stays attached to the sea bottom. An entire colony forms from one polyp. Each member of the colony is so small that it can hardly be seen by the unaided eye. Yet each has a job to do for the colony. Some members are food-gathering polyps. Their tentacles capture food for the entire colony. Other members are reproductive polyps.

Medusa buds develop in the reproductive polyps. In time, the young medusae swim out of the mouths of the parent polyps. When the tiny bell-shaped creatures become adults, the females produce eggs and the males produce sperm. The fertilized medusa eggs develop into polyps. From each polyp a new Obelia colony may begin.

Most hydrozoan colonies, like the Obelia, stay fixed to the sea bottom. But there are some floating colonies. The most familiar example of a floating colony is probably the one formed by the Portuguese man-of-war.

Many people think of the Portuguese man-of-war as a single medusa. But it is really a large colony of medusae and polyps. The balloon is a medusa that is filled with gas. The colony drifts with the ocean currents and is blown by the wind. The balloon, which is usually 6 to 8 inches (15 to 20 centimeters) across, acts like a sail. When the balloon empties, the colony sinks below the surface.

Under the balloon hang long tentacles, as much as 40 feet (12 meters) long. These are made up of many small polyps, which serve the colony in different ways. The reproductive polyps produce little medusae, which swim away and start new colonies.

Stinger polyps, working together, can capture a large fish and deliver it to the feeding polyps. The stinger polyps can be dangerous or even fatal to a human swimmer. When a colony is blown ashore, the stingers remain poisonous for a long time.

The coelenterates called scyphozoans include most of the larger jellyfish. Some jellyfish are shaped like cups or bells. Others look more like umbrellas or saucers. Some are colored a soft pink or purple. Others, such as the lion's manes and sun jellies, are bright yellow and gold. Still others have striped patterns on their upper sides.

Some jellyfish, such as the bright blue-and-orange sea blubbers, are among the largest animals without backbones. They may measure about 12 feet (3.5 meters) in diameter. Their tentacles may extend more than 100 feet (30 meters) beneath them. Moon jellyfish may grow to 10 inches (25 centimeters) in diameter and sometimes even to 2 feet (0.5 meter). Other jellyfish are colorless and the size of a fingertip.

The moon jellyfish is a typical scyphozoan. It swims or floats freely in the water. On the upper side of a fully grown moon jellyfish, you may see a pink or orange pattern like a four-leaf clover. The four "leaves" are the reproductive organs. In male jellyfish they produce sperm cells, which are released through the animal's mouth into the water. In female jellyfish the reproductive organs produce eggs, which remain inside the body until they are fertilized.

The eggs develop in four long, trailing mouth folds. These folds hang down from the mouth and, like the tentacles, bear stinging cells. When the eggs hatch, the young settle on the bottom of the ocean. They develop into a shape very different from the parent animal. They become polyps.

Like the parent jellyfish, the young polyp has stinging cells and catches food with its tentacles. It grows for several months. Gradually the polyp comes to look like a stack of fringed saucers. One by one the saucers pinch off from the polyp and swim away. Each becomes a separate little medusa. In most jellyfish, the medusae go on to produce polyps, and the cycle begins again.

The anthozoans, which are sometimes called flower animals, are polyps with a flowerlike appearance. Included in this group are the sea anemones, corals, sea fans, sea pens, and tube anemones. They are found all over the world in both deep and shallow saltwater environments. These coelenterates, which have no medusa stage, vary greatly in size and may be solitary or colonial.

The sea anemone is an example of a solitary anthozoan. It may be as large as 3 feet (1 meter) in diameter and 3 inches (8 centimeters) tall.

They reproduce in several ways. Sometimes new sea anemones are produced by budding. At other times one sea anemone simply divides into two parts, and each part becomes a new individual. Sea anemones also reproduce sexually. They produce reproductive cells that are thrown out through the animal's mouth. The sperm cells join the egg cells in the water, and the fertilized eggs develop into young animals.

Young sea anemones, which lack tentacles, swim about in search of food. They feed on fish and any other live animal they can manage to snare. Finally they settle on the bottom, attaching themselves to underwater objects with a footlike disk or burrowing in the bottom mud or sand. In time, their tentacles grow.

Corals are the anthozoans that usually form colonies. Although corals appear in a wide variety of forms, hard (or stony), thorny, or soft, they are all tiny polyps. Like other animals in this group, corals reproduce without a medusa stage.

As coelenterates, some corals are unique. They produce a hard outside skeleton made mostly of limestone. Because of this characteristic, colonial corals can form huge structures called reefs. The coral reefs are made up of live corals living atop the limestone skeletons of their ancestors, and they provide shelter for an abundance of marine life.

Anthozoans form some interesting relationships with other sea organisms. Many corals, as well as anemones, house microscopic single-celled algae on and in their tissues. The algae help the corals by providing some of the raw materials they need to grow and build tissue. In turn, the corals give off waste products that the algae use for their life processes. Another mutually helpful relationship exists between some anemones and hermit crabs. The hermit crab places an anemone on the snail shell in which the crab lives. With its ready sting, the anemone helps protect the crab from predators. For its services, the anemone receives particles of food dropped by the crab.

The body of the cubozoan is shaped like a cube or square. A tentacle or group of tentacles is found at each corner of the square. Cubozoans are strong swimmers and aggressive predators, feeding mostly on fish.

Several cubozoans are considered dangerous because of their stings, but one is feared above all others. The cubozoan known as the sea wasp, or box jellyfish, can produce welts on anyone who comes in contact with its tentacles. Its stings are dangerous, and if a person is badly stung, they can even be fatal.