JELLYFISH: Scyphozoa

PHYSICAL CHARACTERISTICS

Jellyfish have one or both of two body forms: bottom-dwelling polyp and freely swimming medusa. The medusa (mi-DOO-suh) is the jelly-like, usually bell- or umbrella-shaped, usually tentacled form. The polyp (PAH-luhp) consists of a tubular sac with a mouth and tentacles on top. Polyps are less than one-eighth inch (4 millimeters) long. Medusae (mi-DOO-see, the plural of medusa) can be as large as 80 inches (2 meters) in diameter. Near the edge of the bell most jellyfish have tentacles used for feeding. The tentacles have millions of stingers that inject toxin into or entangle their prey. Some jellyfish have hundreds of these tentacles. Rather than tentacles, some jellyfish have mouth arms on the underside of the bell. These arms also have stingers for feeding. Other jellyfish have one thick tentacle on the upper surface of the bell. Some jellyfish have a stalk that they attach to seaweed or sea grasses. Stalked jellyfish have eight arms, each bearing a cluster of as many as one hundred short, clubbed tentacles.

GEOGRAPHIC RANGE

Jellyfish live in all the oceans of the world.

HABITAT

Jellyfish medusae live in seawater from shore areas exposed at low tide to the very deep ocean. The polyps are attached to hard surfaces, such as rocks, shells, and plants, at various depths.

DIET

Jellyfish eat plankton, which is microscopic plants and animals drifting in water; fish eggs and larvae (LAR-vee), or young animals that must change form before becoming adults; other jellies; and small crustaceans (krus-TAY-shuns), which are water-dwelling animals that have jointed legs and a hard shell but no backbone.

BEHAVIOR AND REPRODUCTION

The most noticeable behavior of jellyfish is rhythmic pulsing of the swimming bell, which moves them through the water. The swimming pulsations are coordinated by nerve centers around the edge of the bell. Medusae can sense light and dark and can determine their orientation in the water. Some jellyfish swim continuously. This feature is important for oxygen exchange, which occurs over the entire body surface, and for feeding. Several species swim against the current. The result is that they all swim in the same direction and may become concentrated in large masses. Some species move up in the water at night and down in the day. The polyps can move using a "foot" and its extensions.

Most jellyfish catch prey by the tentacles and fold the arm inward to bring the prey to the mouth. Many jellyfish do not swim actively while feeding but remain nearly motionless with their tentacles extended above the bell. For some medusae, the pulsations of the swimming bell force water through the tentacles and create whirlpools that bring prey into contact with the tentacles and oral arms.

LOOK BUT DON'T TOUCH

Because of their beauty and the relaxing effect of their swimming pulsations, jellyfish have been a great success in public aquariums and even as household pets. In Japan jellyfish are kept as pets in special aquariums.

Jellyfish use both asexual and sexual reproduction. Asexual (ay-SEK-shuh-wuhl) means without, while sexual means with the uniting of egg and sperm for the transfer of DNA from two parents. The bottom-dwelling forms of some jellyfish reproduce asexually by budding new polyps from the body or foot. Polyps also produce medusae by another asexual budding process that takes place at a certain time of year and is triggered by environmental factors, such as changes in temperature or light level. During this process the polyp splits in two and forms one to several small medusae. The fully formed medusae break free by swimming pulsations and grow to adults over the course of a month or longer.

The medusae of most species have separate sexes, but in a few species the same animal makes both eggs and sperm. No mating occurs. Sperm strands are released into the water by males and are taken up by females during feeding. In most species the fertilized (FUR-teh-lyzed) eggs, those that have united with sperm, develop into small larvae that swim to a suitable bottom material, attach, and develop into polyps. Some species lack a polyp stage.

JELLYFISH AND PEOPLE

All jellyfish sting, but the stings of small animals and those with short tentacles often are not painful to humans. Jellyfish can be a nuisance to people who catch or farm fish and sell it for food. Besides damaging the fish caught in nets, large masses of jellyfish tear the nets. Jellyfish break up on the enclosures of fish farms and sting and kill the fish. Jellyfish are also an important food in Japan and China, and fishing for cannonball jellyfish has started in the Gulf of Mexico.

HOLD THE PEANUT BUTTER

In some parts of the world the swimming bell of jellyfish is processed in a mixture of salt and a preservative and packaged. The semidried jellyfish is rehydrated, desalted, boiled briefly, and served in a variety of dishes. The prepared jellyfish has a special crunchy texture.

CONSERVATION STATUS

Jellyfish are not threatened or endangered.

SEA NETTLE (Chrysaora quinquecirrha): SPECIES ACCOUNTS

Physical characteristics: The swimming bell of sea nettles can reach 10 inches (25 centimeters) in diameter, but most sea nettles are much smaller. The edges of the swimming bell are petal shaped. One large tentacle emerges from between the petals, and twice as many small tentacles arise from beneath the petals. Eight sense structures are present in alternate spaces between the petals. The narrow oral arms are long and filmy. The colors of medusae range from milky white to white with radiating purplish red stripes on the bell.

Geographic range: Sea nettles live on both sides of the Atlantic Ocean, in the Indian Ocean, and in the eastern part of the Pacific Ocean.

Habitat: Sea nettle medusae live in estuaries (EHS-chew-AIR-eez), the areas where rivers meet the sea.

Diet: Sea nettle medusae eat small crustaceans, comb jellies, and fish eggs and larvae. The polyps eat oyster larvae.

Behavior and reproduction: Sea nettle medusae swim constantly in slow circles. They feed continuously. The life cycle of sea nettles has both a polyp and a medusa stage. The release of eggs and sperm takes place around dawn.

Sea nettles and people: Sea nettles have an irritating sting.

Conservation status: Sea nettles are not threatened or endangered. ∎

NIGHTLIGHT JELLYFISH (Pelagia noctiluca): SPECIES ACCOUNTS

Physical characteristics: The swimming bell of a nightlight jellyfish usually is less than 3.5 inches (9 centimeters) in diameter. It has a bumpy surface caused by clusters of stinging cells, which give a purple or yellowish color to the bell. Four long oral arms and eight long tentacles alternate with eight sense structures in the spaces between folds on the bell. There is no polyp stage.

Geographic range: Nightlight jellyfish live in the Atlantic and Pacific oceans and in the Mediterranean Sea.

Habitat: Nightlight jellyfish live in surface waters of the open ocean but sometimes are carried on the current into shallow coastal waters.

Diet: Nightlight jellyfish eat animal plankton.

Behavior and reproduction: The medusae of nightlight jellyfish emit a blue-green light when they are touched or injured. Mucus released from the damaged area continues to glow, so that at night the jellyfish look like glowing balls in a boat's wake. Nightlight jellyfish do not have a polyp stage. The larvae develop without ever settling on the bottom.

Nightlight jellyfish and people: Nightlight jellyfish have a painful sting, which can cause a severe reaction.

Conservation status: Nightlight jellyfish are not threatened or endangered. ∎

THIMBLE JELLY (Linuche unguiculata): SPECIES ACCOUNTS

Physical characteristics: The medusae of thimble jellies are only 1 inch (2.5 centimeters) high. These jellies are shaped like a thimble, which is a hard cap some people put on a fingertip while sewing. There is a shallow groove near the top of the bell. Thimble jellies have eight very short tentacles and eight sense structures alternating between sixteen folds at the bell margin. The outside of the bell is transparent and has numerous warts of stinging cells. The inner part of the bell is white with greenish brown spots. The polyps form colonies and are covered by a thin, hard sheath.

Geographic range: Thimble jellies live in oceans all over the world.

Habitat: The medusae of thimble jellies live near the surface in warm near-shore waters. The polyps live on coral rubble.

Diet: The medusae of thimble jellies catch a variety of animal plankton on their folds. The colored spots in the bell are filled with algae that transfer nutrients to the medusa. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves.

Behavior and reproduction: Thimble jellies usually live in large groups just beneath the surface. They are very active swimmers, moving in circles. The fertilized eggs of thimble jellies form large larvae that live as plankton for three to four weeks. They settle and form an unbranched colony of polyps. Each polyp can produce as many as forty medusae.

Thimble jellies and people: Stings from the larvae or new medusae of thimble jellies cause a disorder called seabather's eruption. This disorder is characterized by a prickling sensation and red bumps that last for seven to twelve days. It is irritating but not dangerous and becomes worse when the jellies become trapped under a swimsuit.

Conservation status: Thimble jellies are not threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Byatt, Andrew, Alastair Fothergill, and Martha Holmes. The Blue Planet. New York: DK, 2001.

Garcia, Eulalia. Jellyfish: Animals with a Deadly Touch. Milwaukee, WI: Gareth Stevens, 1997.

Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.

Web sites:

"Frequently Asked Questions about Stinging Marine Organisms." Safesea.net. http://www.safesea.net/faq.cfm?cfid=4949282&cftoken=87834841 (accessed on December 17, 2004).

"Things You May Have Been Wondering about Jellies." The Jellies Zone. http://jellieszone.com/jelliesfaq.htm (accessed on January 28, 2005).