Comb Jellies: Ctenophora
COMB JELLIES: CtenophoraVENUS'S GIRDLE (Cestum veneris): SPECIES ACCOUNTS
SEA WALNUT (Mnemiopsis leidyi): SPECIES ACCOUNTS
The body of a comb jelly consists of two clear tissue layers that enclose a jellylike layer. Comb jellies can be as small as a berry or long and ribbon shaped. The outside of the body is covered with eight rows of short fibers that look like the teeth of a comb. The combs are used for swimming and emit flashes of light. Comb jellies have tentacles that do not have stingers but are covered with sticky structures used to capture prey. These animals have a balancing organ that consists of a hard center covered with hairlike fibers. This organ is attached to the comb rows. The digestive system has two openings, but elimination of waste out the anus (AY-nuhs) is rare. Defecation of undigested material occurs primarily through the mouth.
Comb jellies live in all of the oceans of the world.
Most comb jellies live in open ocean. Some live at the bottom.
Comb jellies eat animal plankton or microscopic animals drifting in water, other jellies, crustaceans, mollusks, and fish larvae. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone. Mollusks (MAH-lusks) are animals with a soft, unsegmented body that may or may not have a shell. Larvae (LAR-vee) are animals in an early stage that must change form before becoming adults.
BEHAVIOR AND REPRODUCTION
Comb jellies control water flow around themselves for movement by jet propulsion and "flying," for the capture and eating of prey, and for escaping from predators. Comb jellies either actively seek their prey or wait in ambush for it. Their long tentacles have muscular cores and a covering that contains sticky cells. The tentacles trail through the water or are twirled about by various circular movements of the body. When the tentacles touch prey, the sticky cells burst and discharge a strong, sticky material. Comb jellies that have very short tentacles trap plankton in mucus on their body surface, and the particles are carried to the jelly's mouth by currents produced by hairlike fibers.
Most comb jellies make both eggs and sperm. Only a few species have separate sexes. Most comb jellies release their eggs and sperm into the water. Fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, takes place outside the body. In some species, however, fertilization takes place inside the body. Almost all comb jellies fertilize (FUR-teh-lyze) themselves. The larvae swim freely during their transformation into adults.
COMB JELLIES AND PEOPLE
Some comb jellies are harmful to fish that are caught and sold for food.
Comb jellies are not threatened or endangered.
Physical characteristics: Venus's girdles are ribbon shaped, reaching a length of almost 5 feet (1.5 meters) but a width of only about 3 inches (8 centimeters). The comb rows are all on one side of the ribbon, and the mouth is on the other side.
Geographic range: Venus's girdles live in the Atlantic and Pacific oceans, Antarctic waters, and the Mediterranean Sea.
Habitat: Venus's girdles live in the surface waters of the sea.
Diet: Venus's girdles eat small crustaceans and mollusks.
Behavior and reproduction: Venus's girdles swim across the water 3 to 6 feet (1 to 2 meters) before moving up or down 2 to 4 inches (5 to 10 centimeters) and reversing direction. Using this behavior, the Venus's girdle retraces its original path but 2 to 4 inches (5 to 10 centimeters) above or below it. A Venus's girdle captures prey on tentacles lying over its body, and the combs generate small whirlpools that may increase prey movement and capture as the Venus's girdle moves back and forth through the water. Venus's girdles have an escape behavior that consists of snakelike movements of the long body that allow the animal to move several body lengths in seconds. Venus's girdles make both eggs and sperm, which they release into the water for fertilization and development of larvae outside the body.
Venus's girdles and people: Venus's girdles have no known importance to people.
Conservation status: Venus's girdles are not threatened or endangered. ∎
Physical characteristics: Sea walnuts reach a length of about 4 inches (10 centimeters). Four deep furrows run from the top to bottom of the animal.
Habitat: Sea walnuts live in shallow waters near shore and in bays and estuaries (EHS-chew-AIR-eez), the areas where rivers meet the sea.
Diet: Sea walnuts eat barnacle larvae, tiny crustaceans, fish eggs and larvae, and other animal plankton.
Behavior and reproduction: Sea walnuts spend most of their time actively swimming. Scientists do not know how sea walnuts reproduce.
Sea walnuts and people: Accidental introduction of sea walnuts into the Black Sea during the 1970s caused the collapse of fishing in that area, and many people lost their jobs. The fish could not survive because the sea walnuts had eaten their food. To control the invader, scientists brought in another species of comb jelly—one that feeds on sea walnuts. By 2004 sea walnuts had invaded the Caspian Sea, and scientists were considering the same means of control.
Conservation status: Sea walnuts are not threatened or endangered. ∎
FOR MORE INFORMATION
Carson, Rachel. The Edge of the Sea. 1955. Reprint, Boston: Mariner, 1998.
Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.
Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. Invertebrates. New York: Twenty-First Century, 1996.
"'Alien' Jellyfish Threatening Caspian Sea." U.N. Wire. http://www.unwire.org/UNWire/20040310/449_13861.asp (accessed on January 29, 2005).
Amos, William H. "Venus's Girdle." Microscopy-UK. http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artmay04/wavenus.html (accessed on January 29, 2005).
Mills, C. E. "Ctenophores." University of Washington. http://faculty.washington.edu/cemills/Ctenophores.html (accessed on January 29, 2005).