ECHIURANS: Echiura

PHYSICAL CHARACTERISTICS

Echiurans (eh-key-YUR-enz) are sea creatures sometimes known as spoon worms or fat innkeepers. They have bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree) and can only be divided into similar halves along one plane. Their sausage-shaped bodies are soft, unsegmented, and have two distinct regions, the proboscis and body trunk. The proboscis (pruh-BAH-suhs) is a long, flexible, tubelike snout. The trunk may reach 15.75 inches (40 centimeters) in length, but the proboscis is much longer and can reach up to 3.28 to 6.56 feet (1 to 2 meters). The flexible and muscular proboscis is used to gather food. The lower surface is covered with hairlike cilia (SIH-lee-uh) that help move food toward the mouth located at the base of the proboscis. The proboscis can be extended or retracted, but not withdrawn inside the trunk. It is usually white, pink, green, or brown and can be short or long, scoop- or ribbonlike, and flat or fleshy and spoonlike. The tip may be squared off or notched.

The surface of the thick body trunk is smooth or rough and is gray, dark green, brown, pinkish, or reddish in color. Most species have a pair of hooks on the underside of the body, near the front of the trunk. Some also have one or two rings of bristles around the anus (AY-nuhs), or the opening of the digestive tract at the end of the body. The body wall is wrapped in layers of muscles that run around the body, along its length, or at an angle different from the other muscle groups. The body is supported from the inside by a large, fluid-filled body cavity, or coelom (SIGH-lum). A closed circulatory system may or may not be present. Pairs of kidneylike organs remove wastes from the body's fluid and expel waste from the body.

GEOGRAPHIC RANGE

Echiurans are found in all oceans.

HABITAT

Echiurans are found mostly in the ocean, but a few species live in other kinds of salty waters, such as estuaries (EHS-chew-AIR-eez), where rivers meet the sea. Most species are found in intertidal and shallow waters, but some live at depths of 32,808 feet (10,000 meters). They usually dig burrows in sand, mud, or other debris on the bottom. Some live in rock tunnels bored by other animals, or inside shells, dead corals, or under rocks. Echiurans often host crabs, mollusks, and other kinds of worms in their burrows as commensals (kuh-MEHN-suhls), where they scavenge extra food. Commensals are animals that live on or with other organisms, without harm to either one.

DIET

Echiurans eat bits of dead plants, animals, and microorganisms that live on sand, mud, and rock.

BEHAVIOR AND REPRODUCTION

To feed, some echiurans extend the sticky proboscis out of the burrow and onto the surrounding sea bottom. The tip of the proboscis gathers particles of food and covers them with a sticky coat of mucus. The cilia move the particles back toward the mouth. Other species are filter feeders. Fat innkeeper worms, Urechis caupo, build a sticky net of mucus and place it near the opening of their U-shaped burrow. Both ends of the burrow open to the water. As they flex their body trunks, water is drawn through the burrow, trapping bits of food and small organisms in the net. The worm eventually gathers and eats the food, net and all.

Both males and females are required for reproduction. Most species release eggs and sperm into the water. Fertilization (FUR-teh-lih-ZAY-shun) takes place in the water. In one group the eggs are fertilized inside the female's body. Developing echiurans first go through a larval stage. The unsegmented larvae (LAR-vee) are free-swimming and covered with cilia. They drift with other plankton, or microscopic water-dwelling plants and animals, for up to three months, eventually developing into young worms and settling on the bottom.

ECHIURANS AND PEOPLE

Some echiurans are used by scientists to study how their bodies develop and function and what chemicals they produce, including a skin coloring of some females called bonellin. Bonellin is of interest to people because it may kill certain kinds of bacteria.

STRANGE BUT TRUE

Most animals become male or female based on whether they have a Y-chromosome. But in some echiurans, maleness is determined by environment. Bonellin, a chemical found in the female's skin, turns larvae that settle on her proboscis into males. Those landing elsewhere become females. This way, every male is paired with a female. Bonellin also contains toxins that discourage predators and prevent other animals from attaching themselves to the female's body.

CONSERVATION STATUS

Echiurans are not considered endangered or threatened.

GREEN BONELLIA (Bonellia viridis): SPECIES ACCOUNT

Physical characteristics: The female has an egg- or sausage-shaped body measuring up to 5.9 inches (15 centimeters) long. The proboscis is long and notched at the tip and reaches up to 4.9 feet (1.5 meters). The body is dark green. Green bonellia have one pair of hooks underneath the body. The flattened, colorless males are much smaller (0.039 to 0.11 inches; 1 to 3 millimeters) and do not have a proboscis, mouth, anus, or circulatory system. Their body is made up almost entirely of reproductive organs.

Geographic range: Green bonellia are found in the northeastern Atlantic Ocean, the Mediterranean and Red seas, and the Indopacific region.

Habitat: Females live in burrows built by other animals in coarse sand, rocks, or spaces between rocks. They are found at depths of 33 to 328 feet (10 to 100 meters). They often have a variety of commensals that live with them. The male lives inside the female's body, like a parasite (PAIR-uh-site).

Diet: Green bonellia eat bits of plants, animals, and microscopic organisms found at the base of plants or in sand between rocks. Males rely on females to provide food for them.

Behavior and reproduction: Females move back and forth in their burrows with the help of their proboscis. Muscular contractions of the body wall bring fresh water in contact with the body to renew the supply of oxygen.

Sexes are separate. Fertilization occurs in the genital sac, where a male often lives. The larvae are free swimming. If the larva settles on ocean floor, it develops into a 3.9-inch (10-centimeter) long female. If the larva settles on a female's body (particularly its proboscis), it develops into a 0.039 to 0.078 inch (1 to 2 millimeter) long adult male in about 1 or 2 weeks. Males live as parasites inside the female and produce a ready supply of sperm.

Green bonellias and people: The skin-coloring chemical bonellin is of interest to scientists because it kills many different kinds of organisms, including bacteria.

Conservation status: Green bonellias are not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Kozloff, E. N. Marine Invertebrates of the Pacific Northwest. Seattle, WA: University of Washington Press, 1996.

Ruppert, E. E., and R. S. Fox. Seashore Animals of the Southeast. Columbia: University of South Carolina, 1988.

Stephen, A. C., and S. J. Edmonds. The Phyla Sipuncula and Echiura. London: Trustees of the British Museum (Natural History), 1972.

Periodicals:

Agius, L. "Larval Settlement in the Echiuran Worm Bonellia viridis: Settlement on Both the Adult Proboscis and Body Trunk." Marine Biology 53 (2002): 125-129.

Web sites:

Introduction to the Echiura.http://www.ucmp.berkeley.edu/annelida/echiura.html (accessed on January 6, 2005).

Murina, V. V. Phylum Echiura Stephen, 1965.http://www.ibss.iuf.net/people/murina/echiura.html (accessed on January 6, 2005).

The Echiura-Spoon Worms.http://www.ldeo.columbia.edu/edu/dees/ees/life/slides/phyla/echiura.html (accessed on January 6, 2005).