ARROW WORMS: Chaetognatha

PHYSICAL CHARACTERISTICS

Arrow worms are long, straight, narrow sea worms that use hooks for catching prey. Arrow worms that live near the surface are clear, which helps them avoid predators. The intestines of species that live in middle depths often are yellow or red because they eat prey of those colors. Species that live in deeper water are more muscular and less clear than those that live near the surface. Folds of the body wall in the neck region form a hood that folds over the head, helping the worms to swim smoothly. Tufts of bristles on the body are used for detecting prey. The worms have tail fins and one or two pairs of fins on the sides of the body.

Arrow worms are 0.1 to 6 inches (3 to 150 millimeters) long. The largest species lives in Antarctic waters and is about 3 inches (70 millimeters) long. The bottom-dwelling species are the smallest, one species reaching maturity at a length of about 0.1 inches (3 millimeters).

The body cavity of arrow worms is filled with fluid that is surrounded by muscles and a tough covering. The head has complex muscles that support the grasping hooks. Arrow worms have a mouth, one or two rows of teeth, and eyes.

GEOGRAPHIC RANGE

Arrow worms live in all the oceans of the world.

HABITAT

Arrow worms live in every part of the ocean. Most drift in open water close to the surface. Bottom-dwelling species live attached to objects such as sea grass and rocks.

DIET

Arrow worms eat plankton, including tiny crustaceans, fish larvae, and other arrow worms. Digestion is rapid. Scientists believe each arrow worm eats two to fifty prey animals each day. Plankton is tiny plants and animals drifting in water. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

BEHAVIOR AND REPRODUCTION

Arrow worms position themselves at an angle in the water. When their side bristles detect something moving in the water, the worms quickly sweep their tail, swim rapidly in the direction of the prey, and grab the prey using hooks. Scientists believe arrow worms use venom to immobilize their prey after capturing it. Arrow worms that live in middle depths usually swim to the surface at night to feed and sink to deeper water during the day.

Arrow worms make both eggs and sperm. The male reproductive organs are in the tail, and the female organs are in the trunk. The sperm develop first. Fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, takes places inside the body after one worm places a sperm pouch on another worm, and the sperm move into the second worm's body. In some species the fertilized (FUR-teh-lyzed) eggs are released into the water. In deep-water species the young develop in sacs hanging from the worm. Arrow worms do not have larvae. Small arrow worms hatch from the eggs and then continue to grow.

ARROW WORMS AND PEOPLE

Arrow worms are used as indicators to determine the level of water from the Atlantic Ocean moving into the North Sea.

CONSERVATION STATUS

Arrow worms are not considered threatened or endangered.

NO COMMON NAME (Pterosagitta draco): SPECIES ACCOUNTS

Physical characteristics: Pterosagitta draco worms have eight to ten hooks, six to ten front teeth, and eight to eighteen back teeth. The body length reaches 0.4 inches (11 millimeters), and the tail length is about one-third the length of the body. These worms have one pair of fins on the tail. The eyes are small with T-shaped color spots.

Geographic range: Pterosagitta draco (abbreviated to P. draco) worms live in the Atlantic, Pacific, and Indian oceans.

Habitat: P. draco worms live in warm and cool areas of the open ocean close to the surface.

Diet: P. draco worms eat tiny crustaceans.

Behavior and reproduction: P. draco worms feed at night. They make quick movements over short distances to catch prey. The worms make both eggs and sperm. Fertilized eggs are released into the water, where the worms hatch and grow into adults.

Pterosagitta draco and people: P. draco worms have no known importance to people.

Conservation status: P. draco worms are not considered threatened or endangered. ∎

NO COMMON NAME (Eukrohnia fowleri): SPECIES ACCOUNTS

Physical characteristics: Eukrohnia fowleri worms have eight to fourteen hooks and two to thirty-one back teeth. There are no front teeth. The body reaches a length of about 1.5 inches (40 millimeters), and the tail length is about one-fourth the length of the body. The neck is narrower than the division between the trunk and the tail. There are long fins on the sides of both the trunk and the tail. The head is small with large oval eyes that have diamond-shaped color spots.

Geographic range: Eukrohnia fowleri (abbreviated to E. fowleri) worms live in the Atlantic, Pacific, and Indian oceans.

Habitat: E. fowleri worms live in open water in the middle depths of the sea. They do not live in extremely cold water.

Diet: E. fowleri worms eat tiny crustaceans.

Behavior and reproduction: Scientists do not know how E. fowleri worms behave. These worms grow and reproduce very slowly. The fertilized eggs develop in a sac hanging from the worm.

Eukrohnia fowleri and people: E. fowleri worms have no known importance to people.

Conservation status: E. fowleri worms are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Brusca, Richard C., Gary J. Brusca, and Nancy Haver. Invertebrates. 2nd ed. Sunderland, MA: Sinauer, 2002.

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

Periodicals:

Davis, Cabell S., Carin J. Ashjian, and Philip Alatalo. "Zooplankton Diversity: A Bizarre—and Changing— Array of Life Forms." Oceanus (spring-summer 1996): 7–11.

Web sites:

Sutton, Tracey. "Today's Highlights." Mar-Eco.http://www.mar-eco.no/Shiptoshore/g._o._sars/cruise_journal_gosars/20June (accessed on March 2, 2005).

Thuesen, Erik V. "Chaetognatha." Evergreen State College. http://academic.evergreen.edu/t/thuesene/chaetognaths/chaetognaths.htm (accessed on March 2, 2005).