HEMICHORDATES: Hemichordata

PHYSICAL CHARACTERISTICS

Hemichordates (heh-mee-COOR-duhts) are wormlike sea animals that live alone or in colonies. The type that lives alone, called acorn worms because of the shape of their heads, is a few inches (centimeters) to several feet (meters) long. They have a three-part body plan of a snout, a simple collar, and a trunk. Hairlike fibers covering the body are used for movement and for distributing mucus. The type of hemichordates that lives in colonies is only about 0.04 inches (1 millimeter) long. They also have a three-part body plan, but the snout is short and shield-shaped and the collar is complex, in some species having tentacled arms. Colony-forming hemichordates live in a network of tubes built with mucus from each animal's snout. A third type of hemichordates has only one species, and only its larvae have been found. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

GEOGRAPHIC RANGE

Hemichordates live in all the oceans of the world.

HABITAT

Adult acorn worms usually live in shallow water in burrows at the bottom, but sometimes they live in the sand inside shells, under rocks, in thick seaweed, or between root tangles. Colony-forming hemichordates usually live in deep water in the tubes they make from mucus.

DIET

Hemichordates eat bacteria, microscopic algae, diatoms, and nutrients they scrape from particles of sand and mud or collect from the water. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves. Diatoms (DYE-uh-tahms) are a type of algae that have a shell.

BEHAVIOR AND REPRODUCTION

Acorn worms live alone sheltered in their burrows, under rocks, or in thick tangles of plants. The burrowing species use their snout for digging. They line the U-shaped burrows with mucus to strengthen the walls. One end of the "U" is a cone-shaped dent, and the other is a pile of feces (FEE-seez) a short distance away. The rest of the burrow is underground and sometimes has a few side tunnels. Burrowing acorn worms sometimes stretch their snout and collar out of the tunnel, but they spend most of their time underground.

Some hemichordates gather their meals by stirring up currents with the hairlike fibers on their body and drawing in nutrients from the water. Others take in particles of sand or mud and eat the nutrients sticking to them. Scientists are not sure whether hemichordates use mucus to capture prey. Some scientists believe food sticks to the mucus-covered snout and that hairlike fibers on the animal's body beat in a pattern that draws the mucus and the food together to the mouth. Other scientists have found that the animals use their hairlike fibers to change the direction of their movements and direct food particles to the mouth.

Hemichordates have separate sexes. They release their eggs and sperm into the water, where fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, takes place. In some species the fertilized (FUR-teh-lyzed) eggs develop directly into adults. In most species, however, the fertilized eggs develop into free-floating larvae, which eventually settle on the bottom and transform into adults. Burrow dwellers develop tails that they use to anchor themselves in their mucus-lined tunnels. In some species of hemichordates reproduction is asexual and accomplished by the breaking up of the adult's body or by budding. Asexual (ay-SEK-shuh-wuhl) means without the uniting of egg and sperm for the transfer of DNA from two parents. In budding a bump develops on an animal, grows to full size, and then breaks off to live as a new individual.

HEMICHORDATES AND PEOPLE

Hemichordates have no known importance to people.

CONSERVATION STATUS

Hemichordates are not considered threatened or endangered.

HAWAIIAN ACORN WORM (Ptychodera flava): SPECIES ACCOUNTS

Physical characteristics: Hawaiian acorn worms are yellowish brown and have a small cone-shaped snout, short collar, and long trunk.

Geographic range: Hawaiian acorn worms live in the Pacific Ocean near Japan, Australia, Hawaii, and the Galápagos Islands and in the Indian Ocean near Mauritius and Maldives.

Habitat: Hawaiian acorn worms live in the sea near coastlines.

Diet: Hawaiian acorn worms eat nutrient particles.

Behavior and reproduction: Female Hawaiian acorn worms release mucus-covered eggs into the water, and the males respond by releasing sperm. Fertilization takes place in the water. The eggs hatch into larvae in about two days.

Hawaiian acorn worms and people: Hawaiian acorn worms have no known importance to people.

Conservation status: Hawaiian acorn worms are not considered threatened or endangered. ∎

SPAGHETTI WORM (Saxipendium coronatum): SPECIES ACCOUNTS

Physical characteristics: Spaghetti worms are thin and yellowish white. They can be as long as 10 feet (3 meters). The snout tapers to a soft point toward the front, and the collar is short.

Geographic range: Spaghetti worms live in the Pacific Ocean near the Galápagos Islands.

Habitat: Spaghetti worms live in the deepest ocean loosely attached to rocks near hydrothermal (high-druh-THUR-muhl) vents, which are openings in the ocean floor that serve as smokestacks for releasing extremely hot gases from under Earth's crust.

Diet: Scientists do not know what spaghetti worms eat.

Behavior and reproduction: Spaghetti worms live in tangled coils covered with mucus. Except that fertilization takes place outside the body, scientists do not know how spaghetti worms reproduce.

Spaghetti worms and people: Spaghetti worms have no known importance to people.

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

FOR MORE INFORMATION

Books:

Valentine, James W. On the Origin of Phyla. Chicago: University of Chicago Press, 2004.

Web sites:

Scott, Susan. "Elusive Acorn Worms Are Ocean's Vacuum Cleaners." Ocean Watch.http://www.susanscott.net/OceanWatch2001/aug17-01.html (accessed on March 2, 2005).

Scott, Susan. "Spaghetti Worms Utilize Tentacles in Amazing Ways." Ocean Watch.http://www.susanscott.net/OceanWatch1998/jan19-98.html (accessed on March 2, 2005).