Peanut Worms: Sipuncula

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PEANUT WORMS: Sipuncula

NO COMMON NAME (Sipunculus nudus): SPECIES ACCOUNT

PHYSICAL CHARACTERISTICS

Peanut worms are sea creatures that have bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree). This means that their soft bodies can be divided into similar halves. They are sausagelike and not segmented in any way. Their bodies are gray or brown and are sometimes marked with reddish purple or green. They measure up to 11.8 inches (300 millimeters) long. The front part of the body is called the introvert (IN-treh-vuhrt). The introvert has small hooks that are used to gain traction. At the top of the introvert is the mouth. In some species, a ring of tentacles surrounds the mouth. The introvert can be pulled inside the rest of the body. Muscles pull on the mouth end, turning the introvert inward on itself, in what looks like the opposite of turning a sock inside out. Once withdrawn, the body becomes short, resembling a shelled peanut.

The thicker part of the body is called the trunk and is sometimes covered with small bumps. The soft body wall is supported by two kinds of muscles and a large body cavity filled with fluid. The body cavity is called the coelom (SIGH-lum). Ringlike circular muscles squeeze the body wall and shift fluid forward in the coelom to extend the introvert. Long muscles running the length of the body contract to move the body fluid back and pull the introvert into the trunk. Peanut worms do not have circulatory or respiratory systems. Instead, special cells floating in the fluid carry oxygen and nutrients throughout the body. A kidneylike organ inside the coelom helps filter waste from the body fluid. This waste is expelled from the body through one or two openings opposite the anus (AY-nuhs). The anus is usually located near the upper portion of the body, but in some species it is found on the introvert. The nervous system includes a bundle of nerves inside the tip of the introvert and a nerve cord that runs along the underside of the body.

GEOGRAPHIC RANGE

Peanut worms are found in all of the oceans.

HABITAT

Peanut worms are found in both cold- and warm-water habitats, at all depths between the intertidal zone and 22,510 feet (6,860 meters). Some species live in burrows in sand or mud, while others live in rock crevices, empty seashells, or tubeworm tubes. Still others bore into rock or bone. Some species make their homes in mats of algae (AL-jee) or plantlike growths that live in water, in large sponges, or among the roots of sea grasses or mangrove trees.

DIET

Peanut worms living in sand and mud swallow surrounding sediment collected with their tentacles. Those living in rocks use their introvert hooks to scrape sand, mud, and small organisms from the surfaces of surrounding rocks.

BEHAVIOR AND REPRODUCTION

Most peanut worms quickly withdraw their introverts when disturbed and avoid light by retreating into their burrows or rock crevices. They use their introvert hooks and muscles to pull their bodies forward. Swimming is not common and is accomplished by simply jerking the body trunk in all directions.

Many peanut worms can replace missing tentacles and introverts. Some species can regenerate portions of the digestive tract and body trunk. Others reproduce by purposely dividing their bodies. Each body part then develops all the necessary missing parts. This type of reproduction is called budding, or asexual (ay-SEK-shuh-wuhl) reproduction. Asexual reproduction does not involve mating or male or female reproductive systems.

Most peanut worms require both males and females to reproduce. Only one species has individuals with both male and female reproductive organs. Another species is able to reproduce without fertilization (FUR-teh-lih-ZAY-shun), or the combining of egg and sperm to start development. The sexes are identical to each other, and their reproductive organs are present only while they reproduce. Eggs and sperm are released into the coelom, where they are collected by the kidneylike organs and expelled into the water. Fertilization takes place outside of the body. Peanut worms develop in a variety of ways. Some species develop from eggs directly into miniature versions of the adults, while others must first go through various free-swimming larval stages before becoming young worms.

PEANUT WORMS AND PEOPLE

Larger species of peanut worms are used by fishermen throughout the world as bait. In Java, the western Carolines, and parts of China, they are eaten by humans.

HOME AT LAST

Peanut worms were first illustrated in the mid-1500s and were classified with other wormlike creatures in 1767. Not until 1959 was this unique group of animals placed in their own phylum, Sipuncula. The name of the phylum comes from the Greek siphunculus, meaning "little tube."

CONSERVATION STATUS

Peanut worms are not considered endangered or threatened.

NO COMMON NAME (Sipunculus nudus): SPECIES ACCOUNT

Physical characteristics: Sipunculus nudus measures 6 to 10 inches (150 to 250 millimeters) long. The introvert is short, only one-third the length of the trunk, and lacks hooks. There are 24 to 34 bands of long muscles visible through the skin.

Geographic range: Sipunculus nudus (abbreviated as S. nudus) are found throughout the world in temperate, subtropical, and tropical waters. (Specific distribution map not available.)

Habitat: S. nudus live in burrows in the sand and are found from just below the tidal zone down to 2,953 feet (900 meters) deep.

Diet: S. nudus swallow the surrounding sand to digest bits of plant and animal tissues.

Behavior and reproduction: This species spends its days hidden in its burrow, extending its tentacles to feed at night.

Males and females release sperm and eggs into the water. They pass through two larval stages before becoming young worms.

Sipunculus nudus and people: S. nudus is the best-known species of peanut worm and is often used as a research animal. They are sold as fish bait in some parts of the world.

Conservation status: S. nudus is not considered endangered or threatened. ∎