TURBELLARIANS: Turbellaria

PHYSICAL CHARACTERISTICS

Turbellarians (ter-buh-LAIR-ee-uhns) are free-living flatworms. Free-living means they are not parasites (PAIR-uh-sites), which are animals or plants that live on or in other animals or plants without helping them and usually harming them. Turbellarians have three tissue layers and bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree), meaning the right and left halves of the body match each other. These animals have a complex but incomplete digestive tract, meaning they have no anus (AY-nuhs) and all waste leaves the body through the mouth. Turbellarians have a brain and nerve cords that form a ladderlike nervous system. They have numerous sense organs at the front end of the body and touch receptors all over the body, especially around the mouth, and have organs for eliminating waste and controlling the salt balance in their cells. Turbellarians have no circulatory system, a factor that restricts the size and shape of the animals. Each turbellarian makes both eggs and sperm. The outer layer of turbellarians is covered with hairlike fibers and contains mucus-secreting cells and structures that can produce mass quantities of mucus to prevent the animal from drying out. Most turbellarians have eyelike structures for detecting light. Some species have a pair of these light-detecting structures on their front end, but larger species may have numerous pairs along the body.

GEOGRAPHIC RANGE

Turbellarians live all over the world.

HABITAT

Most turbellarians live in freshwater or seawater, but a few species live on land. Many of the tiny species of turbellarians live between grains of sand in watery habitats. Larger species live in open water or among submerged materials such as rocks, coral, and algae (AL-jee), the plantlike growths that live in water and have no true roots, stems, or leaves. Many species live in invertebrates (in-VER-teh-brehts), which are animals without a backbone, or fishes without harming them. Land-dwelling turbellarians live in damp leaf litter and soil.

DIET

Most turbellarians are predators or scavengers that feed on anything they can fit into their mouths, such as protozoans, crustaceans, worms, and mollusks. Protozoans (proh-tuh-ZOH-uhns) are one-celled living things that resemble animals in that they gets their food from their surroundings rather than making it themselves as plants do. 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 soft, unsegmented bodies that may or may not have a shell.

Some turbellarians eat only certain foods, such as sponges, barnacles, or sea squirts. A few are close to being parasites because they graze on their live hosts. Land turbellarians feed on earthworms and snails. A few species of turbellarians have a relationship with the algae that live in them in which the algae supply the worm with carbohydrates and fats and the worm supplies the algae with nitrogen waste products and a safe haven.

BEHAVIOR AND REPRODUCTION

Some species of turbellarians secrete mucus that may contain poisonous or narcotic chemicals that slow or entangle prey. Turbellarians use a number of behaviors that prevent them from straying beyond their normal habitats and to keep themselves adjusted to their surroundings. For example, most turbellarians move toward something touching their belly and away from something touching their back. This ability allows bottom-dwelling forms to keep their bottom side down. Freely swimming turbellarians have special sense organs for adjusting themselves to gravity. Most species move away from light. This characteristic prevents the worms from coming out in the daylight, when water-dwelling species may be eaten and land-dwelling species may dry out.

All turbellarians have a strong sense of smell that can be used to find food and mates. Some of these worms swing their head back and forth to help determine the proper direction of the food source. Others use trial and error to determine the proper direction to find food. They move in one direction until the signal becomes weaker and then continue switching direction until the signal is strongest.

Turbellarians use both asexual and sexual reproduction. Asexual (ay-SEK-shuh-wuhl) means without and sexual means with the uniting of egg and sperm for the transfer of DNA from two parents. Many species divide asexually by splitting in two from side to side behind the mouth, and each part generates the rest of a body. The rear portion attaches to the material on which the worm lives, and the front portion crawls away. In some species the cells vary in their ability to regrow. The cells in the middle of the body have the strongest ability to regrow. If just the tail is cut off, it will not grow a new body, whereas the main portion of the body will grow a new tail. In some species several crosswise breaks develop that lead to a train of individuals that do not detach until they reach a certain stage. Other species detach fragments that form capsules and eventually develop into new individuals.

Individual turbellarians make both sperm and eggs, and their sexual reproductive systems are quite complicated. Fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, usually occurs when worms align themselves with each other, and the penis of each worm is inserted into the female opening of the other worm and deposits sperm. The worms then go their separate ways with the sperm stored inside. Turbellarians either are born resembling adults and then grow to maturity or produce freely swimming larvae (LAR-vee), or animals in an early stage that change form before becoming adults.

TURBELLARIANS AND PEOPLE

The regrowth abilities of turbellarians have been studied extensively by scientists who want to understand healing and cell regrowth in humans. Several species of turbellarians kill food animals such as oysters, and a few species cause disease in ornamental fishes used in aquariums.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists one species of turbellarians as Extinct, or no longer in existence.

FRESHWATER PLANARIAN (Dugesia tigrina): SPECIES ACCOUNTS

Physical characteristics: The body of the freshwater planarian is lance shaped with ear-shaped structures on each side of the head. These worms are light to dark brown, and some forms have a stripe down the center of the back. This worm has midline, light spots on a dark background or dark spots on a light background. The large mouth is in the middle of the body.

Geographic range: Freshwater planarians live all over North America and are scattered in Europe, where they may have been introduced with water plants.

Habitat: Freshwater planarians live under rocks, plants, and debris in clear freshwater ponds, streams, and springs.

Diet: Freshwater planarians eat various invertebrates, including mosquito larvae.

Behavior and reproduction: Freshwater planarians hide under rocks during the day. When hunting they swing their head from side to side better to sense sources of chemicals coming from food or prey. Freshwater planarians use asexual reproduction by splitting from side to side. Freshwater planarians have a strong capacity for regrowing after splitting. Scientists have not found reproductive organs in some of these worms and believe splitting may be more common than sexual reproduction. When they do mate, each worm deposits several egg capsules. Freshwater planarians do not have larvae. Young worms hatch from eggs.

Freshwater planarians and people: Freshwater planarians are studied intensively as a model for cell regrowth in humans and other animals.

Conservation status: Freshwater planarians are not threatened or endangered. ∎

NO COMMON NAME (Notoplana acticola): SPECIES ACCOUNTS

Physical characteristics: Adults of the species Notoplana acticola (abbreviated as N. acticola) are 1 to 2 inches (2.5 to 6 centimeters) long. They are tan or pale gray with darker markings along the center. The body usually is widest toward the front but is tapered at both ends. There are tentacle-like eyelike structures in rounded clusters with scattered ones in front of, behind, and sometimes beside them. About twenty-five more complex eye structures are present in a lengthwise band.

Geographic range: N. acticola lives in the Pacific Ocean on the coast of North America.

Habitat: N. acticola lives in shallow seawater on rocks.

Diet: N. acticola can eat prey up to half its size. It eats limpets, which are small, shelled mollusks; small barnacles; and worms.

Behavior and reproduction: N. acticola rapidly repairs its nerves if they are severed. Individuals of N. acticola make both eggs and sperm. They deposit their eggs in one another in late spring to early fall.

Notoplana acticola and people: N. acticola has no known importance to people.

Conservation status: N. acticola is not threatened or endangered. ∎

OYSTER LEECH (Stylochus inimicus): SPECIES ACCOUNTS

Physical characteristics: The body of an oyster leech is oval or disk shaped with retractable tentacles. There are three types of eyes. The throat is long and in the middle of the body. Each oyster leech has both male and female reproductive organs close to each other in the rear of the body. Although they are called leeches, oyster leeches are not in the same group as the more familiar blood-sucking leeches, which are segmented worms, the same group that contains earthworms.

Geographic range: Oyster leeches live off both coasts of Florida, United States.

Habitat: Oyster leeches live under rocks and algae, in oyster shells, and in other invertebrates.

Diet: Oyster leeches feed on animal matter, including oyster tissue.

Behavior and reproduction: Oyster leeches tend to hide under debris or in shells of oysters and barnacles. After mating, which can last nine hours and involve more than four partners, an individual oyster leech can deposit seven thousand to twenty-one thousand eggs. Egg masses usually are attached to clean oyster shells. The worms cover their eggs to protect them. Larvae with six eyes hatch from the eggs. The larvae swim up and away from the bottom.

Oyster leeches and people: Oyster leeches enter, devour, and kill oysters, harming the livelihood of people who harvest and sell oysters.

Conservation status: Oyster leeches are not threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Aaseng, Nathan. Invertebrates. New York: Venture, 1993.

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.

Web sites:

"Oyster Leech." Smithsonian Marine Station at Fort Pierce. http://www.sms.si.edu/IRLFieldGuide/Stylochus_sp.htm (accessed on December 20, 2004).

Seifarth, Wolfgang. "Regeneration." Marine Flatworms of the World. http://www.rzuser.uni-heidelberg.de/bu6/Introduction03.html (accessed on December 20, 2004).