Rotifers: Rotifera

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ROTIFERS: Rotifera

NO COMMON NAME (Asplanchna priodonta): SPECIES ACCOUNTS
NO COMMON NAME (Seison nebaliae): SPECIES ACCOUNTS

PHYSICAL CHARACTERISTICS

Rotifers (ROH-tuh-fuhrs) are microscopic animals that have complex jaws and a wheel organ, which is used for movement and feeding. Most rotifers are 0.006 to 0.02 inch (150 to 500 micrometers) long. The body usually is divided into a head, a trunk, and a foot. The largest organ in the head is the wheel organ, which is made up of beating hairlike fibers arranged in bands around the mouth. The jaw has a single sharp tooth or a plate with several strong teeth used to grab and pierce food. The trunk shape varies from long to spherical or baglike. The foot has one or several sections, and it often has two toes with sticky glands at the tips.

GEOGRAPHIC RANGE

Rotifers live all over the world.

HABITAT

Most rotifers live in freshwater in sand or mud at the bottom, in plants, or in open water. Several species live in saltwater, usually among plants but sometimes between grains of sand on beaches.

DIET

Rotifers eat algae and other protists, bacteria, and even other rotifers. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves. Protists (PROH-tists) are one-celled living things that have a nucleus (NOO-klee-uhs), which is the control center of a cell.

BEHAVIOR AND REPRODUCTION

Most rotifers collect food by beating the wheel organ to make a water current. Swimming rotifers move gently through the water in a spiral motion. Crawling rotifers use sticky glands on their foot and head to move much the way inchworms do.

Some species of rotifers use only sexual reproduction; some use only asexual reproduction, and others have both a sexual and an asexual phase. 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. In species that use only asexual reproduction, all the rotifers are females. The female produces eggs that develop into new embryos without fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development. The daughters are genetically identical to the mother.

In rotifers that use only sexual reproduction the males store sperm in a bag that they transfer to females during mating. Fertilization takes place inside the females. Later the females attach the eggs to crustaceans called sea fleas, where they stay until the young hatch. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone.

Among rotifers that have two reproductive phases, the asexual phase has only females that produce more females without fertilization. Certain physical events, however, can produce sexually reproducing females, whose eggs can be fertilized (FUR-teh-lyzed) by a male. If not fertilized, the eggs develop into dwarf males. These males do not live long and immediately after hatching seek a female and fertilize her eggs by injecting her with sperm. The thick-shelled egg that results can survive extreme conditions such as freezing and drying out. After a resting period, an asexually reproducing female hatches from the egg.

ROTIFERS AND PEOPLE

Many species of rotifers are cultured as fish food for aquariums and fish farms. They also can be used in tests for water pollution.

WHAT'S IN A NAME?

The name rotifer comes from the Latin words rota, meaning "wheel," and ferre, meaning "to carry." When these animals are swimming or feeding, the beating of their hairlike fibers gives the illusion that the animal carries two small, rotating wheels.

CONSERVATION STATUS

Rotifers are not considered threatened or endangered.

NO COMMON NAME (Asplanchna priodonta): SPECIES ACCOUNTS

Physical characteristics: Female Asplanchna priodonta rotifers are 0.01 to 0.06 inch (250 to 1,500 micrometers) long, and males are 0.008 to 0.02 inch (200 to 500 micrometers) long. The shape is long in summer and baglike in spring and autumn. The hard parts of the jaw are large, curved, sharply pointed, and tweezerlike.

Geographic range: Asplanchna priodonta (abbreviated as A. priodonta) rotifers live all over the world.

Habitat: A. priodonta rotifers live in freshwater.

Diet: A. priodonta rotifers eat algae, other protists, and other rotifers.

Behavior and reproduction: A. priodonta rotifers produce sexual and asexual females. Fertilization takes place inside the body. These rotifers also may have a third type of female, which can produce eggs that hatch into asexually producing females and eggs that hatch into dwarf males.

Asplanchna priodonta and people: A. priodonta rotifers have no known importance to people.

Conservation status: A. priodonta rotifers are not considered threatened or endangered. ∎

NO COMMON NAME (Seison nebaliae): SPECIES ACCOUNTS

Physical characteristics: Seison nebaliae rotifers are 0.03 to 0.1 inch (800 to 2,500 micrometers) long. Males and females are the same size. The head is egg shaped and has a long neck made up of parts that can retract like the sections of a telescope. The trunk is oval, and the foot is long and made up of sections. Rather than toes, these rotifers have a sticky disk they use to attach themselves to sea fleas.

Geographic range: Seison nebaliae (abbreviated as S. nebaliae) rotifers live in the Atlantic Ocean off the coast of Europe and in the Mediterranean Sea.

Habitat: S. nebaliae rotifers live on sea fleas.

Diet: S. nebaliae rotifers eat bacteria.

Behavior and reproduction: Except that S. nebaliae rotifers live on sea fleas, scientists do not know how they behave. These rotifers use sexual reproduction.

Seison nebaliae and people: S. nebaliae rotifers have no known importance to people.

Conservation status: S. nebaliae rotifers are not considered threatened or endangered. ∎