Orthonectidans: Orthonectida

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ORTHONECTIDANS: Orthonectida

NO COMMON NAME (Rhopalura ophiocomae): SPECIES ACCOUNT

PHYSICAL CHARACTERISTICS

Orthonectidans (or-thuh-NEK-tih-duhns) are parasites (PAIR-uh-sites) that live in the tissues of sea-dwelling invertebrates (in-VER-teh-brehts). A parasite is an animal or plant that lives in or on another animal or plant, called a host, without helping it and usually harming it. Invertebrates are animals without a backbone. Depending on the species of orthonectidan, either the sexes are separate or both male and female reproductive organs are present in the same animal. Adult orthonectidans are
0.002 to 0.031 inch (50 to 800 micrometers) long. The body of an adult orthonectidan consists of a jacket of body cells arranged in rings around an internal mass. Some of these body cells have hairlike fibers and some do not. Contracting muscle cells in the sex organ run in the lengthwise, circular, and oblique directions.

GEOGRAPHIC RANGE

Orthonectidans live in the coastal regions of the northwestern part of the Atlantic Ocean and the Mediterranean Sea, the northwestern part of the Pacific Ocean near Japan, and the northeastern part of the Pacific Ocean along the coast of the United States.

HABITAT

Orthonectidans live in the tissues of sea-dwelling invertebrates, such as flatworms; segmented worms; mollusks (MAH-lusks), or animals with soft, unsegmented bodies that may or may not have a shell; sea stars, sea urchins, sea cucumbers, and their relatives; and sea squirts.

DIET

Orthonectidans absorb nutrients within the host's cells.

BEHAVIOR AND REPRODUCTION

Orthonectidans are parasites that alternate between asexual (ay-SEK-shuh-wuhl) and sexual stages within the host animal. Asexually produced masses of young orthonectidans may develop into sexual forms. Asexual reproduction takes place without and sexual reproduction takes place with the uniting of egg and sperm for the transfer of DNA from two parents. During mating male orthonectidans make brief contact with females and release sperm. Fertilization (FUR-teh-lih-zay-shun), the joining of egg and sperm to start development, takes place inside the female. Embryos form about twenty-two hours later. When the embryos are fully developed, the female ruptures and dies, releasing larvae (LAR-vee) that disperse and enter a new host. Larvae are animals in an early stage that change form before becoming adults.

ORTHONECTIDANS AND PEOPLE

Orthonectidans have no known importance to people.

What's in a Name?

The name orthonectidan means "straight swimming," but these animals usually swim in a spiral motion.

CONSERVATION STATUS

Orthonectidans are not threatened or endangered.

NO COMMON NAME (Rhopalura ophiocomae): SPECIES ACCOUNT

Physical characteristics: Females of Rhopalura ophiocomae (abbreviated as R. ophiocomae) reach a length of 0.01 inch (260 micrometers), males a length of 0.005 inch (130 micrometers). The division of the body is much sharper in males than in females. In females numerous eggs form a compact mass that occupies most of the body. In males the sperm mass is in the middle third of the body.

Geographic range: R. ophiocomae lives off the coasts of France, Great Britain, Italy, and Washington and California in the United States.

Habitat: R. ophiocomae lives in the reproductive and digestive tracts of dwarf brittle stars.

Diet: R. ophiocomae absorbs nutrients in the host's cells.

Behavior and reproduction: R. ophiocomae usually swims with a spiraling motion. To mate, males and females bring their reproductive openings together long enough for the transfer of sperm from the male to the female. Fertilization takes place inside the female. When the embryos are fully developed, the female ruptures and dies, releasing larvae that disperse and enter a new host.

R. ophiocomae and people: R. ophiocomae has no known importance to people.

Conservation status: R. ophiocomae is not threatened or endangered. ∎