Orthonectidans

Phylum Orthonectida Number of families 1

Thumbnail description

Minute dioecious and dimorphic or hermaphroditic parasites found in tissues of a wide variety of marine invertebrate phyla.

Photo: A female Rhopalura ophiocomae. (Specimens in Nouvel collection, Santa Barbara Museum of Natural History. Photo by Hidetaka Furuya. Reproduced by permission.)

Phylum Orthonectida Number of families 1

Thumbnail description

Minute dioecious and dimorphic or hermaphroditic parasites found in tissues of a wide variety of marine invertebrate phyla.

Photo: A female Rhopalura ophiocomae. (Specimens in Nouvel collection, Santa Barbara Museum of Natural History. Photo by Hidetaka Furuya. Reproduced by permission.)

Phylum Orthonectida

Evolution and systematics

The phylum Orthonectida encompasses one family, four genera, and 20 species. Twenty nominal species have been described and variously placed in four genera. Genera are separated on the basis of body shape and the number and arrangement of egg cells in the axial region of the females. Species are separated on the basis of body size; the number, size, and arrangement of rings of ciliated and unciliated jacket cells; the presence and location of pigment and refringent granules in jacket cells; and host specificity. One family, the Rhopaluri-dae, contains all four genera: Rhopalura, Intoshia, Ciliocincta, and Stoecharthrum.

The orthonectids were previously placed in the Mesozoa because of their simple body organization, but recent phylo-genetic studies suggest the orthonectids are closer to myxo-zoans and nematodes. Many scientists now classify Orthonectida at the order level. A more traditional classification; with Orthonectida at the phylum level, is followed here.

Physical characteristics

Depending on the species, orthonectids are either dioecious and dimorphic, or hermaphroditic. The adults are minute, ranging in length from 0.002 to 0.031 in (50 to 800 pm). The body of the adult consists of a jacket of ciliated and unciliated somatic cells arranged in rings around an internal axial mass. Contractile muscle cells differentiate to pack the gonad with longitudinal, circular, and oblique orientations.

Distribution

Orthonectids occur in coastal regions of the English Channel, the Dover Strait, the Strait of Kattegat, the Barents Sea, the White Sea, the northwestern Pacific Ocean (Japan), and the northeastern Pacific Ocean (United States).

Habitat

Orthonectids are found in tissues of organisms in the marine invertebrate phyla Platyhelminthes, Nemertea, Annelida, Mollusca, Echinodermata, Bryozoa, and Urochordata. In the host, infective orthonectid germinal cells penetrate a host cell, and embryos develop and grow into adults within the cytoplasm of the host cell (the "plasmodium").

Behavior

The name Orthonectida means "straight swimming," because orthonectids swim in a straight line, but they generally swim in a spiral motion.

Feeding ecology and diet

Orthonectids may absorb nutrients within the host's cytoplasm.

Reproductive biology

During mating, males make brief contact with females when sperm are released. Fertilization is internal in females. Embryos form about 22 hours after the first cleavage of eggs. When the embryos are fully developed, the female ruptures and dies, releasing ciliated larvae that disperse and enter a new host.

Conservation status

No species of orthonectid is listed by the IUCN.

Significance to humans

None known.

Orthonectids
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment