Orthonectida

(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.

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 Rhopaluridae, 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 phylogenetic studies suggest the orthonectids are closer to myxozoans 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 µm). 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.

Species accounts

List of Species

No common name

Rhopalura ophiocomae

family

Rhopaluridae

taxonomy

Rhopalura ophiocomae Giard, 1877, Wimereux, France.

other common names

None known.

physical characteristics

Reaches length of 0.01 in (260 µm) in females, 0.005 in (130 µm) in males. This genus is characterized by sexual dimorphism, the demarcation of the body into regions being much sharper in males than in females. Males possess conspicuous crystalline inclusions in some epidermal cells. In females, the numerous oocytes form a compact mass that occupies most of the body; in males, the sperm mass is located in the middle third of the body.

distribution

Widely distributed. The usual host species, Amphipholis squamata, occurs off the coasts of France, Great Britain, and Italy, and Washington and California in the United States.

habitat

The perivisceral coelom are closely associated with the walls of the genital bursae or the gut of a parasitized brittle star.

behavior

Generally swims by a spiraling motion.

feeding ecology and diet

May absorb nutrients in the host cytoplasm.

reproductive biology

In males, the genital pore is located between epidermal rings 12 and 14; in females, in ring 19. The male and female may bring their genital pores together long enough for sperm transfer to be effected.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Ciliocincta sabellariae

family

Rhopaluridae

taxonomy

Ciliocincta sabellariae Kozloff, 1965, San Juan Archipelago, Washington, United States.

other common names

None known.

physical characteristics

Body length of females up to 0.01 in (270 µm); usually 38 or 39 rings of epidermal cells. Males up to 0.005 in (130 µm); usually 30 rings of epidermal cells. This genus is characterized by slight sexual dimorphism. Males are smaller than females, but the arrangement of epidermal cells and the pattern of ciliation are similar in both sexes. Males have no crystalline inclusions in epidermal cells comparable to those of male Rhopalura. In females, oocytes are in a single series.

distribution

The host is the sabellid polychaete Sabellaria cementarium, which occurs in the San Juan Archipelago, off the coast of the state of Washington, in the United States.

habitat

Epidermal tissues of the body wall and dorsal cirri of the parasitized polychaete.

behavior

Generally swims in a spiral motion.

feeding ecology and diet

May absorb nutrients in the host cytoplasm.

reproductive biology

In males, the genital pore is located in the boundary between epidermal rings 15 and 16; in females, in ring 14. The male and female may bring their genital pores together long enough for sperm transfer to be effected.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Intoshia linei

family

Rhopaluridae

taxonomy

Intoshia linei Giard, 1877, Wimereux, France.

other common names

None known.

physical characteristics

Body length up to 0.006 in (160 µm); usually 24 rings of epidermal cells. In males, the body of the largest individual is not over 0.0016 in (41 µm) in length. In this genus, more than half the rings of epidermal cells are completely ciliated; the rest lack cilia. Sexual dimorphism usually occurs. Males are much smaller than females, are ovoid, and have two genital pores. Males have no crystalline inclusions in epidermal cells comparable

to those in male Rhopalura. In females, the oocytes fill most of the axial mass, and are packed together so they seem to form two or three rows.

distribution

The hosts are the nemerteans Lineus viridus, L. sanguineus, and L. rubber, which occur on the English Channel coast of France at Wimereux and Roscoff.

habitat

Tissues of a parasitized nemertean.

behavior

Generally swims in a spiraling motion.

feeding ecology and diet

May absorb nutrients in the host cytoplasm.

reproductive biology

In males, there are two genital pores, located in epidural rings 8 and 16; in females, in ring 12. The male and female may bring their genital pores together long enough for sperm transfer to be effected.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Kozloff, Eugene N. "Phyla Placozoa, Dicyemida, and Orthonectida." In Invertebrates. Philadelphia: Saunders College Publishing, 1990.

Periodical

Kozloff, Eugene N. "The Genera of the Phylum Orthonectida." Cahiers de Biologie Marine 33 (1992): 377–406.

Pawlowski, J., J. Montoya-Burgos, J. F. Fahrni, J. Wüest, and L. Zaninetti. "Origin of the Mesozoa Inferred from 18S rRNA Gene Sequences." Molecular Biology and Evolution 13 (1996): 1128–1132.

Hidetaka Furuya, PhD