The Structure and Origin of the Plasmodium of Rhopalura ophiocomae(Phylum Orthonectida)

Abstract

Adult orthonectids develop from germinal cells within a cytoplasmic matrix called a plasmodium. This is generally assumed to be formed by the parasite. In the case of Rhopalura ophiocomae, which lives in the brittle star Amphipholis squamata, the plasmodia occupying the perivisceral coelom are closely associated with the walls of the genital bursae or the gut, and they are covered by peritoneum. They have been reported to contain scattered small nuclei distinct from those within germinal cells, embryos, and adults, but the results of the present study indicate that such nuclei probably do not exist. Furthermore, electron micrographs show that some plasmodia are in continuity with the cytoplasm of contractile cells that lie beneath the peritoneum of a genital bursa or the gut of the host. The matrix of a plasmodium of R. ophiocomaeappears, therefore, to consist of cytoplasm of a contractile cell. It is proposed that after a contractile cell has been entered by an infective cell of the parasite, it hypertrophies, bulging progressively farther into the perivisceral coelom and lifting up the peritoneum, which remains in intimate contact with it.

Structure and supposed feeding mechanisms of the plasmodium of Intoshia linei (Orthonectida)

“…Microvilli on the plasmodium surface are found in I. variabili (Slyusarev, Miller, 1998), as well as in Rhopalura ophiocomae and Ciliocincta sabellaria (Kozloff, 1994(Kozloff, , 1997. In all mentioned species, plasmodium bears microvilli in some separate parts only.…”

“…The feeding strategy of the orthonectid plasmodium seems also to be similar to that of myxosporidian plasmodia (Uspenskaya, 1966(Uspenskaya, , 1984. It may be considered to be an indirect argument for that the plasmodium is an individual organism, but not a transformed host cell (tissue), as Kozloff (1994Kozloff ( , 1997 believed. Ðèñ.…”

“…Metschnikoff (1881), who suggested this term, and many authors after him considered the plasmodium to be a stage of life cycle of a parasite. According to Kozloff (1994Kozloff ( , 1997, the plasmodium is not an individual parasitic organism but a huge host cell (cells) transformed by parasites. The morphology of orthonectid plas- modium is poorly known.…”

“…The morphology of orthonectid plas- modium is poorly known. Only three species have been studied at the ultrastructural level (Kozloff, 1969(Kozloff, , 1971(Kozloff, , 1994(Kozloff, , 1997Slyusarev, Miller, 1998). With the exception of general considerations nothing is known about the way of feeding in the othonectid plasmodium.…”

A New Species of Orthonectida That ParasitizesXenoturbella bocki: Implications for Studies onXenoturbella

“…They have been reported to parasitize acoels, platyhelminthes, annelids, molluscs, nemerteans, echinoderms, and chordates; and it has been suggested that they are present in the host animals as multinucleate plasmodia (Slyusarev and Miller, 1998). However, the plasmodia may also be a mass of host cells that has degenerated due to the parasites, and therefore not a viable stage of the orthonectid life cycle (Kozloff, 1994(Kozloff, , 1997. Male and female adults are produced from germinal cells within the plasmodia, and the ciliated adults swim out from the hosts.…”