Opalinata

Classification

Subphylum of Sarcomastigophora

General Information

These relatively large (up to 1 mm long) protozoa are characterized by numerous close rows of short flagella which move metachronously. Although these flagella look like cilia, the basic organization is quite different. Opalinids have two to numerous identical nuclei; micro- or macronuclei are always absent. Asexual reproduction occurs by longitudinal-oblique binary fission (Fig. 1); in some species, however, additional sexual fusion stages have been reported. In general the opalinids inhabit the colon or cloaca of amphibians, reptiles and fish; their pathogenic effects are low; mostly they feed as commensals on intestinal fluids (by pinocytosis). Transmission occurs mainly by oral uptake of cysts (Fig. 1).

System

Subphylum: Opalinata

Class: Opalinatea

Order: Opalinida

Genus: Opalina (in frogs)

Genus: Protoopalina (in fish)

Genus: Cepedea (in amphibia)

Genus: Zelleriella (in Bufo)

Life Cycle



  
Fig. 1. Life cycle of Opalina ranarum. 1 Cysts are excreted by the adult frog and orally ingested by a tadpole. 2 After hatching the young gamont migrates to the cloaca. 3, 4 Formation of micro- and macrogametes (meiosis). 5 Fusion of the heterogametes. 6 Encystation of the zygote and excretion via feces. 7, 8 After oral uptake of a cyst by another tadpole the trophozoite grows up in the cloaca (up to 0.5 mm). 8.1, 8.2 The small trophozoite may start division inside the tadpole, finally leading to formation and excretion of cysts (1) which give rise to new trophozoites (2 → 8) after ingestion by another tadpole. 9 When metamorphosis of tadpoles to frogs is completed, the trophozoites (agamonts, trophonts) grow up and form up to 2000 nuclei. 10 During the non-breeding season of the frog the trophozoites multiply by binary fission, the axis of which is either longitudinal or olique-transverse. 11, 12 During the breeding season hormones released by the frog induce rapid divisions of the trophozoites without compensatory nuclear divisions and growth. Thus the parasites (precystic forms) become successively smaller. These stages, finally having 2–12 nuclei, encyst (1), are set free with the feces of the host and become infectious for tadpoles. CI, cilia; CW, cyst wall; N, nucleus