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PHYLUM OPALINIDA

INTRODUCTION TO THE OPALINIDA

Opalinida (o-pa-LI-ni-da) means little opal and is derived from the Greek  (opallios -ὀπάλλιος) or the Latin (opalus). The reference is to the tiny opaline trophozoites.

The opalinids are gut commensals of amphibians, particularly anurans, and of some fish.  They are covered by small short flagella (cilia) with parallel basal bodies.  Thus, they superficially resemble ciliates (Figure A).  However, they do not have alveoli nor do they have dimorphic nuclei.   

Opalina is a commensal that has a life cycle that alternates between the mature frog and at least two different tadpoles (Opalina Life Cycle).  Cysts are expelled by the adult frog and taken up by a feeding tadpole.  After the cyst germinates in the gut, the cells (gamonts or gamete-producing cells) move to the cloaca where they undergo meiosis.  In this case, the products of meiosis become gametes that look like the ciliated opalinid, but they are uninucleate and vary in size (microgamete and macrogamete), a condition called anisogamy.  The fusion of gametes forms a zygote that encysts and leaves the host via the feces.  The zygocyst is then taken up by a second tadpole where the cell excysts forming an agamont (a cell that does not undergo meiosis or gamete formation), which stays in the lower bowel and cloaca as a feeding stage, a trophozoite.  The agamonts can form their own cysts which can, in turn, can be taken up by other tadpoles.  As the tadpoles mature and metamorphose into frogs, the agamont trophozoites in them become larger and many times multinucleate (more than 2,000 nuclei).  The large forms of the agamonts persist in the adult frogs until hormonal changes in the breeding season signals a change in the opalinids, which begin to divide without growth and mitosis, making the daughter cells successively smaller and with fewer nuclei with each cytokinesis.  Finally, during the breeding season, the small cells with 2-12 nuclei encyst, are expelled, and the cycle continues.

SYNOPTIC DESCRIPTION OF THE OPALINIDA

SYSTEMATICS OF THE OPALINIDA

Margulis and Schwartz (1988) and Corliss (1990) consider this group to be part of a large, heterogeneous collection of flagellated organisms called the "Zoomastigina" (Pr-8).  Since then, Margulis and Schwartz (1998) removed the amitochondriate taxa and renamed the new phylum Zoomastigota (Pr-30). I have raised the opalinids to the phylum level and am in agreement with Sleigh et al (1984). Older sources such as Kudo (1966) consider the opalinids to be primitive ciliates. However, Margulis and Schwartz (1988 and 1998), Grell (1976), and Lynn and Small (1985) consider that the opalinids only superficially resemble the ciliates. Patterson (1989) suggests that the opalinids have "suffered from taxonomic isolation" long enough and proposes an association with the Proteromonads (a group he considered allied to the Retortomonads).  However, more recently, Patterson (1999) places the retortomonads, and therefore the opalinids, in the Stramenopliles (Heterokonts).  Sogin and Patterson (Tree of Life Project) have placed them near the root of the heterokont tree.  Similarly, Baldauf (2003) in her synthesis have the opalinids clearly in the heterokont line, though at the base and near the divergence of the alveolates.

HIERARCHICAL CLASSIFICATION OF THE OPALINIDA

This page is maintained by Jack R. Holt. Last revised: 02/23/2008.