Subphylum 2. INTRAMACRONUCLEATA: Class 6. COLPODEA — Somatically Conserved but Orally Diverse

Abstract

Ciliates now assigned to the Class COLPODEA were scattered throughout the phylum in unrelated classes and orders. However, it was the structural conservatism of the cortex and its somatic kinetids that enabled the identification of this natural assemblage based on cortical ultrastructure, a fact that has been confirmed by small subunit rRNA gene sequences. Colpodeans are the quintessential cyst-formers in the phylum, and are therefore typical in habitats that have a high probability of desiccation: mosses, soils, and leaf litter are typical habitats. However, colpodeans are found in ponds and lakes, although rarely in marine habitats. Their prey varies with their cell size: smaller col-podeans eat bacteria while the largest colpodean Bursaria can ingest Paramecium. The somatic kinetid is a dikinetid with well-developed overlapping transverse microtubular ribbons derived from the posterior kinetosome and forming what is called the LKm fiber or transversodesma—the strong synapomorphy for the group. Oral structures range from a paroral that almost encircles a prostomatous oral region to a huge deep oral cavity, almost spirotrich-like, adorned with many oral polykinetids. Stomatogenesis ranges from mero- to pleurotelokinetal, and in the colpodids typically occurs within a cyst after dedifferentiation of all parental oral structures. Conjugation has rarely been observed, and it is often assumed that colpodeans are completely asexual. This needs to be tested by molecular genetic approaches.