Description of two species of nassophorean (“Hypostome”) ciliates, Nassulopsis elegans (Ehrenberg, 1833) and Zosterodasys derouxi n.sp., with comments on their systematic and phylogenetic affinities

Abstract

Most of the so-called "lower hypostomes", nassophorean ciliates in the most recent classifications of the phylum Ciliophora, have been little studied in modern times (e.g., employing methods of silver impregnation, a technique widely considered indispensable in comparative taxonomic work on these protists today). In this paper, we present descriptions of two species, a new strain of Nassulopsis elegans (Ehr., 1833) and Zosterodasys derouxi n.sp., based primarily on use of the pyridinated silver carbonate method of Fernández-Galiano. From our own data, especially on the oral hypostomial frange of the first organism and the true somatic synhymenium of the second, and review of the relevant literature, we suggest that the phylogenetic affinities of these forms need to be reassessed. We also tentatively propose some changes in the suprafamilial classification of the ciliate groups involved. While recognizing the need for additional information that can be supplied only by future ultrastructural studies and comparative morphogenetic investigations, we briefly offer the following two speculative ideas at this time: (1) that Nassulopsis be removed from the order Synhymeniida and be considered an evolutionarily primitive genus of the "higher" order Nassulida; and (2) that Zosterodasys be considered a "pivotal" primitive nassophorean that may have given rise, phylogenetically, not only to the more evolved groups of its own class (the Nassophorea) but also to the (primitive groups of the) entire neighboring class Phyllopharyngea.