High Grazing Rates of Marine <i>Askenasia</i> cf. <i>stellaris</i> on <i>Mesodinium rubrum</i> and Insights Into Its Taxonomic Status

Askenasia is a conspicuous ciliate in plankton assemblages of fresh and marine ecosystems; however, many aspects of its biology, taxonomy, and ecology remain enigmatic. Here we provide new information on the behavior, prey preferences, ultrastructure, and phylogenetic diversity of marine Askenasia, from perhaps the first ever cultures established for this genus. Askenasia spp. have long been suspected of being predatory, i.e., feeding on other ciliates, and here we reveal that several isolates of marine A. cf. stellaris had high ingestion and growth rates on the ciliate Mesodinium rubrum. This result supports numerous observations of Askenasia co-occurring with M. rubrum in a variety of ecosystems. While we found no evidence of grazing on several other species of ciliates or photosynthetic flagellates, A. cf. stellaris probably does ingest other small ciliates as prey. Using transmission electron microscopy imaging of Askenasia, we show that the dikinetids of its equatorial ciliary belt resemble those found in Prostomatea ciliates, supporting phylogenetic analysis of partial 18S rRNA genes from two A. cf. stellaris strains. We also provide ultrastructural details of the contractile and food vacuoles in Askenasia. Our results suggest that A. cf. stellaris can be an important predator of M. rubrum in coastal marine ecosystems.