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ABSTRACT Askenasiais 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 marineAskenasia, from perhaps the first ever cultures established for this genus.Askenasiaspp. have long been suspected of being predatory, i.e., feeding on other ciliates, and here we reveal that several isolates of marineA. cf.stellarishad high ingestion and growth rates on the ciliateMesodinium rubrum. This result supports numerous observations ofAskenasiaco‐occurring withM. rubrumin a variety of ecosystems. While we found no evidence of grazing on several other species of ciliates or photosynthetic flagellates,A. cf.stellarisprobably does ingest other small ciliates as prey. Using transmission electron microscopy imaging ofAskenasia, 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 twoA. cf.stellarisstrains. We also provide ultrastructural details of the contractile and food vacuoles inAskenasia. Our results suggest thatA. cf.stellariscan be an important predator ofM. rubrumin coastal marine ecosystems. 

Abstract As chloroplast‐stealing or “kleptoplastidic” lineages become more reliant on stolen machinery, they also tend to become more specialized on the prey from which they acquire this machinery. For example, the ciliateMesodinium rubrumobtains > 95% of its carbon from photosynthesis, and specializes on plastids from theTeleaulaxclade of cryptophytes. However,M. rubrumis sometimes observed in nature containing plastids from other cryptophyte species. Here, we report on substantial ingestion of the blue‐green cryptophyteHemiselmis pacificabyM. rubrum, leading to organelle retention and transient increases inM. rubrum's growth rate. However, microscopy data suggest thatH. pacificaorganelles do not experience the same rearrangement and integration asTeleaulax amphioxeia's. We measuredM. rubrum's functional response, quantified the magnitude and duration of growth benefits, and estimated kleptoplastid photosynthetic rates. Our results suggest that a lack of discrimination betweenH. pacificaand the preferred preyT. amphioxeia(perhaps due to similarities in cryptophyte size and swimming behavior) may result inH. pacificaingestion Thus, while blue‐green cryptophytes may represent a negligible prey source in natural environments, they may helpM. rubrumsurvive whenTeleaulaxare unavailable. Furthermore, these results represent a useful tool for manipulatingM. rubrum's cell biology and photophysiology.