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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.