Species-Specific Prey Preference and Behavioral Responses to Tetraselmis Chloroplast Donors in the Euglenoid Rapaza...

Plastids originated through endosymbiosis and enabled the emergence of photosynthetic eukaryotes. However, in several eukaryotic lineages, plastids are not permanently integrated but are instead acquired transiently through predation, a phenomenon known as kleptoplasty. Although kleptoplasty has been documented across diverse eukaryotic groups, the extent to which prey identity constrains plastid acquisition and subsequent maintenance remains poorly defined. In particular, it is unclear whether kleptoplastic organisms exploit plastid donors broadly or depend on highly selective host-prey interactions. Here, we investigated prey responses and feeding behavior in the kleptoplastic euglenoid Rapaza viridis using a phylogenetically diverse set of Tetraselmis species. Through comparative analysis of ingestion behavior, behavioral interactions, and postingestion chloroplast fate among candidate chloroplast donors, we show that R. viridis exhibits strong prey specificity at the species level. Although R. viridis has been shown to depend on Tetraselmis as a source of kleptoplasts, our results demonstrate that ingestion, functional retention, and growth support are restricted to species belonging to the subgenus Tetraselmis. These findings indicate that chloroplast acquisition in R. viridis is constrained by fine-scale host-prey compatibility, rather than by prey availability alone, and highlight the importance of selective prey recognition and post-ingestion processing in regulating kleptoplasty.