Molecular quantification of differential ingestion and particle trapping rates by the appendicularian Oikopleura dioica as a function of prey size and shape

To investigate whether particles are ingested or merely trapped in the house of the appendicularian Oikopleura dioica, we developed a real-time quantitative polymerase chain reaction (qPCR) assay and applied it in a controlled study where O. dioica was fed three types of algae at different concentrations. At particle concentrations ranging from 100 to 10,000 cells mL(-1), highest ingestion efficiencies (75-96%) were observed with the smallest algal species (Isochrysis sp.) compared to the larger cryptophyte Rhinomonas sp. (2-87%) and a similar sized but spiny diatom, Chaetoceros calcitrans (4-65%). When prey were mixed, the presence of C calcitrans inhibited ingestion of Rhinomonas sp. but not the smaller Isochrysis sp., suggesting that C calcitrans clogged the inlet filter and mechanically hindered ingestion of larger algae. O. dioica is adapted to feed on smaller particles at low (nonbloom) concentrations and may thus be subjected to bottom-up regulation by larger spiny or colonial prey that typically dominate late-stage phytoplankton blooms.