Suspended food particles in oligotrophic waters are scarce and the planktonic community is dominated by minute picoplanktonic (< 2 μm) cells. Consequently, suspension feeders inhabiting such environments are faced with the dual challenge of capturing extremely small particles and efficiently processing large volumes of water. We used direct in situ techniques to characterize the size-dependent capture efficiency curve of four solitary ascidian species (Chordata, Tunicata) from the ultra-oligotrophic East-Mediterranean, and two species from the oligotrophic Gulf of Aqaba (Northern Red-Sea). To control for confounding factors such as particle surface chemistry, we used a suspension of artificial polystyrene microspheres with a wide range of sizes (from 0.3 μm to 10 μm) but identical surface chemistry. This suspension was introduced into the inhalant siphon while the water inhaled and exhaled by the otherwise undisturbed ascidian was cleanly sampled. All the investigated ascidian species captured ≥ 1 μm microspheres at 95% ± 8% efficiency (mean ± 95% Confidence Interval) with no significant difference between species. Moreover, and in contrast to reports from eutrophic waters, both in situ and laboratory experiments revealed an efficient capture of submicron 0.3 μm microspheres at close to 50% efficiency. For most of the species tested, the capture efficiency of microspheres dropped below optimum between 1 μm and 0.5 μm providing an estimation of their mucus mesh pore size. We suggest that ascidians from oligotrophic waters are adapted for capturing submicron particles picoplankton and other. Such adaptation enables “oligotrophic” ascidians to access a much larger food source and facilitates the transfer of planktonic C, N, and P from the water column to the benthos.