Enhanced feeding with structural coordination of ciliary band and oral apparatus*

Oceanic single-cell organisms, particularly ciliates, have specialized oral structures that facilitate nutrient absorption through phagocytosis. These organisms use ciliary bands around their oral surface to generate flow currents, driving nutrients toward their mouths. We previously showed that ciliary currents in both "swimming" and "sessile" organisms can be equally advantageous for feeding. But what are the advantages of having specialized oral structures? If a larger mouth, implying a larger absorbing surface, results in a greater nutrient encounter area, why do ciliates have specialized oral apparatuses occupying relatively small fraction of the cell surface? To address this question, we developed a mathematical model and systematically analyzed the feeding rates of ciliates with various oral apparatus and ciliary band sizes. Our model is based on Blake's envelope model and incorporates an advection-diffusion process of nutrient transport around the model ciliate. Our findings reveal that the feeding rate is enhanced when the ciliary band is positioned adjacent to the oral apparatus. Furthermore, we identified that the optimal designs maximizing feeding rates differ between sessile and motile ciliates. By integrating these results with intracellular nutrient transport mechanisms, our analysis bridges the understanding of outer and inner food transport mechanisms in single-celled ciliates. This study provides insights into the evolution of specialized functions and structures in eukaryotes, highlighting the collaborative adaptation of external and internal mechanisms for efficient nutrient acquisition.