Spherical choanocyte chamber of sponge achieves optimized pumping efficiency

Sponges, the basalmost members of the animal kingdom, exhibit a range of complex architectures in which microfluidic channels connect multitudes of spherical chambers lined with choanocytes, which are flagellated filter-feeding cells. Choanocyte chambers can possess scores or even hundreds of such cells, which drive complex flows entering through porous walls and exiting into the sponge channels. One of the mysteries of the choanocyte chamber is its spherical shape, as it seems inappropriate for inducing directional flow given that some choanocytes flagellate in the opposite direction of flow. To answer this question, we combined direct imaging of choanocyte chambers in living sponges with computational studies of many-flagellum models. Computationally, we found that the flagella beating against the flow play a role in raising the pressure inside the choanocyte chamber. As a result, the mechanical pumping efficiency, calculated from the pressure rise and flow rate, reaches a maximum at the small outlet opening angle. Moreover, a comparison between the experiments and numerical simulations revealed that the chamber diameter, flagellar wave number and the outlet opening angle of E. muelleri, as well as some other species are designed so as to maximize the mechanical pumping efficiency. These results imply that the morphogenesis of the choanocyte chamber is appropriate in terms of hydrodynamics. The findings are useful for understanding the physiology and body design of sponges.