Choanocyte dimensions and pumping rates in the demosponge Halichondria panicea

The individual choanocyte pumping rate in choanocyte chambers (CCs) is important for understanding the hydrodynamics in sponges and has hitherto been based on measured volume-specific filtration rate and estimated CC density. However, the CC density may vary in different regions of the sponge and to circumvent this uncertainty and to get precise measurements of the individual choanocyte pumping rate, a new experimental approach was developed. Here the aim was to measure the individual pumping rate of choanocytes based on live dimensions of CC elements and particle tracking to measure the speed of small particles entering into the CCs. This was done by using combined live-cell imaging in sandwich cultures of the marine demosponge Halichondria panicea and video-tracking of particles. Small 2 μm-beads and cyanobacteria (Cyanobium bacillare) in the incurrent canal enter the CCs via a 3.3 ± 0.9 μm diameter prosopyle to be subsequently captured by the choanocytes whereas larger algal cells (Rhodomonas salina) and 10 μm-beads are captured in the incurrent canals. CC diameters were positively correlated to the diameter of choanocytes, indicating a total of 84 choanocytes per CC with mean diameter 22.9 ± 6.2 μm. The pumping rate per choanocyte (Q_c) was estimated to be between 54 and 68 μm³ s⁻¹. Regardless of demosponge species and based on data in the literature, a choanocyte is suggested to pump between 50 and 100 μm³ s⁻¹.