Aquiferous system, filtration rates and hydrodynamics of the syconoid calcareous sponge Urna sp.

The aim of the present study was to gain insight into the hydrodynamic characteristics of the relatively simple aquiferous system in specimens of a calcareous syconoid sponge, Urna sp. Data on the morphology and ultrastructure of the sponge combined with measured pumping rates were provided and used for subsequent estimates of the pressure drops of water flow through the aquiferous system. The pumping rates were estimated from microscope video-recordings as the product of osculum-cross sectional area and exhalant jet speed. Estimates are given of the sensitivity of pressure drops to dimensional changes associated with observed dynamic, contractile structures (e.g., osculum, apopyles), as well as possible artefacts introduced in the preparations for ultrastructural studies. The estimated pressure losses showed that the choanocyte pumps provide a pressure of 5.5 ± 3.9 Pa at a pumping rate of 1533 ± 1089 µm³ s⁻¹ per choanocyte. Such high pumping rates, comparable to those of some choanoflagellates, have not been reported for syconoid and leuconoid sponges before. However, the corresponding sponge volume-specific pumping rates (about 10–30 min⁻¹) are comparable to values reported in the literature for small sponges and explants that also have relatively higher pumping rates of choanocytes than larger sponges.