The non-hierarchical, non-uniformly branching topology of a leuconoid sponge aquiferous system revealed by 3D reconstruction and morphometrics using corrosion casting and X-ray microtomography

As sessile filter feeders, sponges rely on a highly efficient fluid transport system. Their physiology depends on efficient water exchange, which is performed by the aquiferous system. This prominent poriferan anatomical character represents a dense network of incurrent and excurrent canals on which we lack detailed 3D models. To overcome this, we investigated the complex leucon-type architecture in the demosponge Tethya wilhelma using corrosion casting, microtomography, and 3D reconstructions. Our integrative qualitative and quantitative approach allowed us to create, for the first time, high-resolution 3D representations of entire canal systems which were used for detailed geometric and morphometric measurements. Canal diameters lack distinct size classes, and bifurcations are non-uniformly ramified. A relatively high number of bifurcations show previously unknown and atypical cross-sectional area ratios. Scaling properties and topological patterns of the canals indicate a more complex overall architecture than previously assumed. As a consequence, it might be more convenient to group canals into functional units rather than hierarchical clusters. Our data qualify the leucon canal system architecture of T. wilhelma as a highly efficient fluid transport system adapted toward minimal flow resistance. Our results and approach are relevant for a better understanding of sponge biology and cultivation techniques.