Topology and functional anatomy of sponge aquiferous
systems analysed by corrosion casting and
synchrotron radiation based x-ray micro tomography
J.U. Hammel1,2, F. Wolf1, H. Jahn1, F. Wilde3,
F. Beckmann3, J.A. Kaandorp2 and M. Nickel1
1 Institut
für Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, Friedrich-Schiller-Universität Jena,
Ebertstr. 1, D-07743 Jena, Germany.
2 Section Computational Science, Faculty of Science, University of Amsterdam, PO Box 94248, 1090 GE Amsterdam,
The Netherlands.
3 Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Max-Planck-Str. 1, 21502 Gesthacht, Germany.
Background
Sponges (Porifera) are sessile filter feeding organisms, which acquire their nutrients through a complex,
branching canal system. This system is also called the aquiferous system and is one of the characteristic
features of sponges. The inner canal surfaces represent the interface between the tissue and the environment.
Thus resulting in a close functional relation between the majority of physiological processes and their canal
system. Sponges rely on their ability to pump high volumes of water through their body in order to gain
nutrients, oxygen and export metabolic end products. Filtration is achieved by an efficient particle capture
system in the form of a highly structured 3-dimensional network of canals which are supposed to be
hierarchically organized [1]. However, our present knowledge includes general aspects of canal architecture,
cellular anatomy and limited models on the flow generated by choanocytes [2]. Previous own and other
studies indicate a higher complexity of canal system architecture and flow regimes. However, most studies
so far lack complete 3D reconstructions and morphometric data and are mostly limited to one species.
Variations in growth form and the relation to canal system architecture has until presently never be
considered in such analysis. Our study aimed to acquire quantitative 3D data of the sponge canal system in
conjunction with a topological characterization of key sponge taxa with different canal architectures of the
complex leucon-type aquiferous systems, in order to deepen preliminary studies suggesting a much higher
architectural complexity of the canal system. The obtained data will be used to identify general patterns in
the development and architecture of sponge canal systems in relation to their biological function. 3DModels
for fluid dynamic simulations of key structures of the canal system will be generated and reveal the first
quantitative data of fluid dynamics inside sponge aquiferous systems.
Methodology
We investigated the topology of the canal system using SR-µCT scans of Tethya wilhelma canal system
corrosion casts (cyanoacrylate-based resin casts taken from living sponges, followed by fixation and tissue
maceration) and virtual casts of Spongilla lacustris (created by extracting the canal system in silico from
microtomography scans of complete fixed sponges) [3, 4]. Microtomography was operated by HZG using
the beamlines BW2 and W2 of the storage ring DORIS III at DESY. Entire canal systems were generated
from high resolution 3D reconstructions using ImageJ/Fiji, VG StudioMax and MeVisLab [3]. Hierarchical
classification of canals and additional morphometric parameters were calculated using a custom made
software [5], MeVisLab and Matlab [3].A model of S. lacustris excurrent canal system with apopylar
openings for CFD simulation using Comsol Multiphysics was generated using Autodesk Maya.
Results and discussion
Based on microtomography image stacks, we visualized the 3D canal system morphology of
complete specimens of T. wilhelma (Fig. 1A) and S. lacustris (Fig. 2B). For S. lacustris it was
possible to extract the incurrent and excurrent canal system separately. Quantitative morphometric
and topological analysis revealed a number of so far unknown features in canal system anatomy,
e.g. commonly present highly asymmetric bifurcations (Fig. 1B) [4]. This seems to be a common
feature shared even between different growth forms of sponges with leucon-type aquiferous
systems. Several additional sponge species of different growth forms have been imaged to identify
general patterns in canal system architecture and specific patterns related to growth form. However,
at present the analysis of these data is still in progress. As a preliminary conclusion, our results from
morphometric and topological analyses suggest that the current understanding of current models of
flow in sponges lacks depth and appears oversimplified in several aspects. As the canal system
architecture greatly influences internal flows, it will also affect distribution of nutrients and oxygen
to the tissue. In order to understand these processes we currently conduct computational fluid
dynamic simulations of flow inside sponge canal systems based on aquiferous system models
obtained from SRµCT data (Fig. 2B). However, even though we were able to obtain high spatial
and density resolution scans of S. lacustris specimens it was not possible to extract all relevant data
for a model of the canal system from one specimen. The challenging structures are the flow
generating choanocyte chambers and their openings connecting them to the incurrent and excurrent
canal system. They are just about at the resolution limit. Therefore we were able to identify them
only in a subset of the dataset. The ~600 identifiable choanocytes were used to determine the size
and distribution. Based on these measurements we automatically placed openings into our model of
the excurrent canal system (colored cylinders in Fig. 2B) for flow simulations. These flow
simulation studies are currently under way. However, with the newly available imaging beamlines
at PETRA III we hope to overcome the resolution limit and will be able to perform flow
simulations for entire canal systems in the near future.
A
B
Fig. 1. 3D Morphometric and topological analysis. A. Canal system reconstruction and centerline of
T. wilhelma. B. Analysis of bifurcation asymmetry in relation to the parent canal segment order shows a
functional adaption. Small canals display almost symmetric bifurcations while larger sized canals near the
outflow opening show highly asymmetric ones. Canal diameters are highly adopted to local flow regimes.
A
B
Fig. 2. High resolution microtomography of a juvenile freshwater demosponge Spongilla lacustris. A.
Virtual section through a volme rendering showing canal system elements and choanocyte chambers.
Subgroup of the segmented choanocyte chambers for size and distribution measurements are shown in red B.
Reconstruction of the excurrent canal system from the same dataset with modeled apopylar openings
(colored cylinders) for computational fluid dynamic simulations.
References
[1] Reiswig, H.M., Journal of Morphology, 1975. 145(4): p. 493-502.
[2] Simpson, T.L., The cell biology of sponges1984, New York: Springer. 662.
[3] Hammel, J.U., et al., Acta Zoologica, 2012. 93: p. 160-170.
[4] Nickel, M., et al., Proceedings of SPIE, 2008. 7078: p. 7078W.
Kruszyński, K., et al., Coral Reefs, 2007. 26(4): p. 831-840.