Abstract
Rhythmic body contraction is a phenomenon in the Porifera, which is only partly understood. As a foundation for the understanding of the functional morphology of the highly contractile Tethya wilhelma, we performed a qualitative and quantitative volumetric 3D-analysis of the morphology of a complete non-contracted specimen at resolutions of 5.2 and 6.9 μm, using synchrotron radiation based X-ray computed microtomography (SR-μCT). For the first time, we were able to visualize all three major body structures of a complete poriferan without dissection of the shock-frozen, fixed and contrasted specimen in a near-to-life confirmation: poriferan tissue, mineral skeleton and aquiferous system. Applying a ‘virtual cast’ technique allowed us to analyze the structural details of the complete canal structure. Our results imply an extensive re-circulation of water inside the poriferan due to well-developed by-pass-canals, connecting excurrent and incurrent system. Nevertheless, the oscule region is strictly separated from the incurrent system. Based on our data, we developed a hypothetical flow regime for T. wilhelma, which explains the necessity of by-pass canals to minimize pressure boosts in the canal system during contraction. Additionally, re-circulation optimizes nutrient uptake, within small-sized poriferans, like T. wilhelma. Quantitative analysis allowed us to measure volumes and surfaces, displaying remarkable organizational differences between choanosome and cortex, by means of distribution of morphological elements. The surface-to-volume ratio proved to be very high, underlining the importance of the poriferan pinacoderm. We support a pinacoderm-contraction hypothesis.
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