Dynamics and functional morphology of contraction and expansion in the calcareous sponge Clathrina clathrus

The body of the calcareous sponge Clathrina clathrus (Schmidt, 1864) (Porifera,Calcarea, Calcinea, Clathrinidae) consists of an anastomosing network of even tubes.Like most sponges, C. clathrus is contractile, however its contractile strength exceedsmost other species by far. This enormous shrinkage of the body in contracted animalswas already observed in first in situ studies during the last century and wheremisinterpreted as necrotic events.The contraction-expansion cycle, however, takesmuch more time than in most demosponges (comparative data for other calcareoussponges is not available to date). The aim of our project is to characterize thedynamics and functional morphology of contraction in C. clathrus. Further on we willcompare it to data on well-studied demosponges, like Tethya wilhelma. We usedtime-lapse imaging, histology, SEM and SR-µCT to characterize body dynamics as wellas anatomical traits in 2D and 3D. Our preliminary results confirm the hypothesis ofpinacocyte contractility with a clear body shape change from extended thinpinacocytes in expanded stage to t-shaped pinacocytes in contracted stage. However,in contrast to demosponges, the mesohyl displays prominent changes duringcontraction, too. Surprisingly, the mesohyl increases in thickness during contraction,while the canal lumen almost collapses completely. Whether the volume of themesohyl is altered by incurrent water (swelling = volume increase) or thickening issimply an effect of an overall 3D deformation of the tube walls due to overallshortening will have to be proven