Dynamic structure of the mesohyl in the sponge Chondrosia reniformis (Porifera, Demospongiae)

The common demosponge; Chondrosia reniformis possesses the capacity to unde rgo an unusual creep process which results in the formation of long outgrow ths from the parent body. These shape changes, which have been interpreted as adaptive strategies related to environmental factors, asexual reproducti on or localised locomotor phenomena, are due mainly to the structural and m echanical adaptability of the collagenous mesohyl. This contribution descri bes the morphological correlates of mesohyl plasticisation in C reniformis. The microscopic anatomy of the mesohyl was examined when it was in differe nt physiological conditions: (1) standard "resting" condition, (2) "stiffen ed" condition and (3) dynamic "creep" condition. In this last case four rep resentative regions of the sponge body were analysed: the parent region, th e elongation region, the transition region and the propagule region. The re sults show that the histological modification of the sponge mesohyl during plasticisation is limited and localised. The most significant structural ch anges involve mainly cytological features of specific cellular components c haracterised by granule inclusions (i.e. the spherulous cells) and the arra ngement and density of the collagenous extracellular framework, though the integrity of the collagen fibrils themselves is not affected. Morphological and functional aspects of mesohyl plasticisation invite comparison with th e mutable collagenous tissue of echinoderms. Possible functional analogies between these two tissues are hypothesised.