Comparative analyses of the cuticular and muscular structures of the buccal-pharyngeal apparatus of tardigrades

In spite of the great importance of the tardigrade buccal pharyngeal apparatus in taxonomic and phylogenetic studies, it received little attention as regards its evolution and operating mechanism. To understand the relationships between form and function of the structures acting in the functioning of buccal-pharyngeal apparatus (i.e. cuticular structures, muscular fibers, pharynx), and to increase our knowledge on this apparatus, a comparative analyses using different investigation techniques was performed. The buccal-pharyngeal apparatuses of three species have been studied, Echiniscus trisetosus, Milnesium tardigradum e Paramacrobiotus richtersi, as representative of the two classes and of three orders of tardigrades. The cuticular structures of the buccal-pharyngeal apparatus have been analyzed form a morphological (light microscopy, scanning electron microscopy – SEM, and confocal laser scanner microscopy -CLSM) and chemical (energy dispersive X-ray spectroscopy) point of views. The musculature associated to the sclerified structures of the buccal-pharyngeal apparatus has been analyzed by CLSM to identify the muscular fibers and their relationships with the sclerified structures. The differences in the general anatomy of the buccal-pharyngeal apparatus among the three species were high, even though homologous structures were recognizable. The higher differences among species were found in the organization of the muscular system responsible of the stylet movements. The detailed analyses of the buccal-pharyngeal apparatus allowed a new interpretation of the organization of the stylet systems, and to understand the muscular system related to the feeding. The chemical analyses showed that the piercing stylets were formed by calcium, in form of CaCO3. Heterotardigrada were differentiated from Eutardigrada for the presence of high concentration CaCO3 encrustations in the buccal tube. Within Eutardigrada, Apochela differs from Parachela since they are characterized by the absence of CaCO3 in the buccal tube.