Form and function of the feeding apparatus in Eutardigrada (Tardigrada)

Tardigrade feeding apparatus is a complex structure with considerable taxonomic significance that can be schematically divided into four parts: buccal ring, buccal tube, stylet system, and pharynx. We analyzed the fine morphology and the tridimensional organization of the tardigrade buccal-pharyngeal apparatus in order to clarify the relationships between form and function and to identify new characters for systematic and phylogenetic studies. We conducted a comparative analysis of the cuticular structures of the buccal-pharyngeal apparatuses of twelve eutardigrade species, integrating data obtained by SEM and LM observations. Morphological diversity was observed and new cuticular structures such as the stylet coat of the stylet system were identified. The synthesis of the buccal-pharyngeal apparatus during molting was also analyzed obtaining a clear developmental sequence of its resynthesis. These findings lead us to redefine the previous interpretations of the functioning mechanisms of the buccal-pharyngeal apparatus and provide a more specific relationship between tardigrade diet and the anatomy of their feeding apparatuses. In addition, the detection by energy-dispersive X-ray spectroscopy of calcium in the stylets, buccal tube, and placoids of eutardigrade species (i.e., Milnesium tardigradum, Paramacrobiotus richtersi) indicates that CaCO3 incrustations are not an exclusive feature of heterotardigrades and lead to suppose that this trait was present in the ancestors of both classes.