Ultrastructure of the digestive system of Ramazzottius tribulosus and Macrobiotus richtersi (Eutardigrada) in relationship with diet

The ultrastructure of the digestive system of tardigrades was already described in some species, but it has never been studied in relationship to the diet. Therefore, utrastructural analyses of the midgut and hindgut of the phytophagous Ramazzottius tribulosus and the zoophagous Macrobiotus richtersi have been made. In addition, the foregut of R. tribulosus has been also analyzed. For this part new ultrastructural details have been evidenced. Among them, distinct transverse pillar-like structures, lacking in the electron-dense and compact cuticle of the buccal tube; a hole or groups of holes sometime present in the buccal tube; a large cavity within each salivary glands, where secreted mucus is accumulated; one valve made up by folds of the pharynx and located at the transition from pharynx to esophagus, already found in the zoophagous Isohypsibius prosostomus. In both analyzed species the increase of midgut surface is realized by two orders of folds of the gut wall and by microvilli. In R. tribulosus there are many first-order folds and few second-order fold, whereas in M. richtersi we found an opposite pattern. A perithrophic membrane and microvilli with a well developed glycocalix are found only in the midgut lumen of R. tribulosus. The density of microvilli and the ratio between the real surface with microvilli and the hypothetic surface without microvilli is lower in the zoophagous M. richtersi and I. prosostomus than in the phytophagous R. tribulosus. All these data represent an indirect indication of differences in digestion physiology between phytophagous and zoophagous tardigrade species. The shape of the hindgut is similar in both species and in particular the lumen of the hindgut looks as a heart-like cavity with some narrow cell evaginations.