MORPHO-FUNCTIONAL CHARACTERISTICS OF THE DIGESTIVE SYSTEMS OF RAMAZZOTTIUS TRIBULOSUS AND MACROBIOTUS RICHTERSI (EUTARDIGRADA)

The morphology of the bucco-pharyngeal apparatus is correlated with the food utilized. Nevertheless, even though the ultrastructure of the digestive system of tardigrades was already described in some species, it was not studied in depth, and not in relationship to the feeding. Therefore, comparative utrastructural analyses of the digestive system of the phytophagous Ramazzottius tribulosus and the zoophagous Macrobiotus richtersi were made.In R. tribulosus within the cuticle of the buccal ring there are distinct transverse pillar-like structures. Posterior to the mouth, the cuticle of the buccal tube becomes electron-dense and compact, without pillar-like structures and vacuoles, even though holes or groups of holes are sometime present anterior to the stylet supports. At each side of the buccal tube there are thin processes of the salivary glands, which form a large reservoir where secreted mucous is accumulated, like in the salivary sac of arthropods. Under the reservoir wall there are thin muscle processes and neuromuscular contacts. Stylets and stylet supports are located in the salivary reservoirs. Each stylet has a basal cuticular part and an apical hard calcium part divided by a well visible border. The beginning of the pharynx is characterized by thin valves and cuticular apophyses; the latter are homogenous ultrastructure so as the posterior part of the buccal tube and placoids. The lumen of the pharynx is limited by a clearly tri-layered thin cuticle. At the transition from pharynx to oesophagus there is one valve formed by folds of the gut which hinder the food return. The increase of midgut surface is realized by both large folds of the gut wall and microvilli. Microvilli are relatively short and thick but have well developed glycocalyx and in cross section look disposed regularly forming very compact hexagons (111 per μm2). Epithelial cells of the midgut have large and small vacuoles. In tardigrades fixed after feeding, the midgut lumen exhibits a perithrophic membrane. In the cross section the lumen of the rectum looks as a big heart-like cavity with some narrow cell evaginations. The lumen is lined by a thin cuticle.For M. richtersi, at this moment we have considered only the ultrastructure of midgut and hindgut. Microvilli of the midgut are shorter and thinner than those of R. tribulosus. They form less dense hexagons (73 per μm2). Glycocalyx is present but not particularly developed. The ultrastructure of the midgut (epithelial cells, peritrophic membrane) and that of the rectum, is similar to what is found in R. tribulosus. Comparing these results among them and with published data on the zoophagous Isohypsibius prosostomus, several similarities has been found, such as valves, cuticle lining, ultrastructure of epithelial cells, and peritrophic membrane. They can be related to common feeding processes. The presence of peritrophic membrane indicates that in tardigrades extracellular digestion occurs. Nevertheless, we can identify differences that can be related to the food utilized. Mainly, there is more absorption surface of the midgut in the phytophagous R. tribulosus than in zoophagous M. richtersi and I. prosostomus. Investigations supported by INTAS Grant № 04-83-3807 to A.A.