Ultrastructure of the digestive tract of Gyliauchen nahaensis (Platyhelminthes, Digenea), an inhabitant of the hindgut of herbivorous fishes

Digenean parasites of vertebrates usually amplify the surface area of their gut by increasing the size of the absorptive caeca. Some members of the fa mily Gyliauchenidae, however, have relatively small caeca but have a greatl y expanded foregut. The morphology of the elongate gut of the digenean Gyli auchen nahaensis, an inhabitant of herbivorous fish of the family Siganidae , was examined by light and transmission electron microscopy. The extensive foregut, consisting of a mouth, pharynx, and esophagus, is lined with a sy ncytial tegument-like lining, which is connected to nucleated cell bodies s unken in the parenchyma. The apical cytoplasm in the mouth and anterior reg ions of the pharynx resembles that of the general body tegument, although s ome regional specialization is present. The lining of posterior regions of the pharynx is armed with large apical projections, which are thought to se rve as filtration structures. The lining of the anterior and middle esophag us displays a peculiar form of surface amplification involving the formatio n of elongate flask-shaped invaginations of the apical cytoplasm. The cell bodies associated with these regions are rich in secretory vesicles and it is proposed that these regions of the esophagus are expanded to promote ext racellular digestion. The posterior region of the esophagus lacks the invag inations of other esophageal regions, but displays instead large surface pr ojections. The caeca consists of columnar cells lined by extensive apical m icrolamellae. The peculiar gut morphology of G. nahaensis, coupled with alt erations in the arrangement of suckers, is interpreted to be an adaptation to the predominantly herbivorous diets of the definitive hosts. (C) 2000 Wi ley-Liss, Inc.