Gut glycoconjugates in Sparus aurata L. (Pisces, Teleostei). A comparative histochemical study in larval and adult ages.

This study examined the gut of the euryaline fish Sparus aurata, from the pharynx to the rectum. The specimens were collected from adult animals, both sexes, and several larval and juvenile stages, from 4 to 135 days of age. Histochemical methods of distinguish neutral and acidic glycoconjugates, as well as specific techniques to identify acidic glycoconjugates which contained O-acylated sialic acids were used. The presence and distribution of sugar residues in the oligosaccharide side chain of glycoconjugates were investigated with the use of biotinylated lectins. The pharynx and oesophagus of adult fishes showed the presence of abundant secretory cells which synthesized a large quantity of neutral, as well as sulphated and sialylated glycoconjugates, with different cellular combinations of them in the proximal and distal tract. This may be related to the complex functions carried out by this end of the gut in a marine euryaline fish. Epithelial secretory cells were found in the developing oesophagus during larval life (14 days) earlier than in the stomach and intestine (34 days). The simple columnar epithelium that lined the gastric mucosa of adult fish synthesized a mixture of neutral and acidic glycoconjugates, whereas during larval life it was shown to contain neutral glycoconjugates only. The intestinal goblet cells were shown to secrete both neutral and acidic glycoconjugates, especially sulphated forms. The adherent mucus gel of the gastric and intestinal mucosa contained many sugar residues, as revealed by lectin histochemistry. This work clearly demonstrates that the quality of gut mucosubstances varies in different ages and in regions of the fish alimentary canal. This is possibly caused by changes in environmental conditions and may in turn sustain functional alterations of the digestive apparatus.