A comparative morphological study of the alimentary canal in three cold water pleuronectids

The aim of this investigation was to first determine, whether differences exist in alimentary canal histology between species known to inhabit different environments and to utilize different prey types in the wild and secondly to utilize this information to provide a morphological data base for future studies in digestive physiology and artificial diet development. Samples of each species were captured from wild populations and tissue was removed from five areas along the alimentary canal. Following sampling, tissue was processed for both light and electron microscopy. The existence of a distinct surface secretory cell (ESSC) in association with the esophageal mucosa in the winter flounder and the yellowtail flounder coupled with elaborate mucosal folding and a complex mucous histochemistry suggested that this region may have a role in the initiation of chemical digestion in these species. The stomach epithelia in the Atlantic halibut, the winter flounder and the yellowtail flounder, could be divided into three distinct zones based upon ultrastructural differences in the cell types present. Variation in mucous histochemistry across species as well as between regions suggested that different chemo-types of mucus may be important for different digestive functions. The post-gastric regions of the three pleuronectids could be divided into two main areas based upon the ultrastructural features of the digestion and absorption of specific nutrients: lipid digestion was observed to occur in the intestine and pyloric caeca whereas the intracellular digestion of exogenous protein was observed to occur in the rectum. Numbers of goblet cells within post-gastric regions were not found to be significantly different between species. Numbers were however, significantly different between regions, with a trend indicating an increase in the frequency of goblet cells toward the rectum. (Abstract shortened by UMI.).