Digestive system of the marine blood fluke, Aporocotyle simplex (Odhner, 1900) (Digenea: Aporocotylidae) with consideration of the digenean digestive morphology

This is the first study describing the ultrastructure of the digestive system of a member of the marine teleostinfecting aporocotylid lineage, Aporocotyle simplex (Odhner, 1900), a blood parasite of the long rough dab Hippoglossoides platessoides (Fabricus, 1780) from the Norwegian Sea, Norway. The presence of particular morphological characters of the extensive A. simplex foregut and the four caecal ramifications are described. These show a syncytiality of the digestive epithelial lining with sunken perikarya in the foregut region and intraepithelial nuclei in the caecal region. Based on characteristic features of the pharyngeal muscle patterns in the Digenea, our results support the occurrence of a rudimentary pharynx in A. simplex. The short anterior esophageal portion retains tegumental cytoplasmic inclusions, but the middle-posterior esophageal portion shows considerable morphological modifications of the esophageal cytoplasm and sunken bodies modified into well-developed esophageal glands. These consist of undifferentiated cells, cells in different stages of differentiation and disintegrated cells, showing their renovation and apoptosis, controlled by the nervous system. Our data provide morphological evidence on the digestion of host erythrocytes within the lumen of the middle-posterior esophageal region. Morphological data indicated also the existence of the gastrodermal pathway, completing the digestive process. The revealed differences in the pharyngeal foregut region and in the caecal cytomorphology in marine A. simplex and the previously studied freshwater Sanguinicola inermis may support molecular data on the existence of divergent evolutionary lineages of marine and freshwater teleost-infecting species, or new data assigning Sanguinicola and Aporocotyle as separate families.