Comparative anatomy of the digestive system of the flounder, Syacium papillosum and Syacium gunteri (Pleuronectiformes: Paralichthyidae)

Abstract

In order to understand the anatomical organization of the digestive system of the flounders Syacium papillosum and Syacium gunteri, adult specimens from both sexes for the two species were analysed. By means of dissections of the visceral skeleton, visceral musculature, and the digestive tract, the morphology of the digestive system of the species was found to be affected by the extreme flattening of the body, the asymmetry of the cephalic region, and the reduction in volume of the visceral cavity. The flattening of the cephalic region implies a certain degree of vertical expansion of the visceral skeleton and musculature. The asymmetry was more pronounced in the palatine bones series, mandibles, and in the adductor mandibulae and intermandibularis muscles. The reduction in size of the visceral cavity, due to the forward extension of the anal fin, also implies that the location of the posterior intestine and anus is advanced. The most noticeable characters of the digestive tract were the great size of the stomach, the presence of 4 pyloric caeca, and the intestinal valve. The liver was located on the left side of the visceral cavity. The sharpness of the mandibular and branchial teeth, the great development of the adductor mandibulae muscles and retractor dorsalis muscles, the great size of the stomach and the intestinal ratio correspond to species of carnivorous habits. The anatomical characters that differentiate the digestive system of the species under study are the presence of a small stomach caeca, the greater length of the inferior pyloric caeca and 8 gill rakers in the inferior branch of the first branchial arch in S. papillosum, and the absence of the stomach caeca, the shorter length of the inferior pyloric caeca and 7 gill rakers in the inferior branch of the first branchial arch in S. gunteri. The comparison of the digestive system of both species with other Paralichthyidae and a Bothidae, allows the recognition of the same morphological pattern. The digestive system of Achirus lineatus (Achiridae) markedly differs from that of the Paralichthyidae and Bothidae by the extravisceral character of 2 intestinal loops, while the one from Symphurus civitatium (Cynoglossidae) differs by the presence of the suspension of the esophagus from 3 precaudal vertebrae.