Abstract

Fish are an extremely diverse taxonomic group of animals. They display an extensive amount of variation in the complexity of their alimentary canal, which makes extrapolation of morphological and functional information from one species to another often impossible. Although tilapia are cultured worldwide for food, a detailed description of the normal morphological and histological features of the intestinal tract of this group of fish has not been reported in the literature. This study is the first to describe the normal gross anatomy of the intestinal tract in mature individuals of the Nile tilapia, Oreochromis niloticus.

Fish were spawned and raised to the adult stage, in-house, in a recirculation aquaculture system at optimal temperature and water qualities. When mature, specimens of adult male and female fish were fasted for 12 hours, anesthetized using tricaine methanesulfonate (MS-222, Sigma Chemical Co., St. Louis, MO) and killed by cervical separation. The right and left body wall covering the abdominal cavity of separate fish was removed and the internal organs examined. The in situ relation of the intestinal tract and its associated organs to the esophagus, stomach, and organs of other body systems in the body cavity was then examined.

From cranial to caudal, five principal regions were identified and sequentially designated as the hepatic loop, the proximal major coil, the gastric loop, the distal major coil, and the terminal segment. Except for the terminal segment, each of these areas were further subdivided into two separate regions. On exiting the stomach and progressing caudally, the sequence was as follows: proximal limb of the hepatic loop, distal limb of the hepatic loop, centripetal loop of the proximal major coil, centrifugal loop of the proximal major coil, proximal limb of the gastric loop, distal limb of the gastric loop, centripetal coil of the distal major coil, centrifugal coil of the distal major coil, and the terminal segment. The major coils were arranged in a cone-shaped mass, with each successive loop segment nested internal to the previous one. Aside from their topographical positioning, few external features distinguished one region of the gut tube from another. Internally, the intestinal mucosa was characterized by readily recognizable folds that varied depending on location within the gut. The intestinal looping described in this fish is both novel among patterns previously described in fish, and also among the more complex that have been described to date.