The anatomy of the gastrointestinal tract of fishes follows the same basic plan as in other vertebrates with a degree of variation reflected in phylogeny and ontogeny, diet, and environment. Morphological studies provide us with a context for understanding the spatial organization and relationship of physiological and biochemical data, and the molecular machinery that is rapidly being elucidated through molecular techniques directed at the genome, transcriptome, and proteome. Morphological data are also key to understanding fish nutrition in ecology and aquaculture, and during development as well as mechanisms for physiological adaptations to a changing environment. A number of the multifunctional roles of the fish gut that are discussed in the following chapters (e.g. respiration and ion regulation) of this volume incorporate distinctive morphological features that will be highlighted in this chapter. The stomach represents a significant vertebrate innovation and is also the most highly diversified region of the gut, yet it has undergone a number of independent secondary losses with stomachless fishes accounting for approximately 20% of species. The diversity of fishes makes the identification of the causes of stomach loss elusive. An updated survey is given.

Morphological diversity of the gastrointestinal tract in fishes

Ontogeny of the Digestive Tract

The histology and characteristics of mucins secreted by epithelial mucous cells of the digestive tract in yellow catfish, Pelteobagrus fulvidraco were investigated using light microscope and transmission electron microscope. The digestive tract was divided into a pharynx, oesophagus, U-shaped stomach (with a cardiac, fundic and pyloric part) and intestine, composed of anterior intestine, middle intestine and posterior intestine, which consisted of a mucosa (epithelial layer), lamina propria-submucosa, muscularis and serosa. A large number of isolated longitudinal striated muscular bundles were present in the lamina propria-submucosa of pharynx. Goblet cells were observed throughout the digestive tract, except in the stomach. In the cardiac and fundic stomach, a plenty of gastric glands were observed, whereas they were absent in the pyloric part. Numerous mitochondria and endoplasmic reticulum were observed in the columnar epithelial cells of the intestine, especially of the anterior part. The epithelial mucous cells contained neutral or other two mixtures of acid and neutral mucins, the first being the most common. The neutral mucin was the only type of mucins in the stomach, anterior intestine and middle intestine. The results of this study will be helpful for understanding the digestive physiology and diagnosing some gastrointestinal diseases in yellow catfish.

Histology and mucin histochemistry of the digestive tract of yellow catfish, Pelteobagrus fulvidraco.

Brycon guatemalensis, a Neotropical characid fish, consumes an entirely terrestrial diet, shifting from eating insects as juveniles to fruits and leaves as adults. Juvenile and larger-sized fish collected in the Rio Puerto Viejo at the La Selva Biological Station in Costa Rica were studied to test the hypotheses that, with ontogeny, (1) relative gut length increases, (2) pyloric caeca arrangement and number remain unchanged and (3) pepsin, trypsin and lipase activities decrease, while α-amylase activity increases. These hypotheses were mainly supported in that larger fish had longer guts, unchanged pyloric caeca arrangement but fewer caeca, and, at both environmental and standard temperatures for the enzyme assays, lower pepsin and trypsin activities but higher α-amylase activities than the juveniles. Only lipase, among the digestive enzymes, exhibited the unexpected outcome of either not differing significantly in activity (per g of tissue) between juveniles and larger fish or being significantly higher (per mg of protein) in larger fish. The overall results support the view that B. guatemalensis is specialized morphologically and biochemically to function first as a carnivore and then as a herbivore during its life history.

Insectivore to frugivore: ontogenetic changes in gut morphology and digestive enzyme activity in the characid fish Brycon guatemalensis from Costa Rican rain forest streams

The histological and histochemical characteristics of the intestine and the ultrastructure of it's mucosa were studied in rainbow trout. The histological characterizations revealed that the wall of the intestine is composed of the tunica mucosa, tunica submucosa, tunica muscularis and tunica serosa. Intestinal mucosa displays many villi and a simple columnar epithelium in associated with goblet cells and intraepithelial lymphocytes. Muscularis mucosae and mucosal tubular glands were not seen. Eosinophilic granular cells (mast cells) observed along the intestine mainly beneath the stratum compactum, an acidophilic longitudinal layer. Histochemical analysis showed that the goblet cells have acidic and neutral mucosubstances. At the ultrastructural level, the columnar cells of the intestine have microvilli toward the lumen and are joined together at the apical surfaces by the junctional complexes. Lysosomes, rough endoplasmic reticulum and polymorphic mitochondria are scattered in the supranuclear cytoplasm of enterocytes. Many pinocytotic vesicles were observed in the enterocytes of the middle part of the intestine.

Histological, histochemical and ultrastructural study of the intestine of rainbow trout (Oncorhynchus mykiss).

The estimated mean intestinal coefficients of Leporinus friderici Bloch. 1794 and L. taeniofasciatus Britski, 1997, from the Upper Tocantins River, central-western Brazil, were 1.25 ± 0.15 and 1.14 ± 0.07, respectively. The external surfaces of the lips have taste buds. The histological structure is composed of four layers: mucosa, submucosa, muscular, and serosa. No muscularis mucosa is detected. The oesophageal epithelium is stratified in L. friderici and pseudostratified in L. taeniofasciatus, the latter with PAS + basal cells and eosinophilic apical cells. A thick layer of adipose tissue was found within the submucosa. Leporinus taeniofasciatus has a sphincter separating the cardiac and fundic portions of the stomach. In both species, gastric glands are present in the stomach, except in its pyloric portion. The pyloric caeca possess a simple columnar epithelium with brush border and three types of goblet cells. Rodlet cells were found in L. friderici. The three portions of the intestine showed no histological differences. Anatomical and histological features of the digestive tract are consistent with the omnivorous habit of both these species.

M. P. Albrecht

Papers

Anatomical features and histology of the digestive tract of two related neotropical omnivorous fishes (Characiformes; Anostomidae)