The structural components of a fish's digestive system include the mouth, teeth and gill rakers, esophagus, stomach, pylorus, pyloric caeca, pancreatic tissue (exocrine and endocrine), liver, gall bladder, intestine and anus. Not all components are present in all fish [ha!] A fish's digestive system is adapted to their food habits. In predatory (carnivorous) fishes, the mouth is usually large for engulfing prey whole, or in large chunks, and teeth are present on the jaws (e.g. maxillary and dentary) and tongue (e.g. glossyhyal) for grasping live prey (See Ch. III). Gill rakers are short in carnivorous fish and pharyngeal teeth are short and pointed for moving prey down the throat. In omnivorous and planktivorous fishes, the mouth is smaller and is usually devoid of teeth except for pharyngeal teeth that may be blunt and flat (molariform) for grinding or sharp and long for shredding. Gill rakers in these fish are typically fine to prevent the escape across the gills of small food particles.
The esophagus, which is lubricated by mucus, leads to the stomach. In carnivorous fish, the stomach is muscular and elastic for holding large prey items, while in omnivorous and planktivorous fishes the stomach, if present at all, is small because a more or less constant stream of small food particles can flow directly into the intestine. The pylorus is a sphincter that prevents premature movement of the food bolus out of the stomach. Around the pylorus, many fish have out-pocketings called pyloric caeca. The role of pyloric caeca in digestion has been widely speculated upon. Histologically, they are very similar to the intestine. Around the pyloric caeca (or in that same area in fish lacking them) is pancreatic tissue. In fish the pancreas is usually diffuse, not a discreet body. As in all vertebrates, the pancreas has two digestive functions. It is the source of: 1) exocrine secretion of digestive enzymes (proteases, lipases, and carbohydrases) into the intestine and 2) endocrine secretion of the hormones insulin and glucagon that act to lower and raise blood sugar.
The liver in fish produces bile which is stored in the gall bladder until a bolus passes the stomach, at which time the bile is expelled into the intestine. Bile contains waste products of liver activity which pass out of the fish in the feces. Bile has a digestive function in that it emulsifies lipids, greatly increasing their absorbtion in the intestine. The liver is key in the anabolism and catabolism of amino acids absorbed during digestion and is also the site of storage of food energy in the form of glycogen. Most absorbtion of nutrients occurs in the intestine. The large protein, fat, and starch molecules in food that have been broken into smaller molecules by gastric acid and digestive enzymes move by diffusion or active transport (ATPase pumps) into the network of capillaries surrounding the gut. The intestine is lined with finger-like out-pocketings (villi) that greatly increase the surface area for absorbtion. The intestinal wall, while not as active as the exocrine pancreas, can secrete digestive enzymes, as well. The alimentary canal is typically a short, S-shaped tube in carnivorous fish, but can be long and convoluted in omnivorous fish to aid in the absorbtion of less digestible plant material.
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