A. Digestive System
Though most fish will handle some amount of carbohydrates, they develop signs of ill health if there is a high concentration of carbs in their diet. Protein and fish Protein is the single most important nutrient that the fish needs to grow. Digestive system of a Carnivorous Fish. Such feeds can be crushed to enable easy ingestion. As in all vertebrates, the pancreas has two digestive functions.
Fatty acids and fish
There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas , have cutting teeth for biting chunks out of their victims. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws , for scraping plant and animal growth from rocks. Many fishes such as the Cyprinidae or minnows have no jaw teeth at all but have very strong throat teeth.
Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods gill rakers anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber. Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach.
The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form. Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas.
The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca blind sacs occur in some fishes at this junction and have a digestive or absorptive function or both.
It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency , and a spiral valve is one method of increasing its absorption surface.
Sharks, rays, chimaeras , lungfishes , surviving chondrosteans , holosteans , and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi fingerlike projections somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.
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