THE FUNCTIONAL ANATOMY OF FEEDING
 AND DIGESTION IN FISHES.
By Jim E. Quarles
25-01-2000

While my primary interest is in discus fish the understanding of this subject applies to most fishes and their required feeding habits. The digestive system of a fish is quite simple when compared to that of higher vertebrates. However there is great variations between species. This involves the anatomy of the mouth parts which are frequently modified for unique food habits.

The predators have developed mouths that allows them to capture and swallow fairly large prey. This is generally done by swallowing the prey whole, and most often head first.

Some fish are cannibalistic. One such fish we know and love is the Oscar. It will eat anything it can catch and swallow. And in some cases die trying to ingest food far to large for its mouth to handle.

Some species have teeth others do not. Some use a slash and cut method such as the piranha. On the other hand the pacu which is closely related have developed teeth used to crush nuts and fruit. Regardless of how they capture and ingest food the process of digestion is the same only the finer details very from species to species. As I will explain some species have rather large guts that can contain large amounts of food for digestion, others, such as the discus and angel fish have very small gut areas and this in turn requires feeding far more often in small amounts at any given time. Ingested food passes from the stomach into the intestine, where it is processed further and nutrients are absorbed into the body. Bile salts produced by the liver are stored by the gallbladder and neutralize stomach acids and emulsify dietary fat. Certain dietary acids must be present in order for vitamins to be absorbed. So feeding extra vitamins without these acids should be avoided, as they can cause liver damage. The pancreas secretes enzymes that digest carbohydrates into the intestine.

FOOD NUTRIENTS AND THEIR FUNCTIONS.

Protein and Energy: Protein is the major nutrient required for growth of fish and on a dry weight basis, makes up most of the body structure. The proper protein nutrition cannot be overemphasized in making up a diet for fish of all types.

Proteins contain essential components such as amino acids which are used by fish to synthesize new body tissues and enzymes. Not all protein is of equal value in the body building process. Proteins vary in their ability to support growth, depending upon their source and processing. It has been found that seafood and fish meals provides the best combination of digestible amino acids. Remember feeds that have been dried at high temperature lose nutritional value, because the essential amino acids bind to other components and become unavailable to the fish.

Fish are super efficient at converting food to body tissues, so they need less food to grow than almost any other animal. Over feeding is not only a waste but leads to reduction in water quality as well. This seems to be one of the major problems with those that keep tropcial fish, that and over use of chemicals and drugs which are totally unnecessary in most applocations.

PROTEIN FOR FRY

Fish fry and larvae grow rapidly and require a very rich diet for maximum growth and survival. In the wild protein makes up at least 50 percent or more of their diets on a dry weight bases. High protein foods are important in the hatchery as well. The protein requirements of fish decreases as they gain in size. Young adults can be reared on foods containing 35 to 40 percent protein.

FATS AND ESSENTIAL FATTY ACIDS

Fats can supply energy for normal body needs, sparing proteins for growth. Carbohydrates can also serve this function. If a feed is well designed it will supply just enough energy for maximum growth without producing fatty fish. Tank reared fish are especially prone to fatness because they expend little energy searching for food.

CAROTENOID PIGMENTS

Fat-soluble carotenoid pigments ( carotenes and xanthophylis ) are responsible for the yellow, orange, red and green colors of the skin, flesh, or eggs of many fish. I have read mounting evidence that some of these pigments are important nutrients.

Survival of fertilized eggs to hatching has been shown to be related to the concentration of pigments in the embryonic yolk. And in the pre-spawning diet of the female fish. Carotenoid pigments may serve a protective function for delicate membranes and other sensitive tissues. Also bear in mind that a higher amount of vitamin E and other antioxidants are needed in the diet when carotenoid pigments are included.

One problem to keep in mind in any feeding of fish is that ocular cataracts, that can cause blindness in fish occurs when foods contain inappropriate levels of certain minerals, vitamins and amino acids. The discus hobbyist of today often complains of a white cloudy eye appearance occurs on their fish. In most cases they mistake this for a parasitic problem, and foolishly treat with chemicals or anti-biotics when such treatments are not required.

A well balanced diet using different substances such as brine shrimp, fresh-chopped fish, algae, beef heart well cleaned of fat and gristle or veins. Lots of vegetables added to prepared foods is always a good idea.

PROPER FOOD IS JUST AS IMPORTANT AS PROPER WATER CONDITIONS IN MAINTAINING HEALTH FISH.