Subsections

• • Mouth and oesophagus/gullet
  • Stomach
  • The gut
  • Liver
  • Pancreas
  • Food decomposition and assimilation / digestion and absorption

The alimentary tract

Charr is considered to be an predator, parts of its diet consisting of invertebrates or other fish. The diet can be different depending on the maturation stage, size and stock. The alimentary tract bears witness to the predatory habits, suitably sharp teeth, to hold on to the prey, a defined stomach and the length of the alimentary tract is similar to the length of the fish.

Figure: The alimentary tract in charr.

 

The alimentary tract may be distinguished into mouth and mouth cavity, pharynx, oesophagus, stomach, pyloric caeca, rectum and anus.

Mouth and oesophagus/gullet

The mouth is wide, reaching towards the sides of the head, enabling the charr to take in its food at a perpendicular direction to itself, if not to large. The teeth are on the mandibles and tongue and turn into the mouth. In the mouth there are taste buds and cells producing mucus, that ease the movement of the food down into the stomach.

The gullet is short and thin but elastic and relatively large bits of food may enter it. No digestion takes place in the gullet, but slime production and cells removing salts are found there. There is a narrow duct from the gullet to the swim bladder in charr.

Stomach

The stomach of salmonids is an elongate J-shaped sack, strong, muscular and extremely elastic. A whole lot of food can be forced into it when needed. On the inside there are folds and creases increasing its surface area. The stomach empties food into the gut, in small amounts. The speed of the stomach's processing of the food depends on the rate of feed intake, the nutritive value of the feed, temperature and the size of the fish. The emptying of the stomach proceeds faster in smaller fish, at higher temperature and when the nutritive value is low.

The gut

The gut is between the stomach and the anus. At the front part of the gut, there are many appendici, or small pouches, so-called pyloric cacea, considerably increasing the inside surface area of the gut. Skúflangar are very prominent in fish that consume large amount of fat (e.g. predators such as salmonids and cod). Enzymes from the pancreas and bile from the gall bladder reach the food in the skúflangar. They are therefore organs of food conversion or digestion and its assimilation, especially fat.

In the gut there are many folds to increase the surface area and the endothelial cells exude slime to ease the feed's movement through. There are also found many small digestive glands, exuding enzymes, especially at the front end. In salmonids, the back part of the gut is wider than the front part and is in many ways similar to the colon in land vertebrates. Although the greatest part of the nutritive substances is absorbed in the front part, the absorption continues in the lower intestine, where the fish extracts water from the gut's content. At the end of the gut there is an anular muscle, a strong circular muscle controlling the process of excretion.

Liver

The liver is a relatively large organ in fish as in other vertebrates. In salmonids it is one-lobed and red-brown in colour, often becoming a bit paler in farmed fish. The fish liver, as in mammals, is made up of many small hepatic lobules, commanding a great amount of blood flowthrough. The liver is a multi- functional organ, controlling blood sugar content, assimilation of fatty substances and storing of A- and D-vitamins. Production of many substances, such as urea, cholesterol and blood proteins takes place in the liver, as well as formation of blood cells. In addition, the liver functions as an organ of purification, scouring unnecessary as well as poisonous or harmful substances, even bacteria, from the blood stream.

The liver produces the bile, stored in the gall bladder. The bile ducts pass the hepatic lobules, collecting the gall or bile and passing it on to the gall bladder, which is a diverticulum on the common bile duct. When food enters the alimentary tract, the bile is released through the bile duct, which opens into the gut just below the pyloric caeca. The bile's function is mainly to break up fat into small globules (soap formation), but the bile also increases the acidity (lowers the pH) of the gut content.

Pancreas

In salmonids the pancreas is disconnected and hard to detect, often singular lumps or clusters of cells in the fatty tissue surrounding the pyloric caeca. The distribution and the number is often different, even within the same species or fish stock. The pancreas is a bifunctional gland, with an endocrine and exocrine portion. The exocrine portion is the open gland of the pancreas producing digestive enzymes and pancreatic juice, connected to the gut with ducts or ductules. The digestive enzymes, becoming active in the gut, split or break down proteins (trypsin and chymotrypsin), fat (lipases) and carbohydrates (amylases and chitinases). The endocrine portion, the islets of Langerhans, are small cell clusters scattered throughout the pancreas provided with a rich blood supply. The endocrine portion has (at least) two kinds of cells producing the hormones glucagon and insulin, which control the blood sugar equilibrium.

Food decomposition and assimilation / digestion and absorption

The digestion of food means to decompose the food that enters the alimentary tract and separate it into small units that can be absorbed into the blood stream or released as excrement or waste substance. The breaking down of the food components is accomplished by splitting large molecules into smaller ones. This is mostly carried out by enzymes and bile, which enhances the decomposing action, although it does not contain an enzyme. The enzymes work best at certain conditions, esp. regarding temperature and pH. In the alimentary tract the acidity differs so the enzymes are functional only at specific points in the tract. The mucus covering the inside of the alimentary tract hinders the digestive enzymes working on the endothelial cells.

In the stomach the churning and mixing of the food starts and its actual digestion. Small gastric glands or cells producing hydrochloric acid (HCl) and pepsinogen (an inactive form of the enzyme pepsin, which splits proteins) are found there. The stomach/gastric acid, which is formed when the gastric glands release H⁺ and Cl^-, has many functions. It activates the pepsin, kills most of the bacteria entering the alimentary canal and dissolves partly chalk- like material, such as bone and shells, in the food. Other enzymes, such as chitinase, lipases and amylase have been found in the stomach, but their activity there is uncertain.

When the food has reached the gut or intestine, the digestive enzymes from the glands in the endothelium of the gut wall and from the pancreas partake in the digestion process. Many enzymes are contained in the pancreatic juice as described above. Amylase (and other sugar-splitting enzymes) breaks carbohydrates (polysaccharides) into disaccharides, lipase splits triglycerides into monoglycerides, glycerol and free fatty acids and chitinase breaks down chitin, which is a common hard substance (actually a polysaccharide) in the connective tissue and supportive system of many invertebrates. Chitinase is also found in the stomach. Cellulose is a substance found as a building material in plant cell walls and is mostly indigestible for fish as for many other vertebrates. The limited digestion of cellulose in the fish takes place in the gut by the intestinal bacterial flora.

A few enzymes from the pancreas break down proteins. These are either exopeptidases (splitting amino acids from the end of a protein chain) or endopeptidases (splitting peptide bonds within the protein chain). The main enzymes are trypsin, chymotrypsin and carboxypeptidases. The pancreatic juice also contains bicarbonate, and together with the bile, it changes the pH of the gastric content from being acidic into basic. Thereby the pancreatic enzymes become active, being released into the gut in an inactive state. Aminopeptidases (and di- og tripeptidases (exopeptidases)) are produced in the glands of the gut walls and are active in protein digestion.

When absorbed, the food particles are removed from the alimentary canal into the blood stream. There they are distributed to cells using the material as units for synthesis and build-up of the organism or they are degraded and metabolized further into energy and wastes. A large share of the nutritive elements pass the liver though, which may process them and pass them on or put into storage. Most of the energy-rich metabolites, as well as water-soluble vitamins and salts, are actively absorbed. For this purpose there exist specific cell pumps or transport cells, but this kind of transport costs the organism energy (ATP- ases). The active transport may be in connection with passive diffusion transport of ions, where the ion pump, i.e. active transport mechanism, creates an osmotic differential over the cell walls that carries the molecules and the ions through. The substances may also be carried through the intestinal wall by specific channels or by carrying proteins. It is mostly peptides, amino acids and carbohydrates which are transported in this way. The cells may also to some extent carry whole protein molecules through themselves into the blood stream by covering them in small globules made of the cell wall material.

Substances soluble in fat may be carried inactively through the cell membrane. Free fatty acids and monoglycerides are packed into small fat globules, which can pass into the cell membrane of the endothelium. Short-chain fatty acids can move directly into the blood stream onto the liver. Larger molecules of fatty origin (triglycerides, cholesterol, phospholipids and fat-soluble vitamins) are packed into larger globules in the cells, from where they are transported into the blood stream.