Learn more about the physiology of rainbow trouts
1. The integumentary system Skin and Appendages The skin of the Trout has two major roles, waterproofing the fish and protecting it using the armour plating of the scales. This function of keeping the water out, and fish tissue-fluids in, is performed by the epidermis. This is a very delicate clear covering which is draped over the scales, and possesses tiny glands, the goblet cells. These increase in number when the fish is stressed, and help to secrete the mucus, a protective coating of thick, infection-resistant fluid. Usually it is clear but sometimes skin parasite infections cause the epidermis to secrete a thicker, more viscid mucus, which gives it a bluish tinge.
Beneath the epidermis lie the scales - ovoid plates of bony material which are formed in small pockets, or scale beds. Scales develop on salmonids at the fry stage, and once a fish has its full quota of scales, it does not develop more as it increases in size. Consequently, the scales must grow in step with the fish. Scales grow by accumulation of material round their edges, laid down in the form of concentric rings. When a fish is growing rapidly these rings are spaced far apart so that in the summer (or in the sea) the distance between rings is much greater than in winter. At spawning time, the salmonids do not feed, and in order to obtain enough calcium for eggs or sperm they withdraw calcium from the outermost scale rings. This results in permanent scarring of the scale at that place and by examination of scales, you can assess the age, number of spawning's, and even the fish's size at the end of each year of life. Occasionally a scale may be damaged, and a new scale grows in the scale pocket. This scale cannot recapture the previous life history of the fish and that area of the new scale is therefore blank.
Over the scales are the pigment cells -. melanophores (black cells), iridophores (silver cells), and xanthophores (yellow and red). The black cells are under both nervous and hormonal (ie chemical) control. When fish are on a dark background, they emphasize their black pigment cells, and on a light background, the silver cells are more obvious. When they are depressed due to disease, fish frequently become darker in colour.
The strength of the skin is in the dermis - the layer below the scales. This is a very fibrous layer with considerable tensile strength.
2. The muscular system Muscles and Bones Most of the fish muscle, the muscle which we eat, is known as 'white muscle.' This is a very powerful type of muscle, used principally for escape, or pursuit of prey. Red muscle, which is like the muscle of higher animals, is only found along the lateral line and in some specialized sites such as the base of the fins, the eye and gills.
The main swimming muscles of salmonids are arranged in a series of blocks of white muscle or myotomes. This gives them considerable driving force on the tail. The myotomes are attached to the spine, the central bone, which is very flexible. The fins are moved by small independent red muscles. In addition to the usual fins, the salmonid fish are all characterized by a small appendage on the back, just in front of the tail, which is known as the adipose fin because of its fatty internal structure.
The skeleton of the young fish, as with other animals, is formed of cartilage, which becomes calcified later and can have an important influence on certain diseases.
3. The respiratory system Gills Fish breathe by means of gills, a system of four sets of very fine flattened capillaries or tubes, on either side of the throat, through which the blood flows, and over which water is continually passed. In passing through the gills, the blood gives up its carbon dioxide to the water, and obtains oxygen from the water, through the gill wall.
The respiratory surfaces of the gills, the secondary lamellae, must be very delicate so that the oxygen and carbon dioxide can be readily exchanged. They also contain mucus-producing cells and cells which excrete any excess salt from the blood as it passes through them, and they also excrete ammonia. Such a delicate structure on the outside of the body is highly vulnerable to injury via the water. The gills are protected on the outside by a bony shield called the operculum, and on the inside of the throat, they have a set of comb-like structures called gill rakers, which help to guide the food down the gullet rather than over the gills.
4. The circulatory system Heart and Blood vessels. The circulatory system is the blood-transport system of the fish. The pump in the system is the heart, a muscular organ occupying the area at the base of the throat. It is a two-chambered pump (lacking the auxiliary pump for taking blood to the lungs, which is a feature of man and the higher animals). The blood passes from the triangular, very muscular ventricle, which provides the main pressure, into the white, elastic-walled bulbus arteriosus. This is an elastic pressure-balance, converting the pumping of the heart into a steady surge of blood to the gills, from where it passes to the rest of the body to deliver oxygen to the tissues.
Once it has passed through the gills its pressure is much reduced and its passage through the tissues is relatively slow. In the fine circulatory network of the tissues, the capillaries, the oxygen of the blood is replaced by carbon dioxide and waste products.
The blood then returns via the vena cava or great vein, passing through the kidney on its way back to the heart. As the blood passes through the capillaries, some fluid, known as lymph, is lost to the tissues. This is the watery fluid which runs from a fresh fillet of fish. The lymph is returned to the circulation by a separate set of vessels, the lymphatics, which return it to the bloodstream just before the heart.
5. The digestive system Mouth, Stomach, Intestine and Associated glands This system is a relatively simple tube in the salmonids. It starts at the mouth, where the teeth are designed for capture not chewing. When ingested the food is quickly passed down the gullet or oesophagus to the stomach, a U-shaped organ which can expand greatly to take large meals. It is in the stomach that the food is really chewed, ie it is broken down by the action of acid and digestive enzymes as well as the crushing contractions of the muscles in the wall of the stomach. At the posterior end of the stomach where it joins the small intestine, there is a group of blind-ending sacs, the pyloric caeca. These usually number 30 to 80 in Salmonids, and they lie conspicuously across the stomach when the fish is opened. They are covered with a considerable amount of white fatty tissue unless the fish has been starved.
From the stomach, food passes through a one-way valve, the pylorus, to the intestine, where the disintegrated food is acted upon by further enzymes. These break down the food to its constituent sugars, fats, and amino acids (from proteins), which then pass into the bloodstream of the intestinal wall for transport to the liver. The remaining food - roughage, snail shells, etc, travels on to the large intestine and is voided as faeces.
Associated with the digestive tract are two very important glands. One, the liver, is a large organ situated just in front of the stomach. It is a pinky-brown colour, soft, and easily ruptured. The liver is the main factory of the body, to which food molecules are taken in the blood from the intestine, for manufacture into the proteins, carbohydrates and fats of the fish's body. Inserted in the top of the liver is a small greenish sac - the gall bladder. When incised, this usually releases a greenish fluid called bile, which under normal conditions passes to the intestine through the bile duct, and aids with food breakdown.
Because of its importance in food metabolism, disease of the liver is very significant. The most common liver abnormalities are excessive infiltration by unsuitable dietary fats and parasites. Parasites are also frequently found in the gall bladder.
The other important associated digestive gland is the pancreas. This is a very diffuse structure which cannot be seen with the naked eye as it is scattered throughout the fat surrounding the pyloric caeca. The pancreas has two functions, the production of pancreatic enzymes, which pass via the pancreatic duct to the intestine, and the production of insulin, which controls sugar and protein metabolism and prevents fish from becoming diabetic. The pancreas is very significant in viral diseases, because it is a favourite site of multiplication for two of the most important salmonid viruses.
Many species of fish including salmonids possess a swim bladder, which is a hydrostatic organ used to trim buoyancy at the appropriate depth. The swim bladder may also have a function as a hollow organ for receiving deep, low-frequency sounds. In the salmonids, it has a connection with the back of the throat so that the fish can quickly squeeze out air and drop to the bottom. Any blockage of this duct, or damage to the swim bladder wall, can result in considerable swimming problems for the fish.
6. The excretory system Kidney and Bladder The kidney is the main filter of the body. It filters blood through a sieve-like apparatus called the glomerulus and passes it through tubes to paired ducts, the ureters, which carry it to the bladder. In salmonids this is a small thin-walled structure above the anus. The duct from the bladder drains via the urogenital opening, which is also the exit for eggs.
The kidney of the Trout is a long black structure in the top of the abdomen, extending from the back of the head to the vent. The vena cava runs through the centre of the kidney and on its outer surface may be seen the narrow white ureters, wending towards the bladder. In higher animals the kidney is purely a selective filter, but in the fishes, it also contains the haemopoietic tissue, especially at the front end of the kidney. This is the tissue that makes the oxygen-carrying red blood cells, and defensive white blood cells, and also stores them until they are needed. The other site where this takes place is the spleen, a large black organ attached to the wall of the intestine. This haemopoietic tissue of kidney and spleen is very important in disease as it is affected by a number of serious bacterial and viral agents. It also contains a network of traps, the fixed macrophage cells, which catch any microbes passing through the blood stream and usually succeed in destroying them.
7. The reproductive system Ovaries and Testes The gonads of Trout comprise paired ovaries in the female and testes in the male. In the immature or resting state they lie in the anterior of the abdomen, above and on either side of the stomach. At sexual maturity, under the control of the hormones from the pituitary gland, they develop to extend the full length of the abdomen.
The ovary consists of germinal cells, some of which grow to the size of a pea to form the orange-coloured ova or eggs. Others stay small as the cells for subsequent spawning.
At spawning, the skin of both male and female becomes thicker and shinier, and the urogenital opening swells up. Eggs are released into the abdomen as the supporting capsule ruptures and are pushed on a tide of fluid to the urogenital opening by contractions of the female's abdominal muscles and by small sweeping hair-like cilia inserted in certain parts of the lower abdominal wall. Semen, known as 'milt‘, in fish, is excreted from the testes by bodily contraction and passes into the water as a cloud of living, wriggling sperm cells. In the wild, this occurs in the redd (nest) prepared by the female, but in the hatchery, this process can be done artificially.
8. The nervous system Brain, Spinal cord and Nerves The nervous system of salmonid fish reflects their behaviour. Salmon home on their sense of smell, hunt with their eyes, and are creatures almost entirely of reflex. Consequently, they have a well-developed olfactory area at the front of the brain, which connects directly with the nostrils. These are paired sacs on the snout with a continual flow of water around them, which is completely clear in normal fish, but becomes cloudy in certain disease conditions.
The eye is also one of the sites with extremely delicate blood vessels and is therefore very vulnerable to rupture of capillaries, eg by gas bubbles in certain circumstances. The ears of salmonid fish do not have an outlet and although they may detect some vibrations, their main function is balance. They are located within the skull, just behind the eyes, and when they are damaged by disease the fish is unable to balance properly.
9. The endocrine system Pituitary, Adrenal, and Other hormone-producing glands The endocrine glands are small groups of cells which have a significance for the body way beyond their size. They secrete chemicals into the bloodstream, hormones, which act on distant sites such as the gonads, skin or blood vessels. The most important endocrine gland, the pituitary, which has been called the 'conductor of the endocrine orchestra,' is in a very secure site below the brain.
The adrenal, or inter-renal as it is often called in fish, is a gland producing several important hormones including the fear hormone, adrenaline. It is located within the haemopoietic tissue at the anterior end of the kidney. The thyroid gland, producing growth hormone, is elusive in the salmonids, often scattered randomly around the tissues of the throat area. Salmonids and other fish also have two endocrine structures with as yet unknown functions.
These are the Corpuscles of Stannius, small white spots, placed laterally in the mid-kidney tissue (usually three or four can be seen on the surface of the kidney) and the pseudobranch, a red, vestigial gill-like structure on the inside face of each operculum. The pseudobranch has been associated with an endocrine hormone function but recent evidence suggests it is more likely to be associated with control of oxygen and carbon dioxide levels in the blood.
There are three other endocrine glands of fishes, which do not have equivalents in higher animals. These are the ultimobranchial glands concerned with calcium metabolism, the urophysis, which is a swelling near the end of the spinal cord, and the pineal, at the top of the head, which is thought to be light-sensitive and associated with pigment cell control. The ovary and testis also have an endocrine function, producing sex hormones.
