Feeding Habits and Digestive Physiology of Fishes PDF

FEEDING HABITS AND DIGESTIVE PHYSIOLOGY OF FISHES  Some predators feed upon small fishes or insects found at or near the water surface. This chapter provides basic information on the...

FEEDING HABITS AND DIGESTIVE PHYSIOLOGY OF FISHES  Some predators feed upon small fishes or insects found at or near the water surface. This chapter provides basic information on the feeding habits and  mainly use vision to hunt for prey, although sharks, eels, behavior, and physiology of fishes and crustaceans. The mechanisms and other predatory fishes that feed at night may also that control the movement and digestion of food, methods of rely on smell, taste, and lateral line sense organs to assessing digestibility of feed, factors affecting digestion and locate their prey. absorption of food nutrients, and feeding processes in fish are 2. Grazers discussed. An understanding of the feeding habits, feeding  feed on bottom organisms or planktons that are mechanisms, and the digestion and absorption processes can help selectively consumed. Some grazers feed on algae or fish farmers and nutritionists maximize the use of feed. The rate at nibble on coral reefs to eat polyps. which fish digest their food is of primary importance in determining  The actual taking of food is by bites while browsing feeding rates, frequency, and ration size. Knowledge of the digestive continuously. physiology of fish is also necessary for an effective feed formulation 3. Strainers and in choosing a proper feeding regime. This chapter aims to teach  filter organisms, mainly diatoms and crustaceans from the reader: the feeding habits and behavior of fishes and water. crustaceans; the structural adaptation in the anatomy of the  These fishes swim through rich plankton beds, filter the digestive tract; the various organs of the digestive systems of fishes water, and swallow the soup-like concentrate. and crustaceans and their functions; nutrient digestion and  normally have numerous, fine, and elongated gill rakers. absorption by fishes and the fate of digested and undigested food; 4. Suckers the factors that affect the rate of digestion and absorption; and the feeding process in fish.  suck in mud or food-containing material to obtain their food. Sometimes, food items are separated from the FEEDING HABITS AND BEHAVIOR OF FISHES sediments before being swallowed, although in some catfishes, food is ingested together with flocculent  refer to the process of the search for and ingestion of bottom deposits. food. 5. Parasites  also includes the manner and the stimuli for feeding.  such as lampreys and hagfishes, are very different from other finfishes in their behavior. They obtain nutrients by CLASSIFICATION OF FISH ACCORDING TO THEIR FOOD AND DIET: sucking body fluids of host fish.  Herbivores - those that feed exclusively on plant materials ANATOMY AND PHYSIOLOGY OF THE DIGESTIVE SYSTEM  Carnivores - those that feed exclusively on animal matter  Omnivores - those that derive their nutrients from both  In nature, there is a wide variety of food available on which fish plantsand animals and crustaceans depend.  Planktivores - those that feed on plankton, the microscopic  Fish adapt to their food differences by anatomic as well as plantand animal life in water including bacteria behavioral means. Thus, there are many differences in the  Detritivores - those that feed on decaying matter anatomy and physiology of digestion in fish.  There is a strong correlation between the anatomical structure Food availability = key factor in determining what the fish will eat. of the digestive tract and the feeding habits of the fish.  Herbivorous fish – have an accessory masticatory *Most fishes are highly adaptable in their feeding habits and utilize apparatus or other physiological adaptation to help in the most readily available foods. breaking down plant cell walls before the digestion process starts, and a long, thin gut to increase gut retention time and enhance digestion and absorption. (depends on fibrous foods (e.g. phytoplankton & macrophytes))  Carnivorous fish – have a relatively simple and short gut, with thick mucosa for absorption (consume meat & other more digestible feeds) A. FISHES Digestive system of fish:  Mouth  Esophagus  stomach, CLASSIFICATIONS OF FEEDING BEHAVIOR OF FISHES ACCORDING  pylorus TO THE MANNER OF FEEDING:  intestine  liver 1. Predators  pancreas  fishes that feed on macroscopic animals  either be constantly on the move, hunting and pursuing their prey, or lie-on-wait to catch prey that stray into their territory.   Mucus producing cells are present in the esophagus (The mucus is much thicker anteriorly than posteriorly and is electrically charged.)  esophagus serves only as a passage way, however, enzyme activity has been detected in the esophagus of some fishes indicating a more active role of the esophagus in the digestion process in these species.  Other fishes with long, slim body shapes (e.g. seawater-adapted eels) have a long esophagus.  Osmoregulation may take place in the esophagus if mucus is present.  Some reports have suggested that both passive and active transport of ions into the blood may also take place in the esophagus without addition of water such as the dilution of ingested seawater in freshwater eels. Stomach  An illustration of the digestive tract of four commonly cultured fishes that differ in their food.  vary greatly in their anatomical structure due to  The digestive tract is tubular in nature. adaptations to specific foods.  The whole digestive tract is often referred to as the gut and in  4 GENERAL CONFIGURATIONS OR SHAPES OF FISH fish. STOMACHS: a. a straight stomach with an enlarged portion 4 DIVISIONS OF FISH GUT (Headgut, Foregut, Midgut, b. a U- or J-shaped stomach Hindgut) c. a stomach shaped like a Y on its side where the stem faces the caudal portion 1. HEADGUT d. stomachless fish, such as in carps and other  is the most anterior part includes the mouth (oral or cyprinids buccal cavity), and gills (branchial or pharyngeal activity).  has a configuration or shape which is convenient for  Mouth and various ingestion mechanisms containing food in the shape in which it is ingested. o Ingestion – 1st phase of digestion into the mouth  Food is temporarily stored in the stomach while the o The MOUTH has a variety of adaptations for rest is gradually being processed through the other capturing, handling, and sorting of food before portion of the digestive tract. entry into the stomach.  The size or capacity of the stomach in relation to the  TEETH body weight varies between species and is usually - vary in type, number, and arrangement. related to the interval between feedings and to the - Serve to catch and hold the prey. size of food particles. - The arrangement and structure of the teeth are  Generally, fish that eat relatively small, soft particles related to the kind of food that the fish normally eat. have small stomach whereas fish that eat large food There is a strong correlation among kind of teeth, particles (e.g. whole fish, or eat at infrequent feeding habits, and food eaten. intervals have larger stomach). - active feeders have strong jaws with sharp teeth to  cecum of the Y-shaped stomach = is adapted to bite and shred the food. stretch posteriorly to accommodate large food - Some major kinds of jaw-teeth (cardiform, villiform, particles or prey. canine, incisor and molariform)  the absence of a stomach has been suggested to - Those feeding on mollusks and crustaceans have benefit fish adapted to freshwater (low chloride short heavy teeth, strongenough to crush the mollusk concentration) where stomach acids impose added shell. osmoregulatory pressure. This is to avoid acidifying - Zooplankton feeders and most Planktivores have large amounts of alkaline food, as in omnivorous fish practically no teeth. The shredding of food is most that eat plant sources, corals, shells, and others. often done in the throat or pharynx. Here, another  In milkfish, the stomach can be divided into cardiac set of specialized teeth may be found. Again, the and pyloric portion. structure, size and shape of the pharyngeal teeth are o cardiac portion – more enlarged while the also variable. pyloric stomach is highly muscular - Plankton feeders have fine rows of pharyngeal teeth o pyloric stomach - intensely grind the food - Mollusk eaters have large but flat crowned teeth, particles resulting in chyme (a paste like which is better adapted to crushing their food. mass).  The capacity or volume of the stomach in relation to 2. FOREGUT the body weight varies between species and reflects Esophagus the size of the meal that can be taken voluntarily (it  Most fish have short, wide esophagus that serves as a can vary from as small as 10% of body weight and as transitional area between the striated muscles of the large as 50% of body weight in a single feeding). mouth and the smooth muscles of the gut. 3. MIDGUT absorptive function, an increased secretion of mucus and a Intestine pH near neutral.  digestion process actively continues into the  Histological sections show a sudden change from columnar intestines after preliminary digestion in the stomach. secretory and absorptive to a squamous epithelium that  All fishes have intestines. produces mucus.  Intestine Length varies from = low as 1/5 to as high as 20 times the body length. 5. LIVER  some fish, the intestines may be short and straight  An important metabolic organ. while it can be long, folded, and looped in others.  It aids in digestion by secreting bile, a greenish fluid with  Herbivores have longer intestines than carnivores. strong emulsifying properties. (Although there are some cases of overlaps, some  The bile is stored in the gall bladder and is composed of a general statements can be made on gut length in mixture of bile salts, taurocholate, glycocholate. relation to feeding habits of fish).  Bile acids are derived from metabolism of of cholesterol,  Within the same fish species, the relative gut length and degradation products of hemoglobin, bilirubin and can change as feeding habit of the fish changes. Gut biliverdin. length is directly more related to the amount of  The bile duct opens into the anterior intestines or into the indigestible material in the food rather than whether pyloric caeca if present. the food source is of plant or animal origin.  Fish can reabsorb bile in the hindgut even though most  Fish that ingest large amounts of detritus have gut lipid uptake occurs in the anterior intestine. lengths similar to those of herbivores.  liver is also a storage organ for lipids and glycogen or stored starch.  In some fishes, large amounts of lipid is stored in the liver to help maintain buoyancy. In other fishes, glycogen is the major stored nutrient. 6. PANCREAS  involved in many important functions in digestion. Pyloric caeca  Pancreatic morphology is variable in many bony fishes.  In most fishes, unlike in land animals, there is no discrete  Some fishes possess pyloric caeca. pancreas. The pancreas is diffused, scattered, and  (Conflicting reports) Histologically, the pyloric caeca embedded in the mesenteries, in the liver, and clustered resembles the intestines. Most studies indicate that it around the bile duct, or in combinations of sites. In a serves as an extension of the intestines thus increasing the diffused pancreas, several small ducts open into the effective surface area for digestion and absorption. Other intestine and the pyloric caeca. studies show that it acts as an accessory food reservoir, for  In other cases where the pancreas is found inside the liver, temporary storage, possibly a devise for saving space. the pancreas delivers its secretions directly into the  Rainbow trout caeca takes up amino acids and sugars gallbladder. across the apical membrane of the epithelial cells.  The pancreas produces insulin and digestive secretions,  Electron microscopy has shown that both intestinal and principally proteases and bicarbonates. caecal cells are involved in lipid absorption, with caecal  Insulin stimulates uptake of amino acids from the intestine cells being more active. and may stimulate growth. In the northern pike, insulin  The structure of the absorptive cell of the intestines decreases blood amino acids with uptake into skeletal reflects its specialized function in digestion. It contains the muscles, while in cod, insulin decreases blood glucose. ff: - Many mitochondria (provide energy for metabolic processes) - endoplasmic reticulum (where proteins (including digestive enzymes) are assembled) - golgi bodies (where carbohydrate side chains are attached to proteins). - Tight junctions and desmosomes bind the absorptive cells into a single sheet on the surface of the mucosa. - nucleus lies deep in the cell. - The most striking feature of the cell is the presence of a brush border, a prominent structure on the surface facing the lumen of the intestine. The brush border is composed of minute projections called microvilli. - On the membrane of the microvillus are found several kinds of digestive enzymes and transport proteins. 4. HINDGUT  an extension of the midgut.  Digestion has been shown to continue in the hindgut although with a gradually diminishing digestive or