Digestive System

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The anatomy and function of the digestive system in fish is basically the same as in other vertebrates. It is made up of a combination of mechanical and chemical processes. Food is ingested, broken down, and either absorbed into the blood or remains in the gut and is eventually expelled as feces. Differences arise between fish species in feeding habits, and the environment may influence the presence, position, shape, and size of a particular organ. Fish may be divided into three categories according to the food they eat. Herbivores are specialized and eat detritus and plants. Omnivores eat both plants and animal material, and carnivores feed on larger invertebrates and other fish.

Digestive Organs

The oral cavity is made up of lips, mouth, teeth, and tongue. Fleshy lips are common to herbivores, designed for grazing and straining. Carnivore lips are generally unmodified and thin. Bottom feeders often have barbels, sensory appendages that help guide them when feeding and grazing.

Mouths are large or tubular. Large mouths are usually associated with predatory fish, such as a barracuda, whereas tubular mouths such as that of a seahorse, are better adapted for plankton eaters due to the enhanced ability to suck water and food. Mouth position can be variable depending on the types of feed normally consumed. Bottom feeders have mouths on the ventral surface, surface feeders have a dorsal mouth and bony fish (teleosts) have a terminal mouth.

Teeth are thought to have originally arisen from scales covering the lips. Fish generally have small teeth for gripping and capturing prey. There are four possible types of teeth present in fish, also depending on feeding habits: (1) jaw teeth, (2) palate teeth, (3) tongue teeth, and (4) throat teeth. There is a strong correlation between dentition, feeding habits, and food eaten. Jaw teeth are more developed in carnivores, but they are poorly developed or absent in herbivores, which usually have well-developed throat teeth attached to the gill arches.

The tongue of a fish is very rudimentary. It has no salivary glands but does bear sensory taste buds. Any lubricant fluid in the mouth originates from mucous cells scattered throughout the inner lining, not from the tongue. Plankton eaters also have well-developed gill rakers. This is a sieve-type apparatus attached to the inner edges of the gill arches and serves to filter out phytoplankton, crustaceans, and, in some cases, even diatoms.

A short, broad, and muscular esophagus connects the mouth and elastic stomach. It functions in the transport and taste of food. The musculature of the esophagus tends to be more developed in freshwater fish than in saltwater fish, as it plays a role in minimizing water intake while ingesting food.

The stomach is muscular and motile. The inner surface is lined with gastric glands that secrete digestive enzymes, necessary for the breakdown of food. The size of the stomach is related to the size of food particles and the interval between meals, ie, large food size and infrequent feeding are usually associated with large stomachs, and smaller food particles ingested more constantly are associated with fish with small or no stomachs. The pyloric valve, a ring of muscle, controls the passage of food between the stomach and intestine.

The intestine is a relatively simple tube that has digestive glands and an abundant supply of blood vessels. There are two valves, one at each end. The pyloric valve is at the anterior end and the ilioceacal valve is at the posterior end. Here is where much of the absorption of nutrients takes place. Digestive products are picked up in solution. Intes

Anterior

Snout

Dorsal i

I ^ Dorsalfin

Posterior

Anterior

Snout

I ^ Dorsalfin

Posterior

Caudal fin

Gill cover

Pelvic fin

Ventral

Caudal fin

Gill cover

Pelvic fin

Ventral

Figure 1. External anatomy.

Spinal Swim cord bladder

Kidney Vertebrae

Spinal Swim cord bladder

Kidney Vertebrae

Stomach

Red muscle

White muscle

Heart Gall bladder Liver

Stomach

Red muscle

White muscle

Figure 2. Internal auditing and cross section of trunk.

tine length varies with feeding habits: carnivores have short straight intestines, herbivores have long and coiled intestines. Gut length seems more correlated to the amount of indigestible material in the food rather than the nature of the food. A large surface area is realized by extensive infoldings and ridges. The intestine leads to the rectum where vascularization and secretory cells are sparse, but where more mucous cells are present. The anal opening is called a vent and is also the terminal location for the urinary and reproductive ducts. While the common collection of these ducts suggest a corollary with the avian cloaca, it is not the case. The three ducts in fish all empty to the external environment and not to a common internal chamber. Absorption of plant material is probably poor; probably about 20% or less. Hatchery diets for salmonids may be absorbed up to 80%.

Food Movement

Movement of food through the digestive system is aided by peristaltic waves of muscle contraction. Cilia also line the tract and are especially useful in stressful situations when peristalsis may become reduced or even stop. This suggests that peristalsis is under some nervous control. Mucous membranes in the mouth, esophagus, and stomach provide lubrication to aid the passage of the food along the digestive tract.

Associated Organs

The pyloric caeca is an important appendage that opens into the intestine. It has the same epithelium as the intestine and its purpose is to increase the effective surface area. It may have both an absorptive and digestive func tion. A two or more lobed liver, which serves several metabolic and energetic functions, also produces bile, which aids in digestion. The gallbladder, which opens into the intestine, stores excess bile, which is secreted when required. The pancreas, which is large and distinct in the elasmobranch but diffuse in teleosts, secretes insulin and glucagon in response to nutritional intake, but also secretes digestive enzymes into the intestine. In teleosts, small beads of pancreatic tissue are scattered in the mes-entaries near the digestive tract and are supplied with artery, vein, nerve, and pancreatic ducts. Little is known about the pancreatic secretions and most knowledge is based on histological evidence.

Digestive Enzymes

While goblet cells secrete mucus, gland cells throughout the digestive system contribute digestive enzymes necessary for the breakdown of food. The stomach is lined with secretory cells (gastric glands) that secrete hydrochloric acid (HC1) and pepsinogen. These chemicals break down protein molecules into amino acids and are, therefore, especially important to carnivorous fish. The stomach is very acidic (pH 2-4). Pepsin activity is dependent on pH and temperature, whereas HC1 secretion is dependent on temperature and meal size. Optimum pepsin activity occurs at pH values of 2 and 4. Stomach distension stimulates gastric secretions.

Both the pyloric caeca and intestinal mucosa are sources of lipase, which breaks down fats into fatty acids and glycerine. Absorption of fat may occur in the anterior of the intestine. Intestinal secretions include proteases such as trypsin, lipase, and carbohydrase. These enzymes work best at neutral to alkaline pH. Bicarbonate is secreted (possibly from the pancreas) to raise the pH of the food coming from the stomach. Bile salts, secreted from the gallbladder may also play a role in adjusting the digestive juices to the proper pH to facilitate the digestive process. Bile salts are detergentlike substances formed from the decomposition of cholesterol and other steroids that function in fat emulsification aiding in the digestion of lipids. Fat soluble vitamins (A, D, E, K) are digested by bile. Digestive proteases are also secreted from the pancreatic tissue.

Most of the food absorption takes place in the intestine and pyloric caeca with the undigested material passing on to the rectum. Factors such as temperature, fat content, and presence of indigestible material affect the passage and digestion and absorption of food.

Innervation of Digestive Organs

Innervation of the digestive organs is not well understood, although it is thought that there is sympathetic innervation from paired ganglia, lateral to the spinal cord to the stomach, intestine, and rectum. This adrenergic system has an inhibitory effect. Parasympathetic innervation is through three cranial nerves to various parts of the digestive tract. Glossopharyngeal (IX) and facial (III) nerves innervate the mouth and esophagus areas. The vagus (X) innervates all visceral portions. These cranial nerves are cholinergic and have a stimulatory effect. Intrinsic nerves form a network inside the tissue of the digestive system. These nerves do not originate in the brain or the spine. They may assist in peristaltic movement. Intestinal and pancreatic secretions appear to be also under both nervous and hormonal control.

Feeding Types

Predators have well-developed grasping and holding teeth, ie, sharks, pike, and gar, and well-defined stomachs with strong acid secretions and short intestines (relative to herbivores). They depend on their senses of vision or the lateral line sensory organ to detect their prey. Grazers browse on plants and organisms and sometimes on each other. The trout is an example of this, and in crowded ponds they often nip at the fins of the other fish. Strainers usually have well-developed gill rakers that strain out the plankton and crustaceans in the water that flows through the mouth and gills. Suckers suck in their food. They have mouths on the ventral surface of their heads but adaptive lips. An example of this type of feeder is the sturgeon. Parasites live off the body fluids of other fish. An examples of this type of feeder is the lamprey.

Stimuli for Feeding

Internal motivation is driven by factors such as season and temperature, diurnal light, light intensity, and the time and nature of the last feeding. Fish have been known to feed better at dusk and early morning. Due to patterns of growth, fish also feed in spring and through the summer much more voraciously than in the winter. The hypothalamus is also thought to be involved especially where hunger and satiation triggers are concerned. Stimuli for feeding also comes through the senses, such as smell, taste, sight, and the lateral line system.

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