Digestion in Teleost Fishes ( ANATOMY AND GENERAL PHYSIOLOGY OF THE GUT)

Functional Anatomy of the Gut

The mouth exhibits a variety of fascinating adaptations for capturing, holding and sorting food, ratcheting it into the oesophagus and otherwise manipulating it prior to entry into the stomach. Only two which have possible relevance to digestion Will be discussed.
In milkfish (Chanos), the gill cavity contains epibranchial (suprabranchia) organs dorsally on each side, consisting either of simple blind sacs or elaborate, spirally-coiled ducts. The organs occur in several relatively unrelated families of lower teleosts and apparently relate to the kind of food eaten. Those fish with simple ducts all eat macro-plankton and those with the larger ducts microplankton. Although their function is unknown, concentrating the plankton has been suggested as a possibility.
The common carp provides an excellent example of non-mandibular teeth being used as the primary chewing apparatus. Pharyngeal teeth occur in the most fully developed forms of the Cyprinidae and Cobitidae, although many other groups also show some degree of abrading or triturating ability with some part of the gill bars. In carp, the lower ends of the gill bars have a well developed musculature which operates two sets of interdigitating teeth so as to grind plants into small pieces before swallowing them. The grinding presumably increases the rather small proportion of plant cells which can otherwise be successfully attached by digestive enzymes.
Many fish which chew their food have some ability to secrete mucus at the same time and place. This would have some apparent benefit when ingesting abrasive food. Although one might be tempted to equate such secretions with saliva, enzyme activity in the mucus does not appear to have been demonstrated, so the mucus is only partly comparable to saliva.
The oesophagus, in most cases, is a short, broad, muscular passageway between the mouth and the stomach. Taste buds are usually present along with additional mucus cells. Freshwater fishes are reputed to have longer (stronger?) oesophageal muscles than marine fish, presumably because of the osmoregulatory advantage to be gained by squeezing out the greatest possible amount of water from their food (i.e., marine fish would be drinking seawater in addition to that ingested with their food and freshwater fish would have to excrete any excess water).
The oesophagus of eels (Anguilla) is an exception to this general pattern. It is relatively long, narrow, and serves during seawater residence to dilute ingested seawater before it reaches the stomach. A possible conflict between the osmoregulatory and digestive roles of the gut in marine fish in general will be discussed later (Section 3.5).
Fish stomachs may be classified into four general configurations. These include (a) a straight stomach with an enlarged lumen, as in Esox, (b) a U-shaped stomach with enlarged lumen as in Salmo, Coregonus, Clupea, (c) a stomach shaped like a Y on its side, i.e., the stem of the Y forms a caudally-directed caecum, as in Alosa, Anguilla, the true cods, and ocean perch, and (d) the absence of a stomach as in cyprinids, gobidids, cyprinodonts gobies, blennies, scarids and many others, some families of which only one genus lacks a stomach.
The particular advantage of any configuration seems to rest primarily with the stomach having a shape convenient for containing food in the shape in which it is ingested. Fish which eat mud or other small particles more or less continuously have need for only a small stomach, if any at all. The Y-shaped stomach, at the other extreme, seems particularly suited for holding large prey and can readily stretch posteriorly as needed with little disturbance to the attachments of mesenteries or other organs. Regardless of configuration, all stomachs probably function similarly by producing hydrochloric acid and the enzyme, pepsin.
The transport of food from the stomach into the midgut is controlled by a muscular sphincter, the pylorus. The control of the pylorus has not bean demonstrated in fish, but the best guess at this time is that it resembles that in higher vertebrates. The pylorus is developed to various degrees in different species for unknown reasons, in some species even being absent. In the latter case, the nearby muscles of the stomach wall take over this function, which may also include a grinding function by the roughened internal lining. In fish which lack a stomach, the pylorus is absent and the oesophageal sphincter serves to prevent regress of food from the intestine, i.e., in fish lacking a stomach and pylorus, the midgut attaches directly to the oesophagus.
The digestive processes of the midgut have not been studied extensively, except histo-chemically (see Section 4 for details on enzymes), but so far as known resemble the higher vertebrates. The midgut is mildly alkaline and contains enzymes from the pancreas and the intestinal wall, as well as bile from the liver. These enzymes attack all three classes of foods - proteins, lipids, and carbohydrates - although predators such as salmonids may be largely deficient in carbohydrases. The pyloric caecae attached to the anterior part of the midgut have attracted considerable attention because of their elaborate anatomy and their taxonomic significance. Histological examination has proved them to have the same structure and enzyme content as the upper midgut. Another suggestion was that pyloric caecae might contain bacteria which produce B-vitamins as in the rodent caecum. When tested, this hypothesis had no factual basis either. Pyloric caecae apparently represent a way to increase the surface area of the midgut and nothing more. This still leaves an interesting question of how food is moved into and out of the blind sacs which are often rather lone and slim: e.g., in salmonids.
The demarcation between midgut and hindgut is often minimal in terms of gross anatomy, but more readily differentiated histologically - most secretory cells are lacking in the hindgut except for mucus cells. The blood supply to the hindgut is usually comparable to that in the posterior midgut, so presumably absorption is continuing similarly as in the midgut. Formation of faeces and other hindgut functions appear to have been studied minimally, except histologically.

Peristalsis and its Control

Peristalsis consists of a travelling wave of contraction of the circular and longitudinal layers of muscle in the gut wall such that material inside the gut is moved along. The pharmacology of this system has been investigated in isolated trout intestine demonstrating that an intrinsic nerve network exists to control peristalsis; i.e., cholinergic drugs stimulated and adrenergic drugs inhibited peristaltic movements. The oesophagus arid stomach are also innervated extrinsically by branches of the vagal (cranial X) nerve. No studies appear to have been made so far concerning details of food transport through the teleost gut except for measurements of gastric evacuation time and total food passage time, although gut stasis has been hypothesized to occur in the Pacific salmon, as in domestic animals.

Gastric Evacuation Time and Related Studies

Many studies have been performed relating to developing an optimum feeding schedule, mostly for salmonids, but also including a number of other cultured fish. Variables considered with feeding rate and gastric evacuation time included temperature, season, activity, body size, gut capacity, satiety, and metabolic rate. A relatively consistent finding has been that gastric emptying rate declines more or less exponentially (sometimes linearly) with time. Larger meals first are often, but not always, digested at a faster rate than small meals and the amount of pepsin and acid produced was somewhat proportional to the degree of distension of the stomach. Stomach mobility often increases with the degree of stomach distension also. The appetite, digestion rate, and amount of secretions produced all decreased with decreased temperature, but the secretions also decreased if tested at temperatures in excess of the acclimation temperature. Appetite, i.e., the amount of food eaten voluntarily at one time, appears to be the inverse of stomach fullness, although this does not explain the entire appetite phenomenon. Appetite continues to increase for a number of days after the stomach is empty, indicating that additional metabolic or neural mechanisms are operating. Data on gastric emptying time, digestion rate, and temperature for sockeye salmon have been shown to reflect the underlying phenomenon. Direct comparison of data on digestion among different workers is difficult, because of differences in species, food and methods used.
The total time for passage of food through the gut until the non-digestible portions of a meal are voided as faeces has not commonly been measured. Gastric emptying time and total passage time in skipjack tuna at 23-26 C was about 12 hours with the intestine being maximally filled about five hours after eating and empty after about 14 hours. Defaecation often occurred 2-3 hours after a meal, presumably being material from a previous meal. After a single meal, faeces were found 24, 48 and even 96 hours after the meal. Thus, there is considerable variation in food passage time, presumably relating to the digestibility of the food. Magnuson (1969) commented that the passage rates in skipjack tuna were at least twice as fast as known for any other fish.
The obvious importance of food passage time becomes apparent when one wishes to analyze faeces resulting from ingestion of a specific meal. If one waits to feed a test meal until the gut is completely empty, then the digestion processes observed will be typical only of starved fish. If one feeds the test meal as part of a regular feeding programme, then the problem is to mark the food for appropriate faecal analysis. Thus the problem is not as simple as it might appear at first.

Digestion and Absorption

Digestion is the process by which ingested materials are reduced to molecules of small enough size or other appropriate characteristics for absorption, i.e., passage through the gut wall into the blood stream. This generally means that proteins are hydrolyzed to amino acids or to polypeptide chains of a few amino acids, digestible carbohydrates to simple sugars, and lipids to fatty acids and glycerol. Materials not absorbed are by definition indigestible and are eventually voided as faeces. Digestibility ranges from 100 percent for glucose to as little as 5 percent for raw starch or 5-15 percent for plant material containing mostly cellulose (plant fibre). Digestibility of most natural proteins and lipids ranges from 80 to 90 percent.
Digestion is a progressive process, beginning in the stomach and possibly not ending until food leaves the rectum as faeces. Most studies of digestion simply compare the protein, lipid and carbohydrate content of the faeces with that of the feed. A study on digestion in channel catfish by Smith and Lovell (1973) showed continuing digestion (and absorption) of protein during passage through each part of the gut The methods employed in this . The comparison of faeces collected from the rectum and from the water also points out the hazard of incomplete recovery of faecal matter being likely when collection is done from outside the gut. Most of the protein digestion occurred in the stomach, but also continued in the intestine.
Temperature and pH play major roles in determining the effectiveness of digestive enzymes as a whole (details for specific enzymes are given in Section 4 below). Although most enzyme production decreases at temperatures above or below acclimation temperature, most enzyme activity (for a given amount of enzyme) increases in proportion to the temperature over a wide range of temperatures.
In general, enzyme reaction rates continue to increase at higher temperatures, even though the temperatures increase beyond the lethal temperatures for the species, until the enzymes begin to denature around 50-60°C. On the other hand, enzymes have limited ranges of pH over which they function, often as little as 2 pH units. Data for channel catfish are probably representative of many teleosts. Acid concentrations (pH) in the stomach ranged from 2 to 4, then became alkaline (pH = 7-9) immediately below the pylorus, decreased slightly to a maximum of 8.6 in the upper intestine, and finally neared neutrality in the hindgut (Page et al., 1976). Fish having no stomach have no acid phase in digestion.
The site of secretion in teleost stomachs appears to be a single kind of cell which produces both HCl and enzyme(s). This contrasts with mammals where two types of cells occur, one for acid and one for enzymes. The production of acid in teleosts is presumably the same as in mammals - NaCl and H2CO3 react to produce NaHCO3 and HCl, with the blood being the source of both input materials, which are later mostly reabsorbed in the intestine. One possible explanation for the loss of stomachs in some species of fish is that they live in a chloride-poor environment and that providing large amounts of chloride ion for operating a stomach is bioenergetically disadvantageous. In addition to acid and enzymes, the stomach wall also secretes mucus to protect the stomach from being digested. As long as the rate of mucus production exceeds the rate at which it is washed and digested away, the gut wall is protected from being digested. When mucus production slows or fails, e.g. during gut stasis, during stressful conditions, or post mortem, the gut wall can be eroded or even perforated by the gut's own digestive enzymes.
Two sites produce enzymes in the midgut - the pancreas and the intestinal wall. The intestinal wall is folded or ridged in simple patterns which can be species specific. Secretory cells for both mucus and all three classes of enzymes develop in the depths of the folds, migrate to tops of the ridges (closest to the gut lumen), and then discharge their products. The pancreatic cells produce enzymes and an alkaline solution which are delivered to the upper midgut through the common bile duct. The control of pancreatic secretions (and the pyloric sphincter) in fish is probably the same as in mammals, but there is no information on teleosts yet.
The physical state of food passing through the gut varies with species and type of food. Fish, such as salmonids, which eat relatively large prey, reduce the prey in size layer by layer. Gastric digestion proceeds in a layer of mucus, acid, and enzyme wherever the stomach wall contacts the food. Food appears liquified only in the midgut and solidifies somewhat again during formation of faeces. Pellets of commercial feed seem to be treated similarly, i.e., pellets get smaller and smaller in size with time, although stomachs of some recently-fed salmonids have been found to contain moderate amounts of liquified pellets. Stomachs of juvenile Pacific salmon captured in the open sea contained a thick slurry of pieces of amphi-pods in various stages of solubilization. Fish whose food contains high levels of indigestible ballast, e.g., common carp feeding on a mixture of mud and plants, probably show minimal change in the appearance or volume of their food while it passes through the gut. Microphagous fish, such as the milkfish (Chanos) whose food starts out as a suspension of fine particles, probably also keep it in much the same form all the way through the gut. In general: there seems not to be the same degree of liquifaction of food in fish as is commonly described for mammals.
Absorption of soluble food could begin in the stomach - it occurs in mammals, but has not been investigated in fish - but takes places predominantly in the midgut and probably to some degree in the hindgut. The sites and mechanisms of absorption are largely unstudied, except histologically. Several histologists have identified fat droplets in intestinal epithelial cells following a lipid-rich meal. Increased numbers of leucocytes in general circulation following a meal by the sea bream and increased number of fat droplets in them have been described (Smirnova, 1966). It was hypothesized that leucocytes entered the gut lumen, absorbed lipid droplets, and then returned to the blood stream. It is clear that the mammalian type of villi with their lymph duct (lacteal) inside are absent in fish, although there is some folding and ridging of the gut wall to increase surface area. Lacteals serve as a primary uptake route in mammals for uptake of droplets of emulsified lipids (chylomicra). Teleost fish have a lymphatic system which includes extensions into the gut wall, but its role in lipid uptake is unknown. Absorption of amino acids, peptides, and simple carbohydrates have been little studied, but presumably they diffuse through or are transported across the gut epithelium into the blood stream. What light microscopists identified as a brush border on the surface of the epithelial cells facing the gut lumen, has now been clarified with electron-microscopy as microvilli; i.e., subcellular, finger like projections of the cell membrane whose greatly increased surface area is probably involved in absorption.

Specific Dynamic Action (SDA)

Digested food, particularly proteins, is not fully available to a fish even after it has been absorbed into the blood stream. Amino acids, if used for building new tissue, could be used as absorbed. If amino acids are to be oxidized for energy, however, deamination (removal of the amino group) must occur first - a reaction which requires input of energy. This process, known as specific dynamic action (SDA), can be measured externally in fish as an increase in oxygen consumption beginning soon after ingestion of food followed by an increase in ammonia excretion.
The proportion of amino acids which get deaminated varies with the food and the fish's circumstances. Fish which are not growing because of low temperature or have their ration at maintenance level or below, would deaminate most or all of their amino acids. Fish kept at high rearing temperatures or at high activity levels and therefore having very high metabolic rates would do likewise. On the other hand, fish having rapid growth and high protein intake would deaminate a relatively small proportion of their digested protein, although the absolute quantity of amino acids deaminated could still be large enough to produce a relatively large SDA. The energy for deamination need not necessarily come from amino acids, but will be preferentially taken from carbohydrate or lipid, if available. Thus, salmonid aquaculturists long ago discovered this "protein-sparing" action of limited amounts of inexpensive carbohydrate in the diet as a way of reducing the cost of feed and still achieving a desired level of growth. The protein-sparing action of lipids appears to have been minimally investigated. One can thus minimize SDA costs, but not avoid them completely.

Interrelationship between Osmoregulation and Digestion

Researchers studying osmoregulation and researchers studying digestion have rarely considered each other's data. Marine fishes drink significant amounts of seawater, a relatively-well buffered solution having a pH of about 8.5, while gastric digestion requires a pH of 4 or lower in most fish. The amount of HCl required just to acidify the seawater would be substantial, that is, if the entire stomach gets flooded with seawater. There are several likely alternatives, however. In fish with Y-shaped stomachs, the seawater could travel directly from the oesophagus to the pylorus, and traverse only a small fraction of the stomach surface. If, at the same time, digestion functioned primarily as contact digestion, then it could be largely separated from osmoregulation. On the other hand, marine salmon stomachs have been found to be filled with a liquid slurry which would prevent such separation. In such cases, alternation of digestion and seawater drinking might be possible, although fish whose stomachs seemed continuously filled, and therefore would have no time for drinking, have also been observed.
The pH of seawater should cause little or no problem with intestinal digestion. Too high a salt content in the intestine might exceed the operational range of some enzymes and thus reduce the rate of digestion. However, one of the functions of the stomach (and in eels, the oesophagus) in osmoregulation is to dilute the incoming seawater until it is approximately equal to the osmolarity of blood, thus protecting the intestine.
The final osmoregulatory product of the gut is a rectal fluid composed of magnesium and other divalent ions having about the same total concentration as blood. Preliminary data from scale loss studies indicated that death occurred from toxic levels of magnesium in the blood. A possible cause of the high magnesium is that gut peristalsis stopped, leaving the rectal fluid to accumulate and the magnesium ions to be reabsorbed instead of being excreted.