Digestion in Teleost Fishes ( CHARACTERISTICS OF ENZYMES AND OTHER DIGESTIVE SECRETIONS)

Digestion in the Mouth and Oesophagus

The hard surfaces of the mouths of most teleost; fishes would not lead one to expect any kind of secretion. However, many fish which chew with pharyngeal teeth or similar structures also produce mucus while chewing. Tests of this mucus in a few species for enzyme activity have so far yielded negative results. Likewise, oesophageal mucus cells, when examined histologically, showed no sign of containing any enzymatic granules, although there are reports of gastric-like secretory cells in the posterior oesophagus of a few fish.

Digestion in the Stomach

Pepsin is the predominant gastric enzyme of all vertebrates, including fish. Optimal pH for maximal proteolytic activity has been reported for several species, as follows:
(a) pH 2 - pike, plaice
(b) pH 3-4 - Ictalurus
(c) pH 1.3, pH 2.5-3.5 - salmon, probably similar for tuna
Peptic activity has been shown in a number of cultures and commercial species including Anguilla japonica, Tilapia mossambica, Pleuronecthys, both Salmo and Oncorhynchus species, Ictalurus, Micropterus, Lepomis and Perca. The presence of pepsin is so universal in vertebrates having stomachs that its presence can be presumed in fish for which no data is available.
The histochemistry of gastric secretion has been little studied in fish, although there is agreement on the presence of only one type of secretory cell in fish which stains positively for indicators of pepsinogen (pepsin precursor) cells. There is some question whether there may be more than one pepsin present in some fish, but no chromatographic or other tests have been done to investigate this. Several attempts have been made to identify acid-secreting cells, but results were either negative or confusing.
Other gastric enzymes have been proposed, but not firmly identified. Chitinolytic activity with an optimum at pH 4.5 was claimed for the stomach of Salmo irideus, but in most cases is probably from exogenous sources. If fish are like higher vertebrates, then the stomach wall also produces the hormone gastrin which stimulates gastric secretion. A lipase may also be present.

Digestion in the Midgut and Pyloric Caecae

There are two sources of enzymes for the midgut - the pancreas and the secretory cells in the gut wall - with the pancreas perhaps secreting the greater variety and quantities of enzymes in fish. Because of the variety of enzymes present in different species, there have been some attempts to correlate enzyme activities with diet. However, these enzyme studies are fragmentary and histochemical tests are too general. Much remains to be learned about intestinal digestion in fish.
Trypsin appears to be the predominant protease in the midgut. Since the enzyme appears not to have been isolated, most authors have just tested for proteolytic activity over the pH range of 7 to 11 and reported their results as tryptic activity. The diffuse nature of the pancreas in most cases has limited many researchers to making relatively crude extracts from mixed tissues, hampering localization of the enzyme. Tryptic activity has been found in four stomachless species in Japan: Seriola, two basses and a puffer. Since these fish lack pepsin, some such kind of protease in the intestine would be the primary means of protein digestion. Tryptic activity was found in extracts of both the pancreas of perch and Tilapia and in intestinal extracts of Tilapia, all having a pH optimum of 8.0-8.2. Proteolytic activity has been identified in the pyloric caecae and intestine of rainbow trout. In grass carp, tryptic activity was stronger in the intestine than in the pancreas. In a mixture of pancreatic and pyloric caecae tissue from chinook salmon, casein was digested maximally at pH 9. Tryptic activity has also been demonstrated in extracts of liver of Several species, probably because in fish having a diffuse pancreas, pancreatic tissue extends into the liver, around the portal veins, and around the gall bladder. In several of the cases above, when extracts of pancreas were mixed with extracts of intestine, the tryptic activity increased ten-fold or more, suggesting the presence in fish of the enzyme enterokinase in the intestinal wall which activates in mammals the pancreatic trypsin as it reaches the intestine.
Additional pancreatic enzymes are involved in midgut digestion, many of them yet to be discovered. For example, Japanese workers are studying the occurrence and characteristics of a pancreatic collagenase in several Japanese fishes. There have also been several reports of chitinolytic activity in some fish which eat crustaceans predominantly. This could also have resulted from bacterial activity.
The occurrence of at least one lipase may be assumed in all fishes and has been demonstrated for a number of species. In carp and killifish extracts of intestine showed lipolytic activity. In goldfish, lipase activity occurred in extracts of a mixture of liver and pancreas and in the intestinal contents. Esterase (another lipase) activity has been found in the liver, spleen, bile, intestine, pyloric caecae and stomach of rainbow trout. Use of radioisotope-labeled lipids in cod suggested that the cod's lipase acted in the same manner as mammalian pancreatic lipase, although it was not considered more than a suggestion that fish lipase is of pancreatic origin. Regardless of origin, some kind of lipase is essential to fish because fatty acids are essential dietary components for fish.
Carbohydrases have perhaps excited the most interest of all the enzymes, particularly because salmonids do not handle the large carbohydrate molecules very well, and many workers wanted to determine the reason. Further, because there are several carbohydrases, the possibility that different enzyme combinations might show adaptations to different diets also intrigued some investigators. Also, herbivorous fish might be expected to have more carbohydrase activity and less tryptic activity than carnivores or omnivores.
Amylase is a widespread starch-digesting enzyme which occurs in human saliva and in pancreatic secretions into the small intestine. Amylase activity has been found in goldfish and bluegill sunfish in extracts of mixed liver and pancreas, oesophagus (contamination from regurgitated food suggested) and intestine, but not in large-mouth bass. Similar activity has been seen as well in rainbow trout, perch, Tilapia, Pacific salmon, cod, common carp, eel, and flounder. In fish with a diffuse pancreas there may be no pancreatic duct and so amylase activity appears in the bile. In mackerel. Scomber spp., which have a compact pancreas, the bile had no amylase activity.
Other carbohydrases identified included glucosidases (rainbow trout, chum salmon, common carp), maltase (common carp, red sea bream, Archosargus, marine ayu, Plecoglossidae), and sucrase, lactase, melibiase, and cellobiase, all of the latter in common carp. The hypothesis that carnivores might be deficient in one or more carbohydrases is largely disproved by the widespread presence of amylase in salmonids and other predators and by the presence of maltase in sea bream and ayu. The apparently larger diversity of carbohydrases in common carp than in other fish seems mostly a lack of information about fish other than carp. The question of whether dietary differences influence the kind of enzymes present must remain open but the evidence so far remains largely negative. However, there seems to be some evidence to show that the amounts of various enzymes may relate to the diet. Data in Table 2 suggest that herbivores have de-emphasized the production of proteases compared to the carnivores and the reverse for carbohydrases.
Similarly, in studies of Trachurus, Scomber, Mullus, Mugil, and Pleuronectes, the predatory species, Trachurus and Scomber had the highest proteolytic and lipolytic activities, while the planktivore, Mugil, had the lowest proteolytic and the highest amylolytic activities. Also, stomachless fish (which lack pepsin) are usually herbivores or omnivores, while carnivorous fish have true stomachs with peptic digestion. On the other hand, differences in proteolytic activity between Tilapia and Perca were small, and some other investigations of a variety of species failed to find any species differences. Apparently, where fish are somewhat specialized in their diets, differences in their enzyme activities are apparent. Many fish, however, remain non-specialized and have diversified diets and enzymes.

The Role of Bile, Gall Bladder and Liver in Digestion

The functions of bile have scarcely been studied in fish, but presumably resemble those in higher vertebrates. In mammals bile is composed mainly of bilirubin and biliverdin, which are breakdown products of haemoglobin, and is produced continuously. These salts act like detergents and serve to emulsify lipids, thus making lipids more accessible to enzymes because of the increased surface area, allowing some lipids to be absorbed undigested as micro-droplets. In mammals, about 80 percent of the bile is recycled through the liver and gall bladder.
There are a few studies in fish which suggest that bile serves similar functions in fish. Several histologists have histochemically identified micro-droplets of lipid in midgut epithelium of fishes. That the gall bladder in fish reabsorbs water as in mammals has been confirmed. That bile is produced continuously in fish is suggested by the presence of green mucus in the lumen of the atrophied gut of spawning salmon. There appear to be no studies in fish of gall bladder contraction or other mechanisms controlling the release of bile during digestion. An observation of salmon having impacted gall bladders seemed related to diet because the gall bladders returned to normal when their dry pellet diet was changed to a moist pellet. Fish having impacted (and presumably non-contractile) gall bladders were normal otherwise and were indistinguishable in appearance and growth rates from fish in the same population with normal gall bladders.
Anatomists have tried for many years to correlate the shape of the liver and the position of the gall bladder in the liver with some of its functions. The basic functions of the liver in processing the foods which have been digested and absorbed are entirely cellular and molecular in scope. Thus, there is no functional requirement for shape at any level above the cellular level; i.e., livers basically could be of any shape. On the other hand, some restrictions are created by its position in the circulatory system between the gut and the heart, and the necessary interdigitation of the portal and hepatic veins, hepatic arteries, and bile ducts, all of which must serve essentially every cell of the liver. In common carp, the liver seems to have no shape of its own and simply fills every available space between the loops of the intestine. On the other hand, many fish (e.g., salmonids) have distinctive shape and colour to their livers. Changes in normal size and shape can indicate dietary or other problems. For example, a large, yellowish liver, often with white blotches suggests fatty degeneration of the liver caused by too much starch or by using saturated (mammalian) fats in the diet.