Fish Digestion
Digestion in fishes concerns the breakdown of foods by enzymatic and, in many cases, acidic secretions in the gut. The diversity of foods found in the guts of fishes attests to the variety of morphological and chemical adaptations which have evolved for digestion. The esophagus of fishes often contains many mucus cells and functions as a lubricated transit tube between the buckle pharyngeal cavity and the lower gut. The lower gut of many fishes (especially carnivorous ones) contains a true stomach, characterized by a smooth muscle “muscular is mucosa” layer of tissue. On the other hand, development of a gizzard (for masticator as well assecretory digestive processes) as found in mullets and shad is a stomach specialization for microphages food habits.
The gastric mucosa of the stomachs of carnivorous fishes produces protease (protein breakdown) enzyme (e.g., pepsin) with an optimal activity at a pH of 2 to 4. Hydrochloric acid is also secreted by the gas-trice mucosa glands in these species, creating the low-pH environment. Gastric acid secretions are stimulated by stomach distension, which apparently activates cholinergic (mimicking acetylcholine response) neural fibers. The “secretary” signals of these fibers to the acid glands can be blocked with injections of the neural-blocking agent atropine. Threats of both gastric acid secretion and of pepsin secretion are influenced by temperature. As temperatures increase (up to a point), the rates of secretion also increase. These increased secretions largely account forth threefold to fourfold increase in digestion rate that follow 10°Cincreases in temperature.
Proteins are also broken down in the alkaline medium of the inters-tine by the action of the enzyme try sin. Try sin is secreted by pancreatic tissue, which may be concentrated in a compact organ as in the mackerel (Somber) or diffusely located in the mesenteric membranes surrounding the intestine and liver. Some fishes possess one or more pyloric cacao, which are blind pouches of secretary tissue located near the pyloric valve, at the stomach-intestine junction. Trip-sin may be secreted from the cacao tissue or the pancreatic tissue, which commonly envelopes the cacao.
Fish also have enzymes which break down carbohydrates (carbohydrates) and fats (lipases). The pancreas appears to be the primary site of carbohydrate (e.g., amylase, which breaks down starch) production, although the intestinal mucosa and pyloric cacao represent additional production sites in various species. The pancreas is presumably also the primary site of lipase production. However, lipase activity has been found in extracts of the pyloric cacao and upper intestine as well as the pancreas in mackerel, menhaden, scup (Stenotomus), and sea robin
Both the presence and the quantity of digestive enzymes seem to correlate with the diet of fishes. Herbivorous and omnivorous fishes which have no stomachs also lack pepsin as a low-pH, proteolysis en-same (Kapok et al. 1975). However, omnivorous species have amylase activities in the gut many times that found in carnivorous species (Volya1966).
Compounds broken down by actions of pharyngeal teeth, gizzards, and/or secretions of acid and enzymes are subsequently absorbed through the intestinal wall. Absorption (or assimilation) can be estimated byte difference between the quantity and quality (energy value in kilocalories or joules’) of the food ingested and of the feces excreted. These estimates are made even more precise when fecal nitrogen and calories from nonfood sources (e.g., sloughed gut wall) are taken into account.
As in other animals, the metabolic conversion of biochemical com-pounds, either to provide energy or to synthesize other compounds
Enzymes, structural proteins, stored triglycerides), requires particular cofactors to proceed. The cofactors which are largely unavailable in the body constitute the vitamins. The vitamin requirements offices probably vary somewhat with species, although only a few species (mostly of commercial value) have been investigated (Table 7-1). Di-teary deficiencies of the vitamins that are essential for life and growth provoke a variety of physiological disturbances (Table 7-2). As with the research into essential amino acids, most of the vitamin deficiencies were determined by single vitamin deletions in an otherwise complete diet.