Fishes obtain the energy needed to meet metabolic demands through feeding. The diversity of feeding adaptations foundamong fishes. The emphasis here is on post ingestion processes.
Food is taken into the mouth and passed down the esophagus into the stomach. Secretion of mucus by the epithelial lining of the esophagus helps to lubricate the passage of food along the gut. Most fishes lack a mechanism for chewing food in the mouth, so food items are swallowed whole or in large chunks and much of the physical breakdown takes place in the stomach. However, many fishes, such as minnows (Cyprinidae), suckers, croakers(Sciaenidae), cichlids (Cichlidae), wrasses (Labridae), and parrotfishes (Scaridae), have bony arches or toothed pads deep in the pharynx that are equipped with tooth likeprojections. These pharyngeal teeth grind up food before it reaches the stomach (see Pharyngeal jaws).
The stomach is often highly distensible and can store food. Tough ridges along the internal wall of the stomach,along with contractions of the muscular wall, aid in the physical breakdown of foods. Acidic secretions of the stomach help to further break down foods; proteolytic enzymes also function more efficiently at lower pH. The combined physical and chemical activity of the stomach creates a soupy mixture which is released into the small intestine in small amounts.
Chemical digestion continues in the intestine, aided by bile from the liver, which helps emulsify lipids, and by secretions from the pancreas. Pancreatic juice contains bicarbonate to neutralize the acid from the stomach and a wide variety of enzymes to complete the process of chemical digestion.
The small intestine is also the primary site of absorption of the products of digestion, and mechanisms exist formaximizing this uptake. Elasmobranchs have a short, thick intestine with a large, spiraling fold of tissue (the spiral valve) to increase absorptive surface area. Teleost`sgenerally have longer intestines, often with numerous side pouches (pyloric caecae) to increase the absorptive area (Buddington& Diamond 1987). Herbivorous and microphagous teleost ‘shave particularly long, often coiled, intestines to increase the opportunity to extract nutrients (see Fange & Grove1979; Lobel 1981). Some of these fishes, such as minnows, sucker, and topminnows (Cyprinodontidae), and several tropical marine fishes, including wrasses and parrotfishes, have reduced stomachs or lack them altogether (Fange & Grove 1979; Lobel 1981; Buddington &Diamond 1987). Transgenic Coho Salmon (Salmonidae) have more than two times the intestinal surface area than do their control counterparts, which may help explain how these fish are so effective in extracting the nutrientsneeded to maintain their high rate of growth (Stevens & Devlin2000). Although some nutrient ab sorption may continue inthe large intestine, this last major portion of the gut functions primarily in water absorption.
Once basic metabolic demands are met, excess nutrients can be accumulated. Carbohydrates are stored as glycogen either in the liver or in muscle tissue. Lipids and proteins also are stored, resulting in an increase in mass that we refer to as growth. Lipids tend to accumulate either in the liver, in muscles, or as distinct bodies of fat in the visceralcavity. Protein often goes into tissue growth. All of these potential energy sources are mobilized when needed, although carbohydrates are metabolized first. In prolonged periods of starvation, such as during the migration of salmonids,body lipids and proteins will also be used. Stored lipids yield considerably more energy per gram than stored carbohydrates or proteins.