Energy
intake
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.
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