Digestion Study Guide: FISH 4500

Anabolism vs. Catabolism

Anabolism synthesizes complex molecules from simpler ones; catabolism breaks complex molecules into simpler ones to release energy.

Proteins

Primary building blocks of fish; made of amino acids. Optimal diet for growth contains 35–55% protein.

Amino Acids

Subunits of proteins; classified as essential or non-essential.

Lipids

Fats and oils including triglycerides, phospholipids, and sphingolipids. Optimal diet contains 10–20% lipids.

Carbohydrates

Dietary sugars and starches. Optimal growth diet contains 5–30% carbohydrates.

Early Mortality Syndrome (EMS)

Caused by thiamine (Vitamin B1) deficiency from eating invasive prey high in thiaminase, such as Alewife and Rainbow Smelt.

Thiamine Deficiency

Vitamin B1 deficiency responsible for Early Mortality Syndrome in Great Lakes salmonids.

Fish Alimentary Tract Pathway

Mouth → Pharynx → Esophagus → Stomach → Pyloric Ceca/Intestine → Cloaca/Anus.

Mechanical Digestion in Mouth and Pharynx

Jaw teeth and pharyngeal teeth physically break food into smaller particles.

Carnivore Intestine

Short and simple intestine adapted for digesting animal-based food.

Herbivore Intestine

Long and highly coiled intestine adapted for processing plant material.

Shark Spiral Valve

Internal intestinal structure that increases absorptive surface area in sharks.

Pyloric Ceca

Finger-like projections near the stomach junction that increase surface area and secrete digestive proenzymes.

Digestive Proenzyme Sources

Intestine walls, pyloric ceca, pancreas, gallbladder, liver, and gut microbiome.

Three Primary Stomach Functions

Short-term storage, mechanical mixing, and primary chemical digestion.

Acid Lysis

Chemical rupture of cell walls and membranes by stomach acid.

Fish Stomach pH

Typically ranges from 2.2 to 4.3.

Pepsinogen

An inactive zymogen secreted by gastric glands that is converted to active pepsin by low stomach pH.

Pepsin

Active enzyme that hydrolyzes protein peptide bonds in the stomach.

Proenzyme (Zymogen)

An inactive enzyme precursor that requires pH changes or cleavage to become active.

Endopeptidases

Enzymes that cleave internal peptide bonds. Examples include pepsin, trypsin, and chymotrypsin.

Exopeptidases

Enzymes that cleave terminal peptide bonds to release amino acids. Examples include aminopeptidase, dipeptidases, and tripeptidases.

Optimal Intestinal pH

Fish intestinal enzymes function best at pH 7–9, which is slightly alkaline.

Bicarbonates

Secretions from the pancreas and gallbladder that help maintain the intestine's alkaline pH.

Amylase

Enzyme that breaks down starches.

Disaccharidase

Enzyme that breaks down sugars.

Chitinase

Enzyme that breaks down chitin in exoskeletons.

Cellulase

Enzyme that breaks down plant cell walls.

Lipid-Digesting Enzymes

Lipases, phospholipases, and esterases break down dietary fats; bile salts aid lipid digestion.

Bile Salts

Secretions that aid in the digestion and absorption of dietary fats.

Final Products of Protein Digestion

Free amino acids.

Final Products of Carbohydrate Digestion

Monosaccharides.

Final Products of Fat Digestion

Glycerol and fatty acids.

Villi

Tissue folds in the intestine that increase absorptive surface area.

Microvilli

Microscopic projections on enterocytes that form brush borders and increase absorptive surface area.

Enterocytes

Intestinal epithelial cells responsible for absorbing digested nutrients into blood vessels.

Passive Absorption

Nutrient movement by diffusion through aqueous pores or channels without requiring energy.

Active Absorption

Energy-requiring nutrient transport using ATP, carrier proteins, or sodium pumps.

Pinocytosis

Cellular engulfment that allows larger peptides and proteins to enter enterocytes in vesicles.

Micelle

Small molecular aggregate that carries long-chain fatty acids across cell membranes.

Carnivore Absorption Efficiency

Approximately 80–97%.

Herbivore Absorption Efficiency

Approximately 40–50%.

Specific Dynamic Action (SDA)

The metabolic cost of digestion, including food processing, enzyme secretion, nutrient use, and storage.

Assimilation Efficiency

The percentage of eaten food that is actually used by the fish.

Assimilation Efficiency Formula

Assimilation Efficiency (%) = (Amount of Food Used ÷ Amount of Food Eaten) × 100.

Food Used Formula

Food Used = Food Eaten − Feces Produced.