The knowledge of food and feeding habits and the physiology of digestion of any organism is most essential for development of artificial feed in culture practices. Fish and shellfish belong to the poikilothermous animal. The digestion process is somewhat different than the terrestrial animals. Similarly the mechanism of digestion and absorption process is quite different in fishes and shellfishes. The basic function of digestive system is to dissolve foods by rendering them soluble so that they can be absorbed and utilized in the metabolic process. The system may also function to remove dangerous toxic properties of certain food substances. Fish generally change their feeding habits depending upon availability of food. So according to their feeding fishes are classified into different categories viz., predators, grazers, strainers, suckers and parasites.

1. DIGESTIVE PHYSIOLOGY OF
HERBIVOROUS FISH
Mahendra Pal
M.F.Sc.
College of fisheries,
JAU, Veraval

2. • The knowledge of food and feeding habits and the physiology of
digestion of fish is essential for development of artificial feed in
culture practices.
• The basic function of digestive system is to dissolve foods by
rendering them soluble so that they can be absorbed and utilized in the
metabolic process.
• The digestion process of fish is somewhat different than the terrestrial
animals.
INTRODUCTION

3. FEEDING
Based on the nature of the food ingested, fish are usually categorized into:
 Herbivores, feeding largely on plant material;
 Detrivores, feeding largely on detritus;
 Omnivores, consuming a mixed diet; and
 Carnivores, consuming only animal matter
Herbivores
• < 5% of all bony fishes, no cartilaginous fishes
• Browsers - selective - eat only the plant
• Grazers - less selective - include sediments
The hypothalamus: major control centre of appetite regulation.
 Chemostatic hypothesis
 Thermostatic hypothesis

4. DIGESTIVE SYSTEM
The structural com digestive
system inc
Mouth
Buccal cvity
Pharyncs
Esophagus,
Stomach,
Pylorus,
Pyloric caeca,
Intestine and anus.
Pancreatic tissue
(exocrine and endocrine),
Liver,
Gall bladder,

5. CONT..
milkfish
planktivor
e
carp
omnivore
catfish
omnivore
trout
carnivore
From De Silva and
Anderson, 1995; page
104)

6. FISH DIGESTION: ANATOMY
Two major groups
 Aastric: cyprinids (carps)
 Gastric: cold-water salmonids, warm-water catfish, tilapia, eels,
grouper
Relative gut length (RGL): gut:body length
• High RGL = species consuming detritus, algae (high proportion of
indigestible matter)

7. RGL
Species Feeding RGL
Carp Algae, detritus 15.5
Camaroon
Logsucker
Algae, inverts 4.5
Barbus sharpei Plants 2.8-3.1
Chelethiops
elongatus
Zooplankton 0.7
Rainbow Trout Carnivorous 0.9
From De Silva and Anderson, 1995; page 105

8. DIGESTIVE ANATOMY: MOUTH
Mouth and pharyngeal cavity
Jaws
Teeth - jaw, mouth,
pharyngeal
Gill rakers

9. OESOPHAGUS
The oesophagus
 Commonly referred to as the gullet,
 Short, expandable
 Notclearly demarcated from the stomach,
or intestine in stomachless fish.
Function
 Food storage,
 Trituration and
 Mucus secretion.

10. DIGESTIVE ANATOMY: STOMACH
• In herbivorous fishes no stomach
• The pylorus is a sphincter that prevents premature movement of the
food bolus out of the stomach.
• Around the pylorus, many fish have out-pocketings called pyloric
caeca.

11. DIGESTIVE ANATOMY: INTESTINE
Digestive tract is 3x whole body length,
The intestine is lined with finger-like out-pocketings (villi) increase the
surface area for absorbtion.

12. GASTROITESTINAL TRACT
• Esophagus
• Stomach
Large in carnivores, small in
Herbivores/omnivores
• Pyloric caeca
• Intestine
Long in herbivores-omnivores
• Anus
Separate from urogenital pore

13. Digestive Anatomy: pancreas and liver
Pancreas
• Primary source ofdigestive enzyme
• Around the pyloric caeca (or in that same area in fish
lacking them) is pancreatic tissue.
Function:
 Exocrine secretion of digestive enzymes
(proteases,lipases, and carbohydrases into the intestine.
 Endocrine secretion of the hormones insulin and glucagon.

14. LIVER
• Produces bile which is stored in the gall bladder
• Bile contains salts, cholestrol, phospholipids, pigments, etc.
• Bile has a digestive function in that it emulsifies lipids, greatly
increasing their absorbtion in the intestine.
• The liver is key in the anabolism and catabolism of amino acids and is
also the site of storage of food energy in the form of glycogen

15. After ingestion of food, gut is responsible for:
• Digestion - breaking down food into small, simple molecules
involves use of acids, enzymes
Involves the following processes:
 Mechanical reduction of particle size;
 Enzyme solubilization of organics;
 pH solubilization of inorganics;
 Emulsification of fats
FISH DIGESTIVE PHYSIOLOGY

16. CONT..
• Absorption - taking molecules into blood
 Diffusion into mucosal cells
 Phagocytosis/pinocytosis by mucosal cells
 Active transport via carrier molecules

17. SUMMARY OF DIGESTIVE ENZYMES
Site/Type Fluid/enzyme Function/notes
Stomach HCl Reduces gut pH,
pepsiongen
Gastric secretions Zymogen, pepsinogen, HCl
Amylase
Lipase
Esterase
Chitinase
Proteolysis
COH’s
Lipids
Esters
Chitin
Pancreas HCO3
Proteases
Amylase
Lipase
Chitinase
Neutralizes HCl
Cleave peptide linkages
COH’s
Lipids
Chitin
Liver/bile Bile salts, cholestrol Increase pH, emulsify
lipids
Intestine Aminopeptidases
Lecithinase
Split nucleosides
Phospholipids to glycerol
+ fatty acids

18. DIGESTIVE PROCESSES: FISH STOMACH
• Flow of digesta out of stomach is controlled by the pyloric sphincter.
• Pepsin has pH optimum and lyses protein into small peptides for
easier absorption
• Minerals are solubilized; however, no lipid or COH is modified
• Mixture of gastric juices, digesta, mucous is known as chyme

19. DIGESTIVE PROCESSES: FISH INTESTINE
Digestion is accomplished in: Intestine alkaline pH (7.0 - 9.0)
 Proteolytic enzymes - from pancreas & intestine
 Amylases (carbohydrate digestion) - from pancreas & intestine
 Lipases (lipid digestion) - pancreas & liver (gall bladder, bile
duct)
• Chyme stimulates secretions from the pancreas and gall bladder
• Zymogens for proteins, COH, lipids, chitin and nucleotides are
e.g., enterokinase (trypsinogen --> trypsin)
Others: chymotrypsin, carboxypeptidase, aminopeptidase, chitinase

20. CONT..
Cellulase: interesting in that it is not secreted by pancreas, but rather
produced by gut bacteria
Intestinal mucosa also secretes digestive enzymes

21. DIGESTION OF PROTEINS
Stomachless fish
• Protease compensation is supplemented by some intestinal enzyme
known collectively as erypsin.
• The intestine secretes amino peptidases.
• These act on terminal amino acid called as exopeptidases and those act
on central bonds are called as endopeptidases.

22. DIGESTION OF CARBOHYDRATE
Carbohydrases: break down the carbohydrates
• They are amylase, lactase, saccharases/sucrease and cellulase.
• The starch is digested into glucose by amylase and maltase.
• In those fishes in which sucrose has been reported, the effect as follows:
Starch---------------→ Maltose ---------------→ Glucose
Sucrose --------------- Sucrase → Glucose + Fructose

23. Fat Digestion
• The main enzyme which acts on this lipid is lipase.
• This enzyme is found in pancreas and mucosa of fishes.
• Lipase converts the fats into fatty acids and glycerols.

24. ABSORPTION
• Absorption is the taking up of the compounds by the body either through the
stomach lining or through the intestinal walls.
• The digested food material is thrown out as excreta or faecal matter.

25. METABOLISM
Metabolism refers to the physical and chemical processes that occur
inside the cells of the body and they are necessary for the maintenance of
life.
The transformation of the macro nutrients carbohydrates , fats
and proteins in food to energy and other physiological processes are parts
of the metabolic process.
ATP (adinosene triphosphate) is the major form of energy used for
cellular metabolism.
A number of metabolic processes occur in the liver.

26. PHASES OF METABOLISM
Catabolism
Breakdown of nutrients
Release of energy
Formation of ATP
Anabolism
Synthesis of molecules from smaller and simple molecules.
Use of ATP.
For example: Amino acids, which are simple molecules
(monomers) through a series of anabolic chemical reactions,
build proteins, which are large and complex molecules (polymers).

27. ENERGY FLOW IN FISH

28. SPECIFIC DYNAMIC ACTION (SDA),
This process, known as specific dynamic action (SDA),