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Fish Digestion
Mouth
• Fish are homodonts (All
teeth are the same with no
specialisation)
• There is a wide variety in
shape and size of teeth
depending on the diet.
• Bony fish have connected
premaxillary and mandible
which allows them to open
mouth wide and suck in
prey.
• Some fish bite off lumps
and swallow them but
there is no chewing
premaxillary
mandible
Digestive System
Food enters mouth through the
oesophagus.
Food then moves into the
stomach for digestion.
Some fish have a crop and a
gizzard for storing and grinding
up food
• The stomach is typically
u-curved, and elongated.
It may be modified to
grind up food. Acid and
enzymes secreted in the
stomach digest the food.
• The Liver secretes bile
that is stored in the gall
bladder. Bile emulsifies
fat.
stomach
Pyloric caeca
Pyloric caeca is found between the stomach and the intestines,
they are slender finger-like tubes that secrete digestive enzymes
such as Trypsin and Pepsin. This increases the efficiency of
digestion and absorption of nutrients by creating a large surface
area. Like other organisms, fish intestines contain villi to increase
absorption by increasing surface area
Pyloric caeca
Spiral valve
The intestines of a shark are
much shorter than those of
mammals. Sharks have
compensated for this problem by
having a spiral valve, inside the
intestine to increase the
absorbent surface of the intestine.
By keeping digestible material in
the ileum for an extended period,
maximum nutrient absorption is
ensured. For this reason, many
sharks and related fish feed very
infrequently.
Herbivorous fish
• Herbivorous fish have
bacteria in their guts to
help them digest plant
material.
Digestive lengths
Herbivorous
fish have
longer coiled
intestines
Carnivorous
fish have
shorter, straight
intestines
Comparison of gut length of
herbivorous and carnivorous fish
Heat control
• Fish are Ectothermic – their body temperature
is dependent on the surrounding environment
and not their metabolism.
• Some large predatory fish like sharks, can
maintain a body temperature above that of
the surrounding seawater.
• This enables them to process greater
quantities of food more rapidly, allowing them
to be more active.
How?
Countercurrent heat exchange
Blood flows through the arteries and veins in opposite
directions. The warm blood returning to the heart first
passes through a “net” (rete mirable) that surrounds
the stomach and intestines.