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Fishes
Classification
of Fishes
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Three classes of vertebrates: Agnatha; Chondrichthyes; and Osteichthyes
Fishes are the most numerous of all vertebrates and most widespread in
their distribution
Adaptations
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Because water is 800 times the density of air, it affects both the body and
mobility of fishes: adaptation for buoyancy (trapping of gas inside their body:
gas bladder in order to regulate their vertical position)
Ability to swim: a streamlined shape and muscular tail enables them to
move rapidly through the water; paired fins allow them to maneuver easily
left or right, up or down, and backward and forward; the mucus reduces
friction
Scales limit chemical exchanges through the skin; exchanges occur through
the membranes of the gills: the external respiratory organs
Lateral line system: consists of a row of sensory structures that run the
length of the body and connected by nerves to the brain; detects vibrations.
Osteichthyes
• Osteichthyes make up most of the vertebrate
population in fresh water and in salt water.
• Lobe-finned fishes, or coelacanths, have paddle like
fins with fleshy bases.
• Lungfishes have gills where gas exchange takes
place between water and the blood. They burrow
unto the mud and cover themselves in mucus to stay
moist until the pond refills.
• Ray-finned fishes have fins that are supported by the
long bones called rays. They are the most familiar
fishes and include snakelike eels, yellow perch, cave
fish, herring, and lantern fish.
Morphology of a Bony Fish
External Anatomy
Body Structure
• Distinct head, trunk, and tail regions
• Each side of head is operculum
-Hard plate that opens at rear and covers and protects
gills
• Strong muscles along dorsal backbone thrust tail from
side to side
Fins
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Thin fan-shaped membranes
Richly supplied with blood
By raising and lowering fins, regulate body temperature
Supported by rays or spines
-Rays- bony yet flexible
-Spines- bony and rigid
Adapted for swimming and guiding fish through water
Caudal fin extends from tail
-Moves from side to side and amplifies swimming motions
2 dorsal fins, one anterior and one posterior, and ventral anal
fin
-Help keep fish upright and moving in straight line
Paired pelvic fins and pectoral fins
-Used to steer, brake, move up and down, and even back up
-Orient body when at rest
More on Fins & Tails
The Median Fins
There are four median, unpaired fins. These are: the caudal fin, just seen; the
anal fin, located posterior to the anus; the anterior dorsal fin, beginning at the
anterior end of the trunk; and the posterior dorsal fin, located posterior to the
anterior dorsal fin.
Tail
Tapering from the trunk, the tail ends as a laterally compressed
paddle. The symmetrical caudal fin surrounds the fleshy end of the
tail. This type of symmetrical caudal fin is correlated with the presence
of a swim bladder.
Scales
Almost all fish have scales. Scales are transparent plate-like structures covering the
body usually in an overlapping fashion (sort of like the shingles on a roof). There are four
types of scales: Cycloid, Ctenoid, Placoid and Ganoid.
True bony fish have ctenoid or cycloid scales. Ctenoid scales are rough in texture
because they have tiny teeth along their edges. Cycloid scales on the other hand have a
smooth surface. Fish such as bass, perch and snappers have ctenoid scales (their skin
is rough to the touch) and cycloid scales can be found on fish such as carp, salmon,
herring and sardines.
Ganoid scales are found on more primitive bony fishes such as gars and sturgeon.
These scales are diamond shaped. The rows of bony scutes on a sturgeons body are
modified ganoid scales.
Sharks, skates and most rays have placoid scales which are rough and sharp. Placoid
scales which are also referred to as dermal denticles, look like tiny teeth.
Some primitive fish do not have scales at all. Their bodies are smooth and slimy.
Did You Know?... The scales of some fish can tell scientists how old fish are. By
looking under a microscope scientists can see growth rings on the scales which
represent a season or period of active growth for the fish.
Figure 1: External Anatomy of the Perch (Lateral View).
Internal Anatomy
Digestive System
Find the following internal organs:
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a) Esophagus - located at the extreme anterior end of the body cavity, the esophagus is a
short, straight tube leading from the oropharynx to the stomach.
b) Stomach -the stomach is a larger, thick-walled U-shaped tube. In the perch breakdown
of the food by mechanical and chemical means, begins in the stomach. The size of the
stomach varies according to how much food it contains.
c) Pyloric caeca - the junction of the stomach and the intestine is marked by the presence
of three pyloric caeca. These are blind-ended tubes extending from the gut that serve
secretory and absorptive functions.
d) Intestine - originating at the stomach, the intestine forms an S-shaped loop. At the end
of the loop the intestine constricts and straightens. The intestine extends directly to the
anus. The perch's intestine is less than the length of its body. This correlates with the
animal's carnivorous life style. Herbivorous fish have an intestine, which is longer (2 to 15
times the body length). A longer intestine is required to provide greater digestive and
absorptive surfaces for the herbivores.
e) Liver -situated just anterior to the stomach. On the undersurface of the liver is the gall
bladder. The gall bladder drains bile from the liver, and opens by a number of ducts into
the intestine. Bile is necessary for the proper digestion of fats.
f) Pancreas -the pancreas is a digestive gland, usually found along the ventral border of
the intestine. In some fish the pancreas is embedded in the liver. The pancreas secretes
digestive enzymes into the intestine and hormones (insulin and glucagons) into the blood.
g) Spleen- lying on the posterior dorsal surface of the stomach, the spleen is a football
shaped organ. It functions in the production and maintenance of blood cells.
Figure 2: Perch internal anatomy.
Gill Structure
Circulatory System
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Adapted for rapid swimming and other high-performance
activities
Consists of:
-Two-chambered heart
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Atrium- collecting chambers
Ventricle- pumping chambers
-Blood vessels
-Blood containing red and white blood cells
Heart pumps blood through arteries to small, thin-walled
vessels called capillaries in gills blood picks up oxygen
gas from releases carbon dioxide into water blood
moves to body tissues, where nutrients and wastes are
exchanged blood returns to heart through veins
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Respiratory and Excretory Systems
Gills adapted for gas exchange
Consists of four sets of curved pieces of bone on
each side of head
Each has double row of thin projections called gill
filaments richly supplied with capillaries
Large surface area allows rapid gas exchange
Excrete nitrogenous wastes from body, but task
carried out primarily by kidneys
Kidney’s filter out dissolved chemical wastes from
blood
Swim Bladder
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Swim bladder or gas bladder
-Thin-walled sac in abdominal cavity
-Contains mixture of oxygen, carbon dioxide, and
nitrogen obtained from bloodstream
-By regulating amount of gas in sac, fish adjust overall
density and thus move up or down in water or hover at
given depth
Reproduction
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Sexes are separate
Eggs produced by ovaries in female; sperm produced by testes
in male; both released through opening just rear of anus
Fertilization of eggs takes place externally
Young fish hatch within hours of warm water or after many
weeks in cold water
Number of eggs bony fish may lay varies considerably
Some bear live young
Female may receive sperm during mating, and fertilization in
some is internal and other species it is external
Carries eggs in body until young are born
Spawn- reproduce