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Old 12-25-2010, 09:39 PM   #1 
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A Little Fish Anatomy

Fish are truly amazing creatures. They have been roaming the earth’s
waters for almost 450 million years and have adapted themselves over
time in order to thrive in their watery environment. A species’ body shape,
fin length, and other physical characteristics have been specially formed
through evolution to meet the needs of different types of habitats. As you
find out more about a fish’s physical makeup, you increase the odds of
becoming a successful fishkeeper with that species.
By becoming familiar with the physical characteristics of aquatic species,
you will be able to purchase fish that are healthy, spot problems easier, and
recognize disease quicker.

What Really Makes a Fish Go?
At one time or another, everyone has stood transfixed, watching aquarium
fish glide effortlessly through the water and wondering how they navigate
through their liquid environment with such ease. What makes a fish swim
better than we do? The answer is really quite simple.
Fish have a set of fins (six or seven of them, depending on the species) that
they use for locomotion. They also have a cool organ called a swim bladder
that helps them stay afloat (otherwise they would sink; see the section “The
swim bladder” near the end of this chapter for more). Fish have evolved to
conquer their watery environment with adaptations that have created the
perfect aquatic swimming machine. By contrast, even using rubber fins,
humans can achieve only a pale imitation of their aquatic friends.
To understand what makes a fish go, you must first understand each fin’s function.
Each individual fin has a specific job to do, and the combined effort of all
of a fish’s fins is what propels her through the water and helps her navigate
smoothly. Fin functions are an interaction of muscle power and sheer grace.
The dorsal fin
The dorsal fin is located along the back of the fish between the tail fin and the
head. This is the classic fin you see slicing through the water in the movie
Jaws. If you happen to see a Jaws-type fin while swimming in the ocean, you
may want to take up beach volleyball for a while. Fortunately for your peace
of mind, the dorsal fins on your aquarium fish generally remain underwater.
The dorsal fin provides lateral stability so that your fish can swim in a
straight line. Controlled swimming conserves energy. A fish that cannot swim
well doesn’t live very long because it can’t compete for food with its tankmates.
Each fin consists of a series of individual rays (fin segments, some soft
and some hard) loosely bound together by a membrane web.
A few aquatic pets, such as some goldfish and knifefish, do not have dorsal
fins. They have great difficulty swimming normally because they cannot keep
their movement in a straight line. Other species such as rainbowfish have
two dorsal fins.
The caudal fin
The caudal fin (tail fin) is responsible for sudden forward movement (bursts
of speed) and very fast swimming patterns. Fish also use their caudal fin to
slow forward movement and to help make turns. This fin produces the majority
of a fish’s physical power.
Lengthening the caudal fin of many species (such as the goldfish and betta)
for show purposes through artificial selection (breeding for a specific trait)
produces a slower-moving fish. Fish with very long caudal fins probably
would not survive long in the wild. A 3-inch goldfish with a 6-inch caudal fin
dragging the gravel like the train on a wedding dress is bound to have a few
swimming problems.
Sadly, many species of fish have been selectively bred to have caudal fins
that are so long or unusually shaped, the fish struggle just to stay upright in
the water. Fish that have truncated (Chinese fan-shaped) fins can dash
quickly even though they normally swim slow and easily. Fast-swimming fish
generally have forked fins; rounded fins are found in slower-moving species.
The anal fin
The anal fin is located on the underside of a fish between the pelvic and
caudal fins. The sole purpose of this fin is to provide stability — it keeps your
fish from rolling over in the water and going belly up. In some species, the
anal fin has developed into a double set of fins that are fused together at the
base of the fish’s body.
In species such as the freshwater guppy, the male’s anal fin acts as a sexual
organ and is known as a gonopodium. This rod-shaped organ inserts sperm
into the female’s vent (female organ) during spawning. Many species of
Characins, such as tetras, have small hooks on their anal fins that attach
them to their mate during breeding.
The pectoral fins
Pectoral fins provide stability as a fish moves through the water, hovers, and
makes slow turns. These paired fins are located near the bottom of the fish,
directly beneath the gill openings (one on each side). Pectoral fins are used
for navigation and are constantly in motion.
Many species use the pectoral fins to incubate their eggs with water during
the brooding period. Many flying fish have adapted their pectoral fins into
wings so that they can take short flights through the air. Some species of catfish
can “lock” their pectoral fins into a rigid stance when defending themselves
from predators.
The pelvic fins
Pelvic fins aid fish in braking, stabilizing their bodies, and changing directions.
These fins are located in front of the anal fin on the abdomen of the fish
(one on each side). Other uses of the pelvic fins include searching for food,
carrying eggs, and fighting. These fins are usually smaller in open water
species like the freshwater platy, and larger in some bottom-dwellers.
These fins are often called ventral fins because of their position on the body
near the small body-cavity opening on the bottom of the fish.
The adipose fin
A few species of fish such as tetras and some catfish have an extra fin called
the adipose fin, located on the back between the dorsal and tailfins.
Hobbyists often refer to it as the second dorsal fin. Scientists have not found
any physical reason for this fin to exist. At this point, it has no known use.
But it looks cool, so why not?
Swimming movement
The special body shape of fish helps increase the overall efficiency of their
swimming movement. A fish’s body is usually tapered at the head and tail
and bulky in the middle (like many of us when we hit midlife). This tapering
allows fish to slip through the water without much effort. So, maybe if we can
find a way to live in our bathtub, we’ll have it made.
Looking carefully at your fish, you may notice that most of them swim with
little or no effort, which is surprising because water is much more resistant
than air. But water’s liquid form supports a body’s weight as the body moves.
Because your fish’s weight is suspended in water, she needs only a small
amount of energy to overcome the force of gravity — as opposed to the effort
we humans must put out as we move through atmospheric air on dry land.
A fish’s muscle force is achieved though energy created by short fibers that
run throughout the fish’s entire body. These numerous fibers move in
sequence and create physical energy in a series of s-shaped curves. This
energy is then transferred to the tail to provide locomotion. Finally, the
caudal (tail) fin pushes all the water surrounding it backwards, which in turn
propels the fish’s body in a quick forward motion. This sequence of events
allows the fish to move through the water without creating any turbulence —
which would slow it down.

Last edited by Busted; 12-25-2010 at 09:45 PM.
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Old 12-25-2010, 09:40 PM   #2 
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Just like humans, fish require oxygen for survival. Fish use oxygen that they
strip from the water and produce carbon dioxide as a waste product. Any
living plants in your aquarium use this carbon dioxide, and eventually expel
oxygen back into the water.
The gill method
Unlike land animals, fish don’t get their oxygen from air. Instead, fish take
their oxygen directly from the water through their gills. Gills are lined with a
large number of blood vessels that help retrieve oxygen.
Gills are very similar in structure and form to human lungs, except that they
are a whole lot more efficient: Although fish remove up to 85 percent of the
oxygen from their aquarium water, humans obtain only about 25 percent of
the oxygen in the air they breathe by comparison, so water quality is important!
(Of course, if you live in a crowded city, your oxygen consumption may
drop to about 2 percent.)
Water enters a fish’s mouth and passes across the gills where the oxygen is
extracted by the gill filaments. The oxygen-depleted water is then quickly
Fish with high energy levels who are very active, like the freshwater danio,
must constantly keep swimming in order to force water through their gills
and obtain oxygen. Species of fish with high energy levels would eventually
suffer asphyxiation if kept in a small aquarium that restricted their swimming
movement. You don’t want to live in a sealed elevator with 20 other people.
Neither do your fish.
Make sure that your tank is large enough to provide ample swimming room
for your aquatic friends.
When moving fish from one location to another, you must remember that gills
are made out of fine tissue that can collapse if removed from water. The gills
are structurally supported by the weight of the water itself. So it is very
important that you keep your fish in water while moving them, to avoid causing
any damage to their gills, bodies, and fins which can be crushed by lack
of water support.
The labyrinth organ method
A certain group of fishes (known as the Anabantids), found in Asia and Africa,
are able to breathe air directly from the atmosphere, using a specialized
organ called the labyrinth. The labyrinth, located inside the head behind the
gills, has evolved over time to take oxygen directly from the air as a supplement
to extracting it from the water.
Anabantids include bettas, gouramis, and paradise fish. In the wild, many of
these fish live in dirty, poorly oxygenated waters full of strange-looking creatures.
(Not unlike a good day at our public pool.) These fish tend to have
wide bodies and enlarged fins.
The physical shape of the labyrinth organ gives rise to its name, which literally
means “maze.” The labyrinth contains rosette-shaped plates that have
thousands of oxygen-absorbing blood vessels, which gather air that is
inhaled. The inhaled air is then trapped inside a group of folds (which resembles
a sponge) and is eventually absorbed into the main bloodstream.
Anabantids can survive in a smaller aquarium space than that which is normally
provided (usually 10 or more gallons) because they can extract oxygen
from the air. However, this does not mean that anabantids can or should be
kept in very crowded conditions or extremely tiny tanks. Even though they
have the ability to breathe “extra” air, these fishes still add as much waste to
the water as their tankmates and need proper space and filtration for healthy
You will often see betta fish for sale in small jars. The reason many dealers do
this is to keep the males separated so that they don’t fight. This is not a good
practice for the home aquartist to pick up and use. Don’t keep labyrinth fish
in small bowls or hanging vases for decoration purposes. Give them a good
healthy aquarium environment with plenty of room instead.
Anabantids can develop diseases brought on by crowded tanks with bad
water conditions, just like other species. Take our word for it, they’ll be
healthier and happier in a proper aquarium. (Anabantids should be provided
with the same high quality filtration, heating, and other proper conditions as
is standard with other tropical fish.) They will like you a lot better too.

The Senses
Like humans, fishes have five senses: taste, sight, hearing, touch, and smell.
Fish use all these senses to locate food, communicate with one another,
attract mates, and avoid bigger and meaner fish. Fish have been known to
learn to do without one or more of their senses when they’re injured or born
with a physical defect. We’ve seen fish in the worst possible physical condition
continue to survive. Think how great they can look and feel if we keep
them in the best possible condition!
Here are some fun facts about fish eyes:
Most fish have the ability to see in two directions at the same time. This
physical phenomenon is known as monocular vision.
Fish can’t completely focus both their eyes on a single object at the
same time.
Fish do not have eyelids and sleep with their eyes wide open, resting in
a hypnotic state.
Most fish are nearsighted and see clearly only about a foot away. So, if
you stand across the room, smiling and wildly waving both your hands
to entertain your fish, don’t hold your breath waiting for them to
The lateral line system
Fish have an interesting system known as the lateral line, which helps them
locate objects in their path and in their surrounding environment that they
cannot see normally due to their limited eyesight (see Figure 7-2). This line is
incomplete in some species. The blindcave fish use this system to navigate,
and killfish use it to help locate insects above the surface of the water.

The lateral line is located on both sides of their body and runs from the back
of the eye to the base of the tail fin. These lines are composed of small neuromasts
(receptors) which contain cilia (very fine hairs) in fluid-filled canals.
These canals detect vibrations in the water, and the vibrations form an
“image” inside the fish’s brain.
The eyes
A fish’s eyes are often large to compensate for the poor lighting conditions
that exist under water. Usually the eyes are located on the sides of the head,
and some species can rotate them 360 degrees. In certain species that live in
areas of total or semi-darkness (such as the blindcave fish), the eyes are
absent altogether. Over time, the eyes have been selectively removed
through the evolutionary process.
Some fish do have the ability to see a few colors at various depths, but they
have great difficulty adjusting to rapid light changes because their iris works
slowly. For this reason, fish act “shocked” and may panic when an aquarium
light is suddenly turned on or off without warning in a room that is still dark.
So if you turn on your aquarium light right after you get up in the morning
and then notice that your fish are stuck to the ceiling, you probably frightened
them a little bit.
Within the human eye, the shape of the lens is constantly changing in an
effort to achieve proper focus. The lens in a fish’s eye remains the same
shape, but focuses with help from special ocular ligaments that actually
move the eye forward and backward in its socket.
Fish do not have complex ears like we do because sound travels in water
much faster than in air, so by evolution standards this was not needed. Fish
ears are composed of a simple inner chamber. Vibrations picked up from the
environment are passed over sensory components, which generate sound.
Most ichthyologists (fish experts) believe that a fish’s swim bladder works
together with the components of the inner ear to distinguish specific sound
Smell plays an important role in detecting food and prey, and in locating a
suitable mate. Fish take in smells through their nostrils, which are connected
to their olfactory system. This olfactory system is not completely joined with
the respiratory system and acts as a separate unit.
Fish have taste buds on their mouths, lips, and, in special cases, on their fins.
The complete range of taste for fish is very short, so they must constantly
forage through their environment in hopes that they can “stumble across” the
food they need to survive. Catfish have evolved barbels (whiskerlike appendages)
that contain taste buds for locating food in cloudy or dark water.
The old argument as to whether fish can feel pain or not has been at issue for
many years. We would really hate to find out that our fish could feel pain if
we did something that caused them harm. The safest bet is to assume always
that your fish can feel pain and treat them with respect and great care just
like you would any other pet.

Source of information is from, For Dummies. Pictures are from For Dummies,, Basic Google search for Betta labyrinth organ diagram.

Last edited by Busted; 12-25-2010 at 09:55 PM.
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Old 12-25-2010, 09:51 PM   #3 
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Busted, if you took this from different sites, it's polite to give them credit (you also avoid breaking copyright infringement laws this way).

However, very good info :)

Last edited by JKfish; 12-25-2010 at 09:54 PM.
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Old 12-25-2010, 09:52 PM   #4 
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Originally Posted by JKfish View Post
Busted, if you took this from different sites, it's polite to give them credit (you also aviod breaking copyright infringement laws this way).

However, very good info :)
I did, just had to edit it back in xD i swapped the top and bottom lol.
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Old 12-25-2010, 09:56 PM   #5 
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Oh, okay :)
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Old 12-25-2010, 10:03 PM   #6 
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Was just hoping maybe someone could use this , plus it does contain neat information that not everyone might know
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Old 12-25-2010, 11:25 PM   #7 
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Me being picky but you may want to edit the part about hearing. Sound travels slower in water than in air because it's a denser medium.
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Old 12-26-2010, 09:57 AM   #8 
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Originally Posted by Juneii View Post
Me being picky but you may want to edit the part about hearing. Sound travels slower in water than in air because it's a denser medium.
Sound travels at 1500 m/s in water and 330m/s in air.

Sound waves travel fastest in solids and slowest in gases. This is because the molecules in solids are close together, which makes it easier for the vibrations to pass from one molecule to the next. That being said liquids are more dense in gases. Think about how whales can talk over miles apart, this couldn't happen in air due to the speed the sounds are traveling.
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Old 12-27-2010, 11:26 AM   #9 
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Thanks for sharing Busted. I learned some new things!
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Old 12-27-2010, 11:50 PM   #10 
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Good info. And yes, the credits are there.
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