FISH ANATOMY
External Fish
Anatomy The following illustration of a largemouth bass shows some of the
common external features that are used to describe
the differences between fish that are described in more detail below.
Fish are animals that are cold-blooded, have fins and a
backbone. Most fish have scales and breathe with gills. There are
about 22,000 species of fish that began evolving around 480 million years
ago. The largemouth bass illustrated above has the typical
torpedo-like (fusiform) shape associated with many fishes.
Fins are appendages used by the fish to maintain its
position, move, steer and stop. They are either single fins along the
centerline of the fish, such as the dorsal (back) fins, caudal (tail) fin
and anal fin, or paired fins, which include the pectoral (chest) and pelvic
(hip) fins. Fishes such as catfish have another fleshy lobe behind
the dorsal fin, called an adipose (fat) fin that is not illustrated here.
The dorsal and anal fins primarily help fish to not roll over onto their
sides. The caudal fin is the main fin for propulsion to move the fish
forward. The paired fins assist with steering, stopping and hovering.
Scales in most bony fishes (most freshwater fishes other
than gar that have ganoid scales, and catfish which have no scales) are
either ctenoid or cycloid. Ctenoid scales have jagged edges and
cycloid have smooth rounded edges. Bass and most other fish with
spines have ctenoid scales composed of connective tissue covered with
calcium. Most fishes also have a very important mucus layer covering
the body that helps prevent infection. Anglers should be careful not
to rub this "slime" off when handling a fish that is to be released.
(See Scales for more).
In many freshwater fishes the fins are supported by spines
that are rigid and may be quite sharp thus playing a defensive role.
Catfish have notably hard sharp fins that anglers should be wary of.
The soft dorsal and caudal fins are composed of rays, as are portions of
other fins. Rays are less rigid and frequently branched.
The gills are the breathing apparatus of fish and are
highly vascularized giving them their bright red cover. An operculum
(gill cover) that is a flexible bony plate protects the sensitive gills.
Water is "inhaled" through the mouth, passes over the gills and "exhaled"
from beneath the operculum.
Fish see through their eyes and can detect color. The
eyes are rounder in fish than mammals because of the refractive index of
water and focus is achieved by moving the lens in and out, not distorting
it as in mammals.
Paired nostrils, or nares, in fish are used to detect odors
in water and can be quite sensitive. Eels and catfish have
particularly well developed senses of smell.
The mouths shape is a good clue to what fish eat. The
larger it is the bigger the prey it can consume. Fish have a sense of
taste and may sample items to taste them before swallowing if they are not
obvious prey items. Most freshwater fishes in Florida are omnivorous
(eating both plant and animal matter). Some are primarily piscivorous
(eating mostly other fish). The imported grass carp is one of the few
large fishes that are primarily herbivorous (eating plants). Fish may
or may not have teeth depending on the species. Fish like chain
pickerel and gar have obvious canine-shaped teeth. Other fish have
less obvious teeth, such as the cardiform teeth in catfish which feel like
a roughened area at the front of the mouth, or vomerine teeth that are tiny
patches of teeth, for example, in the roof of a striped bass' mouth.
Grass carp and other minnows have pharyngeal teeth modified from their gill
arches for grinding that are located in the throat.
The lateral line is a sensory organ consisting of fluid
filled sacs with hair-like sensory apparatus that are open to the water
through a series of pores (creating a line along the side of the fish).
The lateral line primarily senses water currents and pressure, and movement
in the water.
The vent is the external opening to digestive urinary and
reproductive tracts. In most fish it is immediately in front of the anal
fin.
Internal Fish Anatomy
The following illustration of a largemouth bass shows some of the
common internal features that are used to describe
the differences between fish that are described in more detail below.
As different as a man may be from a fish, both creatures share
some fascinating similarities in basic
structure and function. And the closer one looks, the
more complex life becomes. The smallest units of
life are microscopic
cells,
and some organisms—such
as an ameba—are no larger than a single cell. In
larger multicellular creatures, individual cells that are
similar in structure and perform a specific function
are grouped into
tissues,
and tissues may be grouped
into even more complex and specialized structures
called
organs. These organs perform the
basic bodily
functions such as respiration, digestion, and sensory
reception.
Man and fish share such organs as the brain,
stomach, liver, and kidneys. Other organs appear in
different forms in different organisms; for example,
the lungs in humans and the gills in fish are very
different but both provide the same basic function of
respiration. Finally, some organs (such as the fish’s
swim bladder) are simply not present in man. Below
are descriptions of some of the organs identified on
the opposite diagram, along with their functions. A
number of other vital organs, such as the spleen and
pancreas, may also be present but are smaller and more
difficult to locate. A largemouth bass destined for the
frying pan makes an excellent specimen because
this species is large enough for easy examination.
For anglers brave enough to do some investigating
while filleting their next fish, a fascinating experience
awaits!
(Note test on internal anatomy is from The
City Fisher (Index), John
Cimbaro editor. Links to specific issues referenced below are to PDF
files, instructions on use are
available, if needed).
SPINE:
The primary
structural framework upon which
the fish’s body is built; connects to the skull at the
front of the fish and to the tail at the rear. The spine
is made up of numerous
vertebrae,
which are hollow
and house and protect the delicate spinal cord.
SPINAL CORD:
Connects the
brain to the rest of the
body and relays sensory information from the body to
the brain, as well as instructions from the brain to the
rest of the body.
BRAIN:
The control center
of the fish, where both
automatic functions (such as respiration) and higher
behaviors ("Should I eat that critter with the spinning
blades?") occur. All sensory information is processed
here.
LATERAL LINE:
One of the
fish’s primary sense
organs; detects underwater vibrations and is capable
of determining the direction of their source. (See
Issue 8 of The City Fisher for more information.)
SWIM (or AIR) BLADDER:
A
hollow, gas-filled balance
organ that allows a fish to conserve energy by
maintaining neutral buoyancy (suspending) in water.
Fish caught from very deep water sometimes need
to have air released from their swim bladder before
they can be released and return to deep water, due to
the difference in atmospheric pressure at the water’s
surface. (Most freshwater anglers in Florida need not
concern themselves with this!) Species of fish that do
not possess a swim bladder sink to the bottom if they
stop swimming.
GILLS:
Allow a
fish to
breathe underwater. These are
very delicate structures and should not be touched if
the fish is to be released! (See Issue 15 of The City Fisher for more
information)
KIDNEY:
Filters liquid
waste materials from the
blood; these wastes are then passed out of the body.
The kidney is also extremely important in regulating
water and salt concentrations within the fish’s body,
allowing certain fish species to exist in freshwater or
saltwater, and in some cases (such as snook or tarpon)
both. (See Issue 11 of The City Fisher for more information.)
STOMACH AND INTESTINES:
Break down (digest)
food and absorb nutrients. Fish such as bass that are
piscivorous (eat
other fish) have fairly short intestines
because such food is easy to chemically break down
and digest. Fish such as tilapia that are
herbivorous
(eat plants) require longer intestines because plant
matter is usually tough and fibrous and more difficult
to break down into usable components. A great
deal about fish feeding habits can be determined by
examining stomach contents. (See Issue 1
of the City Fisher for an
example of a stomach content analysis of the butterfly
peacock.)
PYLORIC CAECA:
This organ
with fingerlike projections
is located near the junction of the stomach and the
intestines. Its function is not entirely understood, but
it is known to secrete enzymes that aid in digestion,
may function to absorb digested food, or do both.
VENT:
The site of waste
elimination from the fish’s
body.
LIVER:
This important
organ has a number of functions. It assists in digestion by secreting enzymes
that break down fats, and also serves as a storage area
for fats and carbohydrates. The liver also is important
in the destruction of old blood cells and in maintaining
proper blood chemistry, as well as playing a role in
nitrogen (waste) excretion.
HEART:
Circulates blood
throughout the body.
Oxygen and digested nutrients are delivered to the
cells of various organs through the blood, and the
blood transports waste products from the cells to the
kidneys and liver for elimination.
GONADS (REPRODUCTIVE ORGANS):
In adult female
bass, the bright orange mass of eggs is unmistakable
during the spawning season, but is still usually
identifiable at other times of the year. The male
organs, which produce milt for fertilizing the eggs,
are much smaller and white but found in the same
general location. The eggs (or
roe)
of certain fish are
considered a delicacy, as in the case of caviar from
sturgeon. (For a related topic, see
Issue
14. of City Fisher)
MUSCLES:
Provide movement
and locomotion.
This is the part of the fish that is usually eaten, and
composes the fillet of the fish.
Common
Measurements
The above image depicts the most commonly used measurements
for fish. All freshwater Florida Fish and Wildlife
Conservation Commission
regulations and the "Big Catch" program depend on
"total length." The total length is the maximum length of
the fish with the mouth closed and the tail fin pinched together.
The best way to obtain this length is to push the fish's snout up against
a vertical surface with the mouth closed and the fish laying along a tape
measure, then pinch the tail fin closed and determine the total length
(see animated illustration), do NOT pull a flexible tape
measure along the curve of the fish.
Conversely, most marine (saltwater regulations) refer to the
"fork length", and scientists often use "standard
length" which is to the end of the fleshy part of the body.
"Standard length" has the advantage of not being affected
by minor damage to the tail fin, nor does it give too much credit to
a fish for the relatively light weight tail when calculating a fish's
condition.
"Girth" is best measured with a fabric ruler, such as
tailors use. It can also be determined by drawing a string around the
fish at its widest point marking where the string overlaps and then
measuring the distance between the overlapping points on a
conventional ruler. Knowing the girth is important when trying to
certify a fish for a record, and provides useful information to
biologists about the relative condition of a fish.
Using total length and girth you can get a rough estimate of a fish's
weight using various formulas.
If you would like more information, Sea
World has a nice site about bony fishes, their anatomy and
physiology. The Florida
Museum of Natural History also has an outstanding site.
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External
Anatomy