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