External Anatomy Internal Anatomy
Common Measurements How To Measure Fish

 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.

External Anatomy

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.

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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.

internal bass anatomy

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).
 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.
 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.
 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.
 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.)
 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.

 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)
 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.)
 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.)
 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.
 The site of waste elimination from the fish’s body.
 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.
 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.
 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)
 Provide movement and locomotion. This is the part of the fish that is usually eaten, and composes the fillet of the fish.

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Common Measurements
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.


Black Bass Stripers Panfish Catfish Pickerels
Gar Other Threatened Name List Anatomy

Restricted Fishes
Prohibited Fishes