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 cold blooded creatures with fins and a backbone. Most fish breath through gills that remove oxygen as water passes through them. Fish use their fins to move and to stop much like we use our legs and feet. Some fish have single fins that run the center of their bodies, such as dorsal fins(on the back) or caudal fins (their tail fins) or paired fins  such as the pelvic fin, These fins are used for steering and stopping the fish.

Scales in some 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 fish also have a mucus layer covering the body that helps prevent infections. When catching and releasing fish, fisherman should be careful not to remove this layer.

Fish can see through their eyes and can detect colors on objects in the water. A fish has rounder eyes than mammels do because they spend their life in th water and have a different reflective need than land animals.

Do fish have a sense of smell? Yes they can.They have paired nostrils, or nares. A fish uses them to detect odors in water, Fish like Eels and catfish have particularly strong senses of smell.

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 man and fish have some fascinating similarities in basic structure and function. And the closer you look, the more detailed animals become. We do share some of the same organs as fish 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. Also some organs (such as the fish’s swim bladder) are  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!

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.

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!

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.

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.

Muscles:

Provide movement and locomotion. This is the part of the fish that is usually eaten, and composes the fillet of the fish.

Information and illustrations on this page in part, courtesy of the Florida Fish & Wildlife Conservation Commission