Internal Anatomy Introduction

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Internal Anatomy

This laboratory exercise will focus on the internal anatomy of bony fishes. Since most fishes have the same organ arrangement, the dissection of several species is not needed. Although we will only be examining members of the class Actinopterygii it should be noted that there are some differences between the different classes of fish. Obviously, the biggest difference is the supportive framework. The jawless fishes (hagfish and lamprey) and all the members of the class Chondrichthyes (sharks, skates, ray, and chimaeras) have a cartilaginous skeleton, while the representatives of Osteichthyes have a bony skeleton. In addition, all representatives of the class Chondrichthyes lack a gas-filled swimbladder, with pelagic individuals relying instead on a large oil-filled liver for buoyancy. After today, you will realize that most bony fishes have a swimbladder that dominates the body cavity. There are numerous other differences between the classes many of which will be covered during lecture.

As a marine technician it is vital that you become comfortable with the dissection and manipulation of fishes. Many physiological studies involve numerous postmortem dissections. Additionally, fisheries technicians are often required to remove the digestive tract for diet analysis and gonads for reproductive estimates. You must be able to identify these structures and remove them quickly to be a valuable asset for an employer.
The focus of this lab is to perform a complete dissection and correctly identify major organ systems and their components. Below is a list with descriptions of the organs you should be able to identify after completing the exercise. Remember, there will be a test requiring you to successfully identify and describe these structures.

  1. Using the attached schematics, identify the structures below

  2. Remove the first gill arch and count the number of gill rakers

  3. After the dissection is complete, remove several structures of interest and examine them further with a dissection microscope (i.e.: gut contents, heart).

  4. When finished, remove the head just behind the operculum and save it in a separate bag.

Digestive system


Starting at the mouth we can easily identify the teeth located on both the upper (maxilla and premaxilla) and lower jaw (mandible). Sharp, pointed teeth are used for puncturing and impaling prey and typically characterize predators that feed on soft bodies animals (i.e. other fish and shrimps). Flat, spade shaped teeth characterize benthic predators who require grinding teeth to crush and process hard-shelled invertebrates.

Mouth and Pharynx

The mouth is where food is manipulated before being passed to the buccal cavity and then the pharynx. Remember, this area is distensible so choking is rare, but a miscalculation of prey size can kill the predator. The buccal cavity often contains the first set of gills and delineates the mouth from the pharynx. The pharynx contains the majority of the gill arches and depending on the species, pharyngeal teeth. These have a similar shape and function as our molars. They are used for additional grinding before food reaches the stomach.


In many fishes the stomach is difficult to discern from the intestine. If present this small sac is similar in function to our stomach. Muscular and lined with cells that secrete pepsin and hydrochloric acid this is the site of chemical digestion.


Used for nutrient absorption, and fecal formation all material is moved through peristaltic muscle contractions. Interestingly, intestine length varies with feeding habits. Predators have a shorter intestine per body length (1/3 -3/4 body length) while herbivores and detritus feeders have longer digestive tracts (2-20 body length). This is correlated to food quality and the surface area needed for digestion. Remember from our class discussions, some members of the class Chodrichthyes have a short intestine but increased surface area due to a spiral valve.

Pyloric caeca

Typically, these fingerlike projections aid in digestion and absorption. They are closely associated with the intestine and can vary in number from hundreds or completely absent. The number of pyloric caeca is used in some species classifications, particularly the salmon.

Liver and Pancreas

These structures function in a similar way to our respective organs. They are both associated with the synthesis of enzymes used in digestion. The liver should be easy to locate with its massive size dominating the anterior portion of the body cavity. In most bony fishes the pancreas is incorporated into the liver and is difficult to identify.

Gas Bladder

In most bony fishes the swimbladder (gas bladder) dominates the body cavity. Located between the digestive system and kidneys, this air filled sac may still be inflated if a careful dissection was conducted. The function of the swimbladder is for buoyancy regulation; primitive fish inflate the swimbladder through the gulping of air at the surface (physostomous). Advanced fish use a capillary system (rete mirabile) and gas gland connected to the blood stream to regulate the volume of their swimbladder (physoclistous).
Circulatory system and respiration


The kidneys clean the blood by removing metabolic waste and creating urine. Unlike advanced vertebrates the kidneys in fish are not globular organs but instead form a thin, soft layer between the gas bladder and vertebrae. As we will discuss in class the kidneys are also vital in osmoregulation. Marine fishes a have a kidney which creates a highly concentrated urine to offset their battle against dehydration, while freshwater fishes produce profuse amounts of urine to avoid becoming waterlogged.


Located under the operculum the numerous gill arches function in respiration, osmoregulation and the removal of metabolic wastes. The gill filament is suspended from the gill arch posteriorly and is highly folded to increase surface area. Anteriorly, the gill rakers point into the mouth and are used by ichthyologist for classification. Typically, predatory fishes have short gill rakers; while filter feeding fishes have numerous, long gill rakers that are used to capture planktonic prey.


The heart is located just below and between the left and right gill complexes. Covered in a pericardial sac the heart contains four chambers, which lie in a series. Blood moves through the heart in one circuit. Heart  Gills  Body  Back to the heart.

Accurate sex determination in the field is critical in many reproduction studies. Many species are not sexually dimorphic and therefore require inspection of the gonads for sex identification. Below are some general rules for differentiating between testes and ovaries. Remember, immature individuals are difficult to identify and usually require histological examination of the gonads.


When viable these paired structures can swell significantly and constitute up to 30% of the fish’s total body weight. Testes are usually flattened, folded around the edges and white in color. Mature individuals who are not spawning have testes with a brown color.


When spawning the ovaries can constitute up to 70% of the fish’s total body weight. Ovaries can vary in color from pink, yellow or orange and usually have a tubular look. Mature ovaries are loaded with eggs contained in a clear sac.

Internal anatomy of a typical bony fish

Fishes (Cailliet et al.)

Digestive system of a bony fish

Fishes (Cailliet et al.)

  1. Lateral view of a fish heart showing four chambers

  2. Ventral view of the heart

Fishes (Cailliet et al.)

Internal Anatomy

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