Internal Anatomy
Introduction
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 subclass 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 class 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 quiz requiring you to successfully identify and describe these
structures.
Tasks
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
Teeth
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 bodied 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.
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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.
Intestine
Used for nutrient absorption and fecal formation, all material is moved through by peristaltic muscle
contractions. Interestingly, intestine length varies with feeding habits. Predators have a shorter intestine per
body length (1/3 -3/4 of body length) while herbivores and detritus feeders have longer digestive tracts (2-20x
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 Chondrichthyes 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 to 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.
Buoyancy
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
Kidneys
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. The kidneys are also vital in osmoregulation. Marine fishes have a kidney which creates highly
concentrated urine to offset their battle against dehydration, while freshwater fishes produce profuse amounts of
urine to avoid becoming waterlogged.
Gills
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 ichthyologists 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.
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Heart
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
Gonads
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.
Testes
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.
Ovaries
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.
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Internal anatomy of a typical bony fish
Fishes (Cailliet et al.)
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Digestive system of a bony fish
Fishes (Cailliet et al.)
A. Lateral view of a fish heart showing four
chambers
B. Ventral view of the heart
Fishes (Cailliet et al.)
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