Spring 2007
Marine Vertebrate Biology
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 test 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 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.
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Spring 2007
Marine Vertebrate Biology
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.
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).
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Spring 2007
Marine Vertebrate Biology
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 a 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 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.
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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Spring 2007
Marine Vertebrate Biology
Internal anatomy of a typical bony fish
Fishes (Cailliet et al.)
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Spring 2007
Marine Vertebrate Biology
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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