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Biology 11: Dogfish Dissection

Introduction: In this lab you will explore and observe the internal and external structures of the dogfish (Squalus acanthias) anatomy. Dogfish are cartilaginous fish.

Your sharks have been double injected with latex to make identification of internal structures easier. Red latex indicates arteries and blue veins.

Background: Dogfish are one of the most common types of migratory sharks, travelling in large schools. Dogfish can reach sexual maturity around 6-12 years old and can live

up to 30 years. They eat small fish and marine animals and are eaten by larger fish and marine mammals. Dogfish have a poisonous spine located on the dorsal fin for protection against predators.

Materials:

Preserved Dogfish Dissecting Tray Dissecting Guide Safety Glasses

Gloves Lab Apron Scalpel Scissors

Probe Tweezers Dissecting Pins

Procedure:

Through this dissection you will dissect and analyze the various body systems that characterize your dogfish (and sharks in general). You will start out with the external features of your dogfish and work your way inwards.

Follow the instructions for each system below and use the checklist (at the end of the package) and dissecting guide to identify each systems’ structures.

At the end of each system call Mr. Fox over to make sure that you have identified everything correctly.

Biology 11: Dogfish Dissection Lab

Part A: Dogfish External Anatomy

1. Place your dogfish on your dissecting tray and identify the structures listed below:

a) Rostrum: Pointed snout.

b) Nostrils: Water is taken into the smaller of these openings, passes by a sensory membrane that detects chemicals and then expelled out the larger opening.

c) Spiracles: Small openings just behind the eyes. Allow water to pass over the gills even if the mouth is closed.

d) Mouth: Functions in obtaining food and allowing water to pass over the gills.

e) Gill Slits: Five vertical slits allowing water to exit the gills.

f) Lateral Line: Pale line running from the pectoral fin past the pelvic fin on the side of the shark. Is a collection of small pores leading to sensitive nerve receptors below the skin that sense motions in the water.

g) Cloaca: Exit for the digestive tract and entrance for the reproductive system.

h) Clasper: Found only in male sharks, these extensions of the pelvic fin are used to transfer sperm to female sharks during mating.

i) Dorsal Spines: Located just behind each dorsal fin and contains a toxin to ward off predation.

j) Placoid Scales: Modified teeth that cover the shark’s skin.


2. a) Run your hand from head to tail and then tail to head. Do you notice any difference in texture?

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b) What benefit might the configuration of the placoid scales have to the shark’s swimming ability?

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2. Based on the shape and location of each fin, what type of function or movement do you think it might provide the shark?

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3. How would the spiracles benefit a bottom-dwelling shark or ray?

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4) What adaptations do dogfish have for their lifestyle?

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Part B: Dogfish Internal Anatomy

1. Place your dogfish ventral side up on the dissecting tray. You will need to remove the spines using scissors before you do so.

2. Use a scalpel (or scissors if the skin is too tough) to create three left to right incisions: One just posterior to the mouth, one just posterior to the pectoral fins and the third just anterior to the pelvic fins. Be careful not to cut too deeply or else you will damage the organs.

3. Use a scalpel or scissors to continue the existing ventral incision up to your anterior horizontal incision and down to your posterior horizontal incision. Cutting through the pectoral and pelvic girdles will be tough and likely require the use of scissors.


4. Either remove the flaps of skin you have created or pin them back using dissection pins. This should let you view the organs in the shark’s two body cavities: pericardial cavity (containing the heart) and pleuroperitoneal cavity (containing the digestive organs).

Digestive Tract

-The digestive system is responsible for chemically and mechanically breaking down found into smaller compounds that can be released into the bloodstream and transported to body cells.

-To better see the digestive system cut from the middle of the lower jaw on a diagonal towards the gills. Make sure that you are cutting around the heart!

-Identify and pin the following structures involved in the digestion of food in the mouth and transfer of these materials to the stomach:

1. Teeth: Derived from the scales that cover the shark’s body! Adapted to function as cutting structures and are replaced regularly as they wear out.

2. Pharynx: Cavity posterior to the spiracles. Gill slits open onto either side of the posterior portion of the pharynx. Gill rakers branching off the gill arches prevent large food particles from entering the gills from the pharynx.

3. Esophagus: Short connection between the phanrynx and the stomach. It is very short and wide in sharks.

4. Stomach: This J-shaped organ is composed of two portions, the cardiac (portion near the heart) and limb (portion after the bend in the stomach), and ends in the pyloric sphincter, a muscular ring that can be felt.


5. It can be difficult to tell the esophagus and stomach apart by their external features, but is very easy if you view their internal structures. Create an incision through the ventral wall of the esophagus and stomach and open both organs. You may need to remove part of the liver before you do this to get better access to the esophagus and stomach.

a) What were the stomach contents?

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b) Wash out the contents of the stomach if necessary and examine the lining of the esophagus and stomach. The esophagus is covered in papillae projections, whereas the stomach is lined with deep folds called rugae. Based on your observations, which organ is more suited for mechanical digestion? Why?

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6. Duodenum: First section of the small intestine immediately after the stomach. Digestive fluids from the gallbladder (bile, originally produced by the liver) and pancreas (pancreatic juice) are secreted into it and they chemically digest food. Locate the liver, gallbladder and pancreas.

7. Spiral Intestine: Second section of the small intestine. Distinguishable by an extensive network of arteries and veins on its surface.

-Cut open the spiral intestine and observe its interior structure. What do you think is the purpose of the spiral valve? How do humans accomplish the same thing in our intestines?

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8. Colon: Third section of the small intestine. It ends in the anus, which opens into the cloaca.

9. Rectal Gland: Finger-like structure leading into the colon that


excretes salt in order to regulate it’s concentration in the blood.

*Get approval from Mr. Fox before you carry onto the next section!

Respiratory and Circulatory System

-The respiratory system functions in gas exchange and the circulatory system in the transportation of respiratory gases, nutrients, hormones, defensive cells and wastes throughout the body.

1. The gills should already be exposed from your previous explorations, but if they are not trim some of the excess tissue out of the way using your scissors.

2. When water enters through the shark’s mouth or spiracles it travels into the internal gill slits to the gill pouches, which contain the gill lamellae and secondary gill lamellae that are responsible for gas exchange. The water then exits the shark’s body through the external gill arches.

3. How are the gill lamellae modified for their function?

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4. How are the gills protected from damage from materials that may be inside the shark’s mouth?

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5. Once oxygenated, blood travels from the gills through arteries to the rest of the body. Make sure that you see the high degree of vascular tissue (arteries and veins) surrounding the gills!

6. Now examine the pericardial cavity and the two chambered heart. It may be necessary to gently remove some tissue or membrane obscuring the organ.

*More on the next page!


6. Identify and pin the following heart structures. You may remove the heart and cut it open to see its different chambers.

a) Sinus Venosus: A thin walled, -non-muscular sac that collects deoxygenated blood from the body.

b) Atrium: Collects deoxygenated blood from the sinus venosus.

c) Ventricle: Main contracting chamber of the heart.

d) Conus Arteriosus: Muscular reservoir that empties after the ventricle contracts. It adds pressure to the blood to send it to the gills.


Urogenital System

-The urogenital system includes both the reproductive and excretory organs and structures. These systems are studied together as they share common ducts.

Excretory System

-The excretory system filters wastes from the blood, producing urine.

-You will need to remove the digestive tract from its anterior end and the digestive tract from its anterior end (esophagus) and posterior end (colon).

-Identify and pin the following excretory structures:

1. Kidneys: These two organs extract urea from urine and returns it to the blood. This ensures that the shark’s body fluids have the same amount of solutes as the surrounding seawater. This prevents seawater from trying to invade the shark’s tissues and cells (which would kill them).


2. Rectal Gland: Finger-like organ leading into the colon. It secretes excess salt into the colon for expulsion from the body.

Reproductive System

-The reproductive organs are responsible for the production of egg and sperm and for fertilization.

Male Sharks

-Identify and pin the following structures:

1. Testes: Produce sperm. Oval shaped and located next to the kidneys and on either side of the stomach.

2. Epididymis: Anterior portion of the kidney that receives the sperm from the testes. *Too small to see.

3. Vas Deferens: A highly coiled tube in adult sharks (straight in juveniles) that carries sperm to the seminal vesicle.

4. Seminal Vesicle: Enlarged section of the Vas Deferens that adds secretions to the sperm.

5. Sperm Sacs: Receives the sperm containing fluid and passes it through the cloaca to exit the body.


6. Claspers: Modified section of the pelvic fin that transfer the sperm into the female cloaca. Sperm transportation is facilitated by dorsal grooves in the claspers and lubricating fluid secreted by the siphon

sac. *Get approval from Mr. Fox before you

carry onto the next section!

Female Sharks

-Identify and pin the following structures:

1. Ovaries: Produce the eggs. Found on either side of the kidneys and actually looks quite a bit like the liver. In immature sharks they are smooth and small. If you shark is mature sharks the ovaries may contain eggs.

2. Oviducts: Elongated tubes that contain the eggs when they are ready to be fertilized.

3. Shell Gland: Eggs pass through here on their way to the uterus. The gland secretes a thin membrane around fertilized eggs. *Too small to be seen.

4. Uterus: Enlarged end of the oviduct where eggs develop. As the offspring develop they are attached to a yolk sac. You will need to cut through the pelvic girdle to see these structures.



Nervous System: The Brain

1. Remove the skin from the dorsal section of the head (you will need to flip your shark onto its ventral side to do this).

2. With your scalpel, carefully shave the shark’s cranium down from the rostrum to the back of the gill slits to expose the brain, olfactory lobes and major brain nerves. Make sure you shave of thin sections so that you don’t cut into the brain or nerves! *See image below!

3. Now that you’ve exposed the nervous system, you should be able to identify and pin the following:

a) Olfactory Sacs: Two large bulbous nerve sensors that detect chemical changes in the water. Reached through the nostrils.

b) Olfactory Lobes: Area of the brain that receives and processes nerve signals from the olfactory sacs.

c) Cerebrum: Two hemispheres between the olfactory lobes associated with sight and smell.

d) Optic Lobe: Prominent lobes in the mid-brain that receive nerves from the eyes.

e) Cerebellum: Just posterior from the optic lobes. Control muscular coordination and position.


f) Medulla Oblongata: At the base of the brain. A widening of the spinal cord that controls many spinal reflexes.

Analysis Questions

1. Identifying Relationships What shark organ that you

observed is most closely related in function to a human

ear? Explain. (3 Marks)

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2. Identifying Relationships What are two evolutionary advantage jaws provide cartilaginous and bony

fish? (2 Marks)

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3. Identifying Relationships Sharks do not possess a swim bladder. How do they regulate their

bouyancy? (2 Marks)

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4. Interpreting Information What adaptations for life in the water did you observe in the shark? Name

three and describe what that adaptation does. (3 Marks)

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5. Drawing Conclusions Describe the differences between a gastrovascular cavity and a complete one-

way digestive system. Which type of digestive system do fish have and why is it considered to be an

evolutionary advance? (5 Marks)

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6. Making Predictions Predict how a fish’s behaviour would change if it had an open circulatory system

instead of a closed circulatory system. (2 Marks)

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