3 Sharks and FishSharks are fish. But unlike the bony fish they do not have gas bladders. This means that if they stop swimming, they sink.
4 Myth: if a shark stop swimming it dies. Truth: There are some sharks that live in open water and because sharks don’t have gas bladders if they stop swimming they sink. If the ocean floor is too deep the pressure may crush the shark.
5 Ion BalanceSharks release urea and other solutes into their body which keeps their internal fluids at relatively the same salinity as the ocean. This means that they do not need to ingest water or find a source of salt.
6 BirthSome sharks, like the dogfish shark are ovoviviparous. This means that they lay “eggs” inside their own body. The babies hatch inside the mother and continue their development until they are ready to enter the ocean as smaller versions of their parents.
7 Quick Facts about Dogfish Sharks Found Worldwide, from temperate to subpolar.Extremely destructive to fishing industry.Lifespan is yearsGestation is up to 2 yearsSpecies name “acanthias” refers to mildly poisonous spines on the dorsal fin
8 Terminology Anterior – toward the head Posterior – toward the tail Dorsal – toward the backboneVentral – toward the bellyLateral – toward the side
9 Overall DescriptionFusiform – shape of the body is streamline. Built for swimming in the sea with least possible resistanceThe Head (cranial) – from the point of the rostrum to the pectoral fins.The Trunk – from the pectoral fins to the pelvic fins.The Tail (caudal) – from the pelvic fin to the end of the caudal fin.
10 SkinMade of Placoid scales, or scales that are modifications of teeth. Their structure and development are similar to teeth.Fun Fact: Shark skin has been used as an abrasive in furniture building and to coat the hilt of swords and handles of tools.
11 Body Color Sharks are dark above and much lighter below. This is referred to as counter shading and is common to aquatic vertebrates.This tends to neutralize the effects of natural lights which come from above.
12 Lateral LineOn the side of the body near the dorsal surface is a light-colored stripe. This is the lateral line, below the surface is a line of nerve receptors that have pores opening to the surface. These receptors are sensitive to the mechanical movement of water, disturbances in the water, and sudden changes in pressure. This warns sharks of vibrations and movements even when visibility is reduced.
13 Ampullae of LorenziniThese are patches of pores on the head near the eyes, snout, and nostrils.They are sense organs which are sensitive to changes in temperature, water pressure, electrical fields, and salinity
14 Nares These are the openings for the external nostrils. Located on the ventral surface of the rostrum this is where water is drawn into the nares to moisten the sensory cells of the olfactory sac.Water passes into the and out of the olfactory sac, permitting the shark to detect the odors of the water.
15 JawsA testing device, called the gnathodynamometer, was used to measure the force exerted by the jaws of a typical eight-foot shark.The measured force came out to 18 tons per square inch!Beside the visible teeth you will find several rows of flattened teeth ready to replace the visible teeth when they are lost or worn out.
16 SpiraclesThese large openings posterior and dorsal to the eyes are actually reduced first gill slits.These serve as incurrent water passageways leading into the mouth.This allows water to be brought in for respiration even when the shark’s mouth is closed or when it is feeding.
17 Gill Slits Most sharks have 5 external gill slits. Water taken in by the mouth is passed over the internal gills, oxygen is removed and carbon dioxide excreted.The water is then forced out of the body by way of the gill slits.
18 Dorsal Fins Dogfish sharks have two dorsal fins. A feature peculiar to spiny dogfish, is the presence of two spines. One anterior to each of the dorsal fins.These spines are used when captured to inflict puncture wounds and carry poison secreted by glands at their base.
19 Caudal Fin Each tail fin is divided into two lobes. Dogfish sharks have a type of tail known as heterocercal.HeterocercalHomocercal
20 Pectoral and Pelvic Fins Pectoral fins: located at the anterior end of the shark these are vital for the necessary lift to keep a shark horizontal.Pelvic fins: located at the posterior end of the shark. These are different in males and females.
21 Cloaca Cloaca means “sewer” This is the chamber between the pelvic fins. It receives the products of the intestine, the urinary and the genital ducts.
22 ClaspersFound only on the males these are extensions of the pelvic fin.They are used during copulation.
24 Dissection Procedures During Dissection rely heavily upon your dissecting needles, probes, and fingers rather than your scalpel.When using scissors advance with the rounded end and not the sharp pointed end.Dissecting Needles
25 Dissection Procedures Identify the following:Pectoral FinsPelvic FinsCaudal FinDorsal Fins
26 Dissection Procedures Run your hand over the body of the shark from head to tail.Now try the other way.
27 Dissection Procedures Look for the lateral line.It will be a light-colored horizontal stripe on the side of the body nearer the dorsal surface than the ventral surface.Use a magnifying glass and observe the pores.
28 Dissection Procedures Locate the ampullae of LorenziniThese will look like black heads on the nose of the shark.Press firmly upon the skin near the nares (nostrils).Note the jelly-like material you have squeezed out of the pores.
29 Muscle Later, if you have time, remove the skin from the gills Make a shallow incision into the skin along the spine of the dogfish just behind the second dorsal finContinue to cut caudally for about two inchesAt each of the ends cut the skin ventrally along the sides of the body until you reach the middle of the bellyUsing a blunt instrument remove the section of skinMake observations about the muscleLater, if you have time, remove the skin from the gills
30 Opening the Body Cavity Turn your specimen ventral side upMake an incision anterior to the cloacaCut through the skin and muscle in an anterior direction just to the right of the middleContinue your cut until your reach the anterior portion of the pectoral finsUse your scissors and cut laterally toward the right and leftReturn to the cloaca and cut laterally toward the right and left
31 The Liver The largest organ in the cavity is the liver. Has 3 lobes. There are 2 main lobes,the right and left and a third median lobe that is much smaller located in the middle.The gall bladder is the green sac on the right edge of the median lobe.
32 Esophagus Move the large lobes of the liver laterally. Behind them you will find a thick muscular tube coming from the top of the cavity and extending posteriorly to the left.This is the esophagus.
33 StomachThe esophagus leads to the “J” shaped stomach and ends in the “U” shaped duodenum.Cut the stomach open along its axis. Open it and note its contents.The duodenum is the first portion of the small intestine and is where the gall bladder and bile duct enter the digestive system.
34 Pancreas and SpleenThe pancreas is the whitish glandular tissue that partially obscures the duodenum. The larger part is found on the dorsal side.The spleen is found near the posterior end of the stomach and looks like a dark triangle.
35 Vascular IntestineThe next part of the small intestine is easily found because it is marked by rings.Cut away the outer tissue to find a symmetrical spiral shape within, called the spiral valve.This spiral shape adds surface area to increase digestion and absorption.Higher vertebrates have finger-like projections called villi.
36 Non-Digestive OrgansGonads: these can be found by moving the liver and digestive organs to one side and looking to the anterodorsal portion of the body cavity.The kidneys are the elongated structures running the length of the body on either side of the mid-dorsal line.
37 Opening the Oral Cavity Insert your scissors into the side of the shark’s mouth.Begin cutting posteriorly through the angle of the jaws across the gill slits as far back as the pectoral girdle.Cut across the bottom of the sharks mouth.Then open the mouth and lay it flat.
38 Oral Cavity Note the teeth Find the tongue. It is not a true tongue and is practically immovable.Find the spiraclesFind the gills
39 The Circulatory System Back in the body cavity find the heart it will be located between the pectoral fins.It may be hidden beneath a thin membrane.
40 Enjoy the rest of your time I suggest that you look at the eye.
41 Shark Dissection Analysis Boney fish have swim bladders that allow them to remain buoyant. How have sharks adapted to the same situation?How have sharks overcome the dangers of leaving offspring in the open ocean to develop?Why is the fusiform body shape an important adaptation in sharks? Would that be beneficial in a terrestrial ecosystem?
42 Shark Dissection Analysis Why is the Lateral Line an important adaptation for fishes?The counter shading that sharks have is an important adaptation in a marine ecosystem. Would this adaptation be beneficial in terrestrial or freshwater ecosystems? Why?
43 Created by Scott Johnson and Pam Simmons-Brooks Sloan Creek Middle School, October 2010