Download Dissection of the Common Mackerel Objectives

Dissection of the Common Mackerel
Objectives
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Identify the scientific and common name of the fish you are dissecting.
Determine the sex of the fish.
Determine the age of the fish.
Locate and name the main features of the fish.
Locate the main divisions of the fish musculature.
Locate the gills, show the main parts of a gill, and explain the function of each part.
Locate the heart of the fish and locate its main parts.
Identify the visceral organs.
Identify the main features in a urogenital system of a mackerel.
Materials
One fish
Field guide
Wash basin
Microscope slide
Cover slip
Medicine dropper
Probes
Dissecting scissors
Dissecting scalpel
Procedure
First lay the fish out on a large wash basin so that the left side is facing you. Use the
field guide to identify your fish and to identify the characteristics of your fish. Because
it is a fish, the phylum is Chordata and the subphylum is Vertebrata. Since it is a bony
fish with a skeleton, it is in the class Osteichthyes. Mackerels belong in the
Scombridae family.
1. Determine the genus and species, as well as the common name. Record your
answer. Include the scientific name, its common name, a brief description of the fish, its
habitat, its distribution (range), and any other interesting information.
Scales
Examine the scales. Most fish are covered with scales and a covering of mucus, which
serves to protect them from parasites, abrasion, and infection. The mucus also aids the
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mackerel by lubricating the fish to cut down friction. This saves the mackerel valuable
energy. Mackerel swim at high speeds and it is important to have the least amount of
frictional resistance when they are swimming. Mackerel possess ctenoid scales that
almost universally characterize the spiny rayed bony fishes.
You can determine the age of the fish by examining a scale. Scales grow as the fish
grows. Every year many bands of growth called circuli form around the older portion
of the scale. Fish are cold-blooded (ectotherm) and their growth rate is affected by the
water temperature, as well as by the amount of food eaten. Summer growth is usually
faster than winter growth. So, the summer circuli are more widely spaced. During the
colder winter months, the fish’s growth rate slows down. Therefore, the circuli that are
formed in the winter are much closer together. The winter circuli, because they are so
close together, make a dark ring that appears on the scale. The ring is called an annulus.
By counting the number of annuli, you can determine how many years the fish has
lived. For example, if you can count three rings of annuli, starting from the focus
(where growth starts), on your fish, then the fish has lived through three winters, and
consequently the fish is three years old. Sometimes the annuli are hard to see clearly.
This may be because the fish has been under environmental stress. Fish that live under
environmental stress may not exhibit normal body growth and scale growth.
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Now find the area below the notch in the dorsal fin about midway between the dorsal
and ventral surface. This is about in the middle of the left side of the fish and is also the
area about at the end of the pectoral fin when it is pulled straight back. Be certain that
you are below the lateral line.
Use forceps to grasp the toothed end of three scales in the area previously described.
Rub the scales between your fingers. This will bring you good luck for three years. Just
checking to see if you were actually reading this. This actually should be done under
running water to remove the mucus. Rinse the scales in a watch glass to remove any
dirt and grime.
Make a wet mount slide with at least two of your scales. Examine the slide using the
lowest power on a compound microscope. Compare your scales with the above
diagram of the ctenoid scale.
2. Select the best scale and draw it. Label the circuli, each annuli, the spiny posterior
portion, and the anterior portion. Determine how old your fish is and record its age.
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External Anatomy
Study your mackerel. Be one with it. Note the streamlined fusiform (spindle-shaped)
body, which is thickest one-third of the distance from the mouth to the tail and tapers in
both directions. Pick up the fish and look directly at the mouth from the front. Observe
the ovoid cross section of the fish.
3. How would this shape facilitate movement in the water?
Identify the three regions of the body: the anterior head, which extends to the rear of
the bony operculum covering the gills; the trunk, extending from the operculum to the
anus; and the caudal region, extending from the anus to the posterior.
Find the two double nostrils, eyes (no eyelids), and the large mouth equipped with
teeth on the head. A bony operculum covers the gills on each side of the head. Under
each operculum are four gills. The operculum is attached at the front and on the dorsal
side, but is open behind and ventrally for the release of water.
Observe the small fins attached to the body: four unpaired median fins (two dorsal fins,
one anal fin, and one caudal fin) and two sets of paired fins (two pectoral fins and two
pelvic fins). All of the fins are membranous extensions of the skin supported by
numerous fin rays. The fins are important aides in swimming; they help stabilize the
fish and direct its movements through water.
One each side of the fish, extending from the operculum behind the eye to the base of
the tail, is the lateral line. The lateral lines are specialized sense organs, which detect
vibrations and current direction. They appear to aid fish in orientation, help to avoid
obstacles, and avoiding predators.
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4. Draw an external diagram of your fish. Give your drawing a title. Be sure to
include the head, trunk, caudal region, eyes, mouth, nostrils, operculum, gills, fins, and
finlets (five dorsal and five anal fins right before the caudal fin).
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Dissection
Make a longitudinal cut along the ventral abdominal wall with the scalpel. Start the
incision just anterior of the anus and carefully up to the level of the pelvic girdle. After
making the mid-ventral incision, make a second incision from the posterior of the first
incision and cut dorsally to the level of the lateral line. Make a similar incision from the
anterior end of the mid-ventral incision. Raise this portion of the body wall.
You should now be able to clearly view much of the internal parts of your fish. Notice
the muscular system in your fish. It is relatively simple compared to most vertebrates.
Most of the body musculature consists of segmental muscles. Contractions of these
muscles, from the front to the rear, result in the flexing of the body, which aids in
swimming. Locomotion is made possible by muscle contractions on the bone. Next to
these muscles is a myoseptum of connective tissue. These myoseptum separate the
myotomes. A horizontal septum separates the upper muscles (epaxial) and the lower
muscles (hypaxial). Fish have a higher percentage of muscle than any other animal.
Now cut through the peritoneum (the thin transparent tissue that lines the abdominal
cavity) if you have not already done so. Observe the large liver (*1 in photo below).
Beneath the liver, locate the stomach (*5 in photo below) and the small intestine (*5 in
photo below). Posteriorly, the small intestine empties into the large intestine, which
terminates at the anus. The pyloric caeca (*4 in photo below) connect to the large
intestine and aid in digestion. Dorsal to the digestive tract, find the large swim bladder
(*2 in photo below). The swim bladder is a hollow, gas filled sac, which serves as a
buoyancy organ. Adjusting the volume of gas within the swim bladder compensates for
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the differences in specific gravity between its body and that of the surrounding water
while swimming at various depths.
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Two long, dark kidneys are above the swim bladder. Other organs in the peritoneal
cavity include the spleen (an elongate organ lying along the posterior surface of the
stomach), the pancreas (on the ventral surface of the intestine), the gonads (posterior to
the stomach and dorsal to the intestine; (*3 in photo above), and the urinary bladder
(posterior to the gonads).
The kidneys filter nitrogenous wastes from the blood and empty posteriorly through
the archinephric ducts, which lead to the urinary bladder. From the bladder, urine
passes into the urogenital sinus and out through the urogenital pore. In males, the
urinary pore and the genital openings are separate. In females, there is a common
urogenital pore through which both systems empty.
5. Determine the sex of your fish by observing the urogenital cavity. The urogenital
cavity is located in the posterior right portion of the fish. It lies directly under the
second dorsal fin. Record the sex of your fish here.
Now draw your attention towards the elements of your fish’s skeletal system. The
scales, the fin rays, and some of the bones on your skull represent elements in the
dermal exoskeleton. The chief supporting structure of the body consists of the bony
exoskeleton. Locate the axial skeleton, which consists of the skull, the vertebral
column, the ribs, and the medial fins. The appendicular skeleton is made up of the
pectoral girdle, the pectoral fins, and a small pelvic girdle, which supports the pelvic
fins. Locate it. The vertebral column is made of many individual vertebrae. The trunk
vertebrae have a large cylindrical centrum with a dorsal neural arch (through which the
dorsal nerve chord passes) and a single neural spine.
6. Make a drawing of the internal anatomy of your fish. Give your drawing a title.
Label the epaxial and hypaxial muscles, myoseptum, stomach, small and large
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intestines, swim bladder, liver, gonads, urinary bladder, urogenital pore, and the
vertebral column. Use the figures below to help you label your diagram.
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Now carefully cut through the bony operculum (which protects the operculum) from
one side of the mackerel to expose the gills. Locate the four gills within the gill chamber.
Observe the numerous fingerlike gill filaments extending posteriorly from each gill. The
large surface area of these filaments facilitates gas exchange with the capillary beds
within each filament. Mackerel require a large supply of oxygen, so the surface of your
fish’s gill filaments are ten times larger than the outside of the fish.
Remove one gill and locate the bony gill arch, which supports the gill and the hard,
finger-like projections, the gill rakers, which protect the gills and prevent the passage of
coarse material through the gills. Each filament consists of many thin lamellae, which
contain the capillaries and provide a large surface area for gas exchange.
7. Draw a diagram of the removed gill. Label the gill arch, gill rakers, and gill
filaments. Observe your gill using a dissecting scope. Notice the lamellae.
The heart is located in the pericardial cavity, which lies ventral to the gills and anterior
to the pelvic fins. Carefully cut through the pectoral girdle, a bone that attaches the
backbone of the fish to the pectoral fins, and the muscles anterior to the girdle. Cut
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away part of the lateral body wall to expose the heart and main blood vessels. This will
be difficult to see. Try to locate it anyways, unless you want to be like a hagfish.
The mackerel has a two-chambered heart with a thin walled atrium and a thick walled,
muscular ventricle. Blood passes from the sinus venosus to the atrium and from the
atrium to the muscular ventricle. Contraction of the ventricle forces the blood into a
short conus arteriosus (or bulbus arteriosus) and out through the short ventral aorta.
From the ventral aorta, the blood passes to the gills via four different pairs of branchial
arteries. Once the blood is oxygenated in the gills, arteries carry oxygenated blood to
the organs and tissue of the head, trunk, and caudal regions. Blood is carried to these
organs from the heart by arteries, then capillaries, and then is taken back to the heart by
veins. A diagram of a typical two-chambered fish heart is show below.
Questions
1. Name the line that runs across this fish and enables it to detect vibrations in the
water.
2. Do mackerel have eyelids?
3. What type of scales do mackerel have? Describe the functions of the mucus that
covers the scales.
4. If a fishes scales lack annuli, besides environmental stress, what other factor might
cause this?
5. What is the white bony structure that supports the gill? Why is it important?
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6. Why do you think fish gills have such a high surface area? Relate this to oxygen
concentrations in water and air.
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