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TODAY'S CREATURE FEATURE-THE HORSESHOE CRAB
MASTER TEACHER Helen Mebane
GRADES 5 - 8 SCIENCE
OVERVIEW
Students will be introduced to "living fossils" which
have been around even before dinosaurs roamed the earth. These
harmless creatures are called the horseshoe crabs, or Limulus. They are not
really crabs but are related to spiders in the phylum Arthropods. They are
in their own classification Class Merostomata. Students will view a video
showing their interesting anatomy, life cycle, and unique blood properties which
are used for pharmaceuticals and in the medical field. Students will be given
the opportunity to both observe a live and preserved horseshoe crab.
ETV SERIES
Wonders Under the Sea- 404 Horseshoe Crabs
LEARNING OBJECTIVES
Students will be able to:
*observe and draw the dorsal and ventral parts of a preserved
horseshoe crab
*observe a live horseshoe crab
*state the life cycle of a horseshoe crab
*state the scientific uses for a horseshoe crab
MATERIALS
per student
Activity sheet with 23 questions taken from the video
per three or four students
one preserved horseshoe crab
*Preserved horseshoe crabs may be purchased from Carolina
Biological Supply Company:
AA-22-5360, 8 inches + body length, plain color injection,
$5.25 each, Carosafe package
AA-22-5361, 6-8 inches in body length, plain color injection,
$5.25 each, Caropak Single
AA-22-5370, 6-8 inches in body length,
single color injection, $6.95 each, Carosafe
AA-22-5371, 6-8 inches in body length, single color injection,
$6.95 each, Caropak Single
AA-22-5381, 3-6 inches in body length, plain color injection,
$4.30 each, Caropak Single
AA-22-5390, 2-3 inches in body length, plain color injection,
$3.40 each, Carosafe
AA-22-5400, 1-2 inches in body length, plain color injection,
$2.55 each, Carosafe
Horseshoe crabs are available in odorless Carosafe or in
damp-packed Caropak, plain color injection means no color injection, and single
color injection means the circulatory system is injected with a colored latex.
Their address is P.O. Box 6010, Burlington, NC 27216-6010 and their toll free
fax number is 1-800-222-7112.
*Preserved horseshoe crabs may be purchased from Frey
Scientific, A Division of the Beckley Cardy Group:
S22650, 6 to 8 inches in length at $6.75 each,
S22651, 3 to 6 inches in length at a cost of $39.60 for a bucket
of ten
S22652, 2 to 3 inches in length at a cost of $24.55 for a bucket
of ten. Their address is 100 Paragon Parkway, Mansfield, Ohio 44903
and their toll free fax number is 1-888-454-1457 and their toll free phone is
1-888-222-1332.
**Live horseshoe crabs may be purchased from Aqualand Pet
Center at a cost of one for $6.95 or a pair for $12.50. Their
address is P.O. Box 55-7365, Miami, Florida 33255-7365 and their telephone
number is 305-823-1266 and their fax number is 305-823-1202.
VOCABULARY
horseshoe crab a marine animal with a large
oval shell and a stiff, pointed tail that is more closely related to spiders.
omnivores animals which get their energy from
eating both plants and animals.
endangered species animals whose population
is so small that it is in danger of extinction.
spawning the behavior of the female to
release large number of eggs into the water or sand.
bulk head a man-made barrier between water
and land usually to prevent erosion.
living fossils the name given to trilobite
larvae. Example: Baby horseshoe crabs
pheromones a chemical stimulant.
Example: A perfume to attract the opposite sex horseshoe crabs.
satellite males the other males which mate
with the female during mating season.
trilobites the name given to the horseshoe
crab after they hatch and up to one year of age.
molting in animals, the periodic shedding and
replacing of the old body covering, such as the exoskeleton.
exoskeleton on all arthropods, the hard,
external skeleton of an animal. Example: a grasshopper
rudder the limb used by the animal to steer
with while in the water.
nanobase the mouth of the horseshoe crab.
gizzard the muscular, digestive organs used
to grind up the food.
fertilizer the food (nutrients)
necessary for plant growth.
pharmaceutical medicines or the science of
drug preparation, uses and composition.
endotoxins the poisons inside of a
cell.
protein granules the small bits or chucks of
protein.
prosoma the part of the body covered by the
shell and includes the head.
hemalysate the copper-based blood of the
horseshoe crab.
PRE-VIEWING ACTIVITIES
Ask the students, "What is a horseshoe crab? Where
would you go to find one? Do these animals live in freshwater or saltwater?
Today, we will see a video on horseshoe crabs learn how scientists use their
blood to test medical equipment."
FOCUS FOR VIEWING
Say, "As part of our study of marine animals, we will view
the external parts of a preserved horseshoe crab and learn how they are helpful
to man." To give the students a specific responsibility while viewing
say, "As we view this video, find some fascinating facts that will be
answered from the handout-Activity Sheet. Record your answers as the tape
is viewed. Activity Sheet is at the end of the lesson.
VIEWING ACTIVITIES
Begin the tape where Kate Brown introduces Tom Schmid
from the Texas State Aquarium. Tom is talking to a group of children who have
several questions about horseshoe crabs. Pause the tape where you
see a boy wearing a blue cap and after you hear, "They are mostly related
to spiders." This is the answer given to the first child's question,
"Why don't horseshoe crabs look like other crabs?" On the
Activity sheet, review the first question on the handout. (The truth is
they are not really crabs; they are in Phylum Arthropods. They are more
closely related to spiders.) Resume the tape. Pause
the tape where you see a girl wearing a black cape and after you hear,
"Atlantic coast." This is the answer to the second child's
question, "Where do horseshoe crabs live?" On the Activity
sheet, review the second question on the handout. (They live all along the
Atlantic coast from Maine to Florida. They are found in the Gulf of Mexico
to the Yucatan peninsula, and off the coast of Texas.) Resume the
tape. Pause the tape where you see a boy wearing a red cap and
after you hear, "omnivores." This is the answer to the third
child's question, "What do horseshoe crabs eat?" On the Activity
sheet, review the third question on the handout. (They are called
omnivores because they eat anything that they come across such as clams, worms,
dead fish or algae on the bottom of the ocean.) Resume the tape.
Pause the tape where you see a girl with a yellow cap and after you hear,
"they are not endangered." This is the answer to the fourth
child's question, "Are horseshoe crabs endangered?" On the
Activity sheet, review the fourth question on the handout. (They are not
endangered but there's not as many as there was in the past. They come to
the beach to spawn and if there is no beach, a bulk head, or some type of
erosion there is no place for the animal to spawn. They have been used as
bait or chicken feed. We have to watch their numbers because they may
become endangered.) Resume the tape. Pause the tape
where you see a boy with a black cap and after you hear, "...two feet in
diameter." This is the answer to the fifth child's question,
"How big do horseshoe crabs get?" On the Activity sheet, review
the fifth question on the handout. (They may be 2 feet in diameter when
they mature.) Resume the tape. Pause the tape where
you see a girl wearing a blue cap and after you hear, "...chicken
feed." This is the answer to the sixth child's question, "What
are horseshoe crabs used for?" See the Activity sheet, review the
sixth question on the handout. (At one time they were used as chicken feed
but when they were ground up and eaten by the chickens, the eggs tasted like
fish so the poultry farmers don't do that anymore. Fishermen used them as
bait. Presently, scientists use their very special blood to run tests on
medical equipment to make sure it's safe. They also have very unusual
eyes, similar to our eyes, so scientists are now researching that.) Resume
the tape.
Pause the tape where you see Kate with both her hand with
palms up and after you hear, "survivor that dates back several hundred
millions of years." This is the answer to the next question by Kate
to Julie Galbraith. Kate asks, "Why are horseshoe crabs known as the
living fossils?" On the activity sheet, review the seventh question
on the handout. (The horseshoe crab is a survivor that dates back several
hundred million years, even before the dinosaurs roamed the earth.) Resume
the tape. Pause the tape where you see Kate cup her hands together
and after you hear, "...four species." This is the answer to the
next question by Kate. She asks, "How many species of horseshoe crabs
are there?" On the activity sheet, review for the eighth question on
the handout. (There are four species, three live in the southeast Asia-
Vietnam, Thailand, and Japan.) Resume the tape. Pause
the tape where you see several horseshoe crabs swimming in shallow water and
after you hear, "come to the beach each spring." This is the
answer to the next question by Kate. She asks, "How do horseshoe
crabs mate?" See the Activity sheet, review the ninth question on the
handout. (They come to the beach each spring. Nesting occurs at
night. The males wait for the females to release the pheromones-a chemical
stimulant. It's horseshoe crab "perfume" to attract the males.
One male climbs on the female and she drags him to the coastline.
The female digs five or seven shallow nests and begins to lay a couple of
thousand eggs in each nest for a total of 20,000 eggs. Then the attached
male and satellite males, other males that gather around fertilize the eggs as
they are laid by the female. The males mate with several females during
the spawning season.) Resume the tape. Pause the tape
where you see a black bird pecking at the shoreline and after you hear,
"Birds like the eggs." This is the answer to the next question
by Kate. She asks, "Do the birds eat the eggs?" On the
activity sheet, review the tenth question on the handout. (Spawning
usually occurs at night because the birds like to eat the eggs. Birds
consume 320 tons of horseshoe crab eggs. Some birds, up to as many as
twenty species, can't store enough energy to fly to the Arctic nesting grounds.
They increase their weight two to three times during their vacation on the
beaches.) Resume the tape. Pause the tape where you
see one female horseshoe crab digging into the sand and after you hear,
"...two to four weeks." This is the answer to the next question
by Kate. She asks, "How long does it take for the horseshoe crabs to
hatch?" On the Activity sheet, review the eleventh question on the
handout. (If everything goes as plan, then the sun keeps the eggs warm and
the tides keep them moist, the crabs hatch in two to four weeks later.) Resume
the tape. Pause the tape where you see Julie holding a one inch
horseshoe crab and after you hear, "year old trilobites." This
is the answer to the next question by Kate. She asks, "What do baby
horseshoe crabs look like?" On the Activity sheet, review the twelfth
question on the handout. (Actually, these are year old horseshoe crabs
which are called "trilobite larvae". They are 1/8 inch across
and in a few days they begin to resemble the horseshoe crab. It takes 9 to
10 years for them to reach maturity and they grow to two feet in length.
They molt to grow. They have an exoskeleton (hard outer skeleton) which
opens up on the front edge and the crab crawls out forward to increase in size.
After each molt they will increase 25 % in size. When they mature,
they only molt once a year.) Resume the tape. Fast forward
the tape through the "Marine Science and You" segment with Natalie
Jenkins which shows the "Ocean in Motion." Resume the
tape. Pause the tape where you see Julie holding a five inch
horseshoe crab with a moving tail and after you hear, "...nasty tail."
This is the answer to the next question by Kate. She asks, "Is the
horseshoe's tail dangerous?" On the Activity sheet, review the
thirteenth question on the handout. (Its nasty tail is harmless.
It's used to right itself, flip itself right side up, used as a rudder, and to
dig in the sand.) Resume the tape. Pause the tape
where you see Julie holding her hands together and after you hear,
"...claws are harmless." This is the answer to the next question
by Kate. She asks "Are the claws dangerous?" On the
Activity sheet, review the fourteenth question on the handout. (The
claws are located at the ends of the five pairs of walking legs. They are
harmless. They are only used to grasp food and begin to break it up.)
Resume the tape. Pause the tape where you see Julie pointing
at the center area of a horseshoe crab and after you hear, "they have no
teeth." This is the answer to the next question by Kate. She
asks, "Do horseshoe crabs have teeth?" On the Activity sheet,
review the fifteenth question on the handout. (They have no teeth,
only claws to break up the food. Their nanobase is at the center of the
walking legs. The food is shredded and then sent to the muscular
gizzard to be ground further. Horseshoe crabs can't poke, poison, pinch or
bite you.) Resume the tape. Pause the tape where you
see Kate look at the camera and laugh and after you hear, "They are useful
for..." This is the answer to the next question by Kate. She
asks, "How are horseshoe crabs useful?" On the Activity sheet,
review the sixteenth question on the handout. (1. They are a limited
source of food in some parts of the world. 2. In the past, they've been
used as fertilizers. 3. Also in the past, they've been used to feed
chickens and hogs, but that has been eliminated. 4. They've used as bait
for conches, lobsters, and eels. 5. They've used for pharmaceuticals and
in the medical field because of their unique blood.) Resume the tape.
Pause the tape where you see a blue screen with blood cells and after you
hear, "...horseshoe crab blood." This is the answer to the next
question by Kate. She asks, "What is so unique about their
blood?" On the Activity sheet, review the seventeenth question on the
handout. (Horseshoe crab blood has a clotting mechanism that causes it to
clot when it comes in contact with endotoxins. Protein granules, present
in blood cells, clump together and begin to fuse surrounding plasma. This
protects the horseshoe crab because the ocean water is full of endotoxins.
If the shell is injured, it allows the blood clot to form quickly.) Resume
the tape. Pause the tape where you see the technician flip the test
tube upside down and after you hear, "...pharmaceutical and the medical
field." This is the answer to the next question by Kate. She
asks, "How is the blood beneficial to pharmaceutical companies and the
medical field?" On the Activity sheet, review the eighteenth question
on the handout. (Intravenous drugs should be tested for endotoxins.
Scientists used to use the "rabbit fever test" where the contaminated
solution was injected into the rabbit. If the rabbit developed a fever it
meant there was a bacteria or a contamination in the solution in its body.
Using blood from the horseshoe crab was quicker, more accurate, and live animals
(rabbits) were not needed anymore.) Resume the tape. Pause
the tape where you see the technician place the horseshoe crab into the rack and
after you hear, "horseshoe crabs were taken to the lab." This is
the answer to the next question by Kate. She asks, "How is the blood
extracted from the horseshoe crabs?" On the Activity sheet, review
the nineteenth question on the handout. (The horseshoe crabs were taken to
a lab and inserted in racks where a needle was placed in the membrane between
the abdomen and the prosoma. A small amount of solution is removed which
is the horseshoe crab's blood. It is a blue, copper-based blood called
hemalysate. The healthy, mature live crabs used are from the Northeast of
the United States. After a small amount of blood is drawn, they're trucked
back to the coast which is not harmful to the crabs. After 3-4 days, the
plasma volume of the crab returns to normal and in 3-4 months, the blood count
is back to normal. Horseshoe crabs can be identified as having already donated
blood by the small scar where the needle was inserted. Horseshoe crabs are not
used twice during the same season.) Resume the tape. Pause
the tape where you see Kate point both her hand toward her chest and after you
hear, "...protect the horseshoe crab." This is the answer
to the next question by Kate. She asks, "What can we, the ordinary
citizen, do to ensure their survival rate?" On the Activity sheet,
review the twentieth question on the handout. (We, the ordinary citizen,
can protect the horseshoe crab by not spoiling the coastline, never picking them
up by the tail, and not bothering them at the beach.) Fast forward through the
"Creature Feature" segment with Mike Peyton on blue crabs. Pause at
the end of the segment. Resume the tape. Pause the tape
where you see at the bottom of the screen 1-800-799-7901 and after Kate
introduces Karen Ryan and you hear, "...Nine years for males."
This is the answer to the next question by the first caller. He asks,
"How long does it take for the horseshoe crabs to be fully grown?"
On the Activity sheet, review the twenty first question on the handout.
(It takes nine years for the males and ten years for the female. She
produces eggs which is hard work.) Resume the tape. Pause
the tape where you see Karen holding a model of a horseshoe crab and after you
hear, "...male and female horseshoe crabs." This is the answer
to the next question by Kate. She asks, "How do you tell the
differences between the male and female horseshoe crabs?" On the
Activity sheet, review twenty second question on the handout. (You can
tell the difference because the males have a more pronounced forehead; it's
bigger. The first set of walking legs have claspers to hang on to the
female to get a free ride to the coastline). Resume the tape.
Fast forward the tape though all these questions where Sams asks, "
Do horseshoe crabs shed their shells even though they are like Arachnids?"
Next, Jenny asks, "Why are horseshoe crabs called crabs even though they
are not crabs?" Next, Blake asks, "What do horseshoe crabs
eat?" Next, Zachary, "Are horseshoe crabs and spiders related to
trilobites?" Next, Austin, "Why do horseshoe crabs look like
manta rays?" Pause the tape after Austin's question is answered.
Resume the tape. Pause the tape to where you see Karen
holding the horseshoe crab upside down (like a frying pan). You will hear,
"some places hold them upside down." This is the answer to the
last question by Kate. She asks, "How would you eat them?"
(Some places hold the horseshoe crab upside down by the tail and roast over a
fire. Then they pick the meat out of the shell and put in a spicy stew.
Others eat the abdomen and the tissues that hold the tail.) Stop
and eject the tape.
POST-VIEWING ACTIVITIES
Explain, "Now you have a better understanding of the
external anatomy of the horseshoe crab, their life cycle, and unique scientific
uses. Let's observe these preserved horseshoe crabs and label their parts.
Use the handout adapted from the National Aquarium in Baltimore- 1997. The
website is http://www.aqua.org/education/teachers/
horseshoe.html
ACTION PLAN
Students could either take a field trip to the Texas State
Aquarium or write Sally Hoke-Marine Biologist from the Texas State Aquarium to
invite her to our classroom. Ask her questions: Are horseshoe crabs
easy to care for? Is it true that they go a year without eating? How do
they swim? How do they breathe? Do they have good eye vision like we do?
What is the latest research using the horseshoe crabs? Also visit
the Wonders Under the Sea website at www.televentures.org for more material on
marine science topics.
Note: The Texas State Aquarium is located at 2710 North
Shoreline, Corpus Christi, Texas 78402-1097
Have students visit the following website that have more
information on horseshoe crabs:
http://www.faseb.org/opar/poster/crab.html
http://www.enviroweb.org/oceanwatch/news/crfr2.html
EXTENSIONS
Science
Students may visit the website of the Texas State Aquarium at http://www.txstateaq.com
for more information on the horseshoe crabs.
Students may use the horseshoe crab anatomy guide to review the
animal's dorsal and ventral anatomy provided from the National Aquarium in
Baltimore website. It's very useful as the students observe the preserved
horseshoe crab provided by the teacher.
Math
Remember, after each molt, horseshoe crabs will increase
25 % in size.
Problem #1-Using a calculator and ruler, estimate how many times
they would molt until they become fully grown adults. Remember, they grow from
an 1/8 of an inch to 24 inches (from the tip of the tail to the front end of the
carapace) in 9 years for males and 10 years for females. Also, they only molt
once a year when they are mature. (16 molts on an average)
Problem # 2-Using calculator or a ruler, determine the molting
(growth) sizes in problem #1 in centimeters.
Art
As stated in the post-visit activities from the National
Aquarium in Baltimore website, teachers could enlarge the pictures of the
horseshoe crab body parts using the school copy machine. If it can't do
this then it is suggested you make a transparency of the horseshoe crab.
Then use an overhead projector to trace the parts onto a posterboard. Then cut
out each part and laminate or use clear contact paper to make a model of a
horseshoe crab. The students could then make their own model labeling body parts
and explaining their functions to each other in their group.
Language Arts
Research information of the websites given or read these books
below. Write your own short story or poem about horseshoe crabs.
Teachers Resources
Fotheringham, Nick and Brunemeister, Susan. Beachcomber's Guide
to Gulf Coast Marine Life. Houston, Texas: Gulf Publishing Company
Berrill, N.J. and Jacquelyn. 1001 Questions Answered about the
Seashore. New York: Dover Publications.
Meinkoth, Norman A. The Audubon Society Field Guide to North
American Seashore Creatures. New York: Alfred A. Knopf.
Miner, Ray Waldo. Field Book of Seashore Life. New York: G.P.
Putman's Sons
Children Resources
Cook, Joseph J. The Curious World of the Crab. Dodd, Mead &
Company. 1970.
McClung, Robert M. Horseshoe Crab. William Morrow and Co. 1967.
Mars, W. T. The Crab from Yesterday: the Life Cycle of a
Horseshoe Crab. Frederick Warne and Co., Inc. 1970.
Tate, Suzanne. Harry Horseshoe Crab. Nags Head, NC: Nags Head
Art. 1991.
Social Studies
By using a overhead projector, draw a huge world map which may
cover a wall outside your classroom. Draw the outline of horseshoe crabs
about two inches in length and find all the geographic distributions of all four
species of horseshoe crabs. Glue an outline of all the areas the
horseshoe crabs may be found. Next, hot glue your story or poem (which you
wrote in your Language Arts class) all around the world map for the student body
to read.
Activity Sheet
Answer these questions to the best of your ability.
1. Why don't horseshoe crabs look like other
crabs?______________________________
________________________________________________________________________
2. Where do horseshoe crabs live?
____________________________________________
________________________________________________________________________
3. What do horseshoe crabs
eat?______________________________________________
________________________________________________________________________
4. Are horseshoe crabs endangered?
__________________________________________
________________________________________________________________________
5. How big do horseshoe crabs get?
___________________________________________
________________________________________________________________________
6. What are horseshoe crab used for?
_________________________________________
________________________________________________________________________
7. Why are horseshoe crabs known as "living
fossils"?____________________________
________________________________________________________________________
8. How many species of horseshoe crabs are there?
______________________________
________________________________________________________________________
9. How do horseshoe crabs mate?
____________________________________________
________________________________________________________________________
10. Do the birds eat the eggs?
_______________________________________________
________________________________________________________________________
11. How long does it take for the horseshoe crabs to hatch?
________________________
________________________________________________________________________
12. What do baby horseshoe crabs look like?
___________________________________
________________________________________________________________________
13. Is the horseshoe crab's tail dangerous?
_____________________________________
________________________________________________________________________
14. Are the claws dangerous?
_______________________________________________
________________________________________________________________________
15. Do horseshoe crabs have teeth?
___________________________________________
________________________________________________________________________
16. How are horseshoe crabs useful?
__________________________________________
________________________________________________________________________
17. What is so unique about their
blood?_______________________________________
________________________________________________________________________
18. How is the blood beneficial to pharmaceutical and the
medical field? ____________
________________________________________________________________________
19. How is the blood extracted from the horseshoe crabs?
_________________________
________________________________________________________________________
20. What can we-the ordinary citizen do to ensure their survival
rate?________________
________________________________________________________________________
21. How long does it take for the horseshoe crabs to be fully
grown? ________________
22. How do you tell the difference between the male and
female horseshoe crab? ______
_______________________________________________________________________
23. How would you eat them?
_______________________________________________
_______________________________________________________________________
HANDOUT
Updated: April 01, 2008
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