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Asian Shore Crab Population Study
Hemigrapsus sanguineus
Invasive Species Population Study
Please Note: This activity has been adapted by Kristin Van Wagner, Education Coordinator with
the Narragansett Bay Research Reserve (www.nbnerr.org) from the, “Hemigrapsus sanguineus
Invasive Species Population Study” by David J. Welty, Ph.D. Fairhaven H.S.
I. Student Objectives:
Students will be able to:
1. Define and give examples of invasive species.
2. Explain how invasive species are introduced into a new environment.
3. Discuss how an invasive species has an impact on an ecosystem.
4. Identify the species and sex of different species of crabs that inhabit the
intertidal zone.
5. Conduct a population sampling technique to determine the population
of crabs at a site.
6. Graph and analyze the population data for sex ratios, size ratios, and
distribution within the intertidal zone.
II. Resources
A. Teacher Background Information:
1. Location of Hemigrapsus sanguineus
Hemigrapsus sanguineus was first found in Cape May, New Jersey in 1988
(Williams and McDermott, 1990) and quickly spread along the eastern seaboard
from North Carolina to Maine. It is found in most rocky intertidal coastal and
estuarine environments in southern New England (Ledesma, M.E., and N.J.
O’Connor. 2001). Hemigrapsus was found in salinities as low as 24 ppt and was
more abundant in lower and middle intertidal elevations. It can be found year
round under rocks.
2. Identifying the Hemigrapsus sanguineus
Hemigrapsus is easily differentiated from the indigenous Atlantic mud crab
Panopeus herbstii and an earlier intertidal invasive species, the European green
crab Carcinus maenas. The first distinguishing characteristic is the coloring of
the carapace. H. sanguineus has a banding pattern on the walking legs and a
dark carapace ranging from brownish orange to greenish black (Williams and
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Asian Shore Crab Population Study
McDermott, 1990). In comparison, the carapace of P. herbstii is dark bluegreen, brown or buff with purplish speckles on the front portion of the carapace.
C. maenas's carapace is dark green, mottled with black and brownish spots.
The most reliable way to differentiate the crabs is by the number of anterolateral
teeth on the carapace. H. sanguineus has three anterolateral teeth in
comparison to the five anterolateral teeth of P. herbstii and C. maenas.
See the diagram below.
Another distinguishing factor are the claws. The male H. sanguineus has a fleshy
knob at the base of the chelipid, which neither of the other two crabs have.
P. herbstii can be identified by dark brown claw fingers.
See the crab anatomy diagram in Appendix 1. for reference.
3. Handling C. maenas, H. sanguineus, and P. herbstii.
Crabs differ in their response to being handled. Each crab species has its own
characteristic temperament. The handler must respect the crabs’ claws.
Remember, crabs do not bite they pinch. As a rule, the larger the claw, the
more powerful it is and the more caution should be taken. In general, the best
way to capture and handle a crab is to pin down the crab’s carapace with
your index finger. Then, using your thumb and middle finger, grasp the outside
of the carapace at its widest point. Have the students practice handling crabs
before starting their studies in the field.
4. Determining the Sex of the Crab
After your students have mastered handling the crabs, they can practice
determining their sex. Secure a crab and turn it over to examine the telson. The
wider abdomen, often compared to the Washington Capital building, indicates
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Asian Shore Crab Population Study
a female, while the thinner telson, resembling the Washington Monument,
indicates a male.
5. Determining the Size of Crabs
To determine the size of crabs, purchase calipers from a science supply house.
Both stainless steel and plastic models were evaluated. The plastic model had
many advantages over the metal calipers:
1) Lower cost ($6.80 compared to $12.95,Carolina)
2) Does not require immediate rinsing,
3) They do not rust, and
4) Easier to adjust graduated dial
The calipers used in this study were purchased from Carolina: Plastic Vernier
Caliper (ER-70-2647, $6.80) and Plastic Vernier Caliper (ER-70-2651, $12.95).
Explaining how to use calipers to determine the width of an object can be
difficult so practice with a peer. After instructing the students how to use a
caliper, have the students practice determining the width of a standard sized
object such as a penny. If they can be located, collect the molted shells of
crabs from local beaches that have washed up in the strandline. These will give
the students practice determining the width of the carapace without the
added challenge of a wiggling, pinching crab. After the students have
mastered measuring the carapace, bring out live crabs for the students to
practice on. The crabs can be kept in a plastic cooler with seawater from the
same area they were caught. Purchase an air pump to keep the water
oxygenated.
6. Planning a field study for H. sanguineus
Consult a tide chart before scouting a site (for Narragansett Bay, RI a tide
calendar can be found at www.nbnerr.org). Since the crabs inhabit the middle
and lower intertidal zone, it is important to plan a visit as close to low tide as
possible. Plan to visit the survey site at low tide and conduct an informal survey
to see if the site will be satisfactory. Check that Asian shore crabs are living
there. They prefer cobble, rocky beaches along the shores of Narragansett Bay
and other estuaries. Collect enough crabs for students to practice handling
them, and to learn how to sex and measure the crabs. You can use regular
kitchen food storage containers with some water in the bottom to store the
crabs in a cool, dark place for a limited amount of time. Be sure not to over-fill
the containers as this may drown the crabs. The crabs will prefer shallow water
and perhaps a rock for cover. Make sure you use a lid with air holes punched in
it on the container.
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B. Lesson Plan:
Materials:
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0.5 m quadrants (easily made from PVC pipe) 1 for each group
2 buckets or containers for each group
Gloves for those students who would rather dig with gloves on
Container to bring sea water back in order to maintain crabs in the lab
Several sheets or grid paper to record location of quadrants
Pencils and clipboards – one for each group
Calipers – one for each group
Containers to hold crabs (must be covered if transporting the crabs)
New York times article “Tiny Invader Becomes a Bully in Local Waters”
Nail polish (for the extension only)
Day 1: Reinforce Marine Food Web Concepts
Introduce ecosystems and food webs from established curriculums.
Have the students read an article from the New York Times entitled,
“TINY INVADER BECOMES A BULLY IN LOCAL WATERS”
(http://www.nytimes.com/2001/06/10/nyregion/tiny-invader-becomes-a-bully-inlocal-waters.html) and discuss the significance of the article.
A copy of this article is provided in Appendix 2. for reference.
The following questions may be used to prompt discussion:
- What impacts might these invasive crabs have on native crab
populations?
- Think about how these impacts might move up through the food chain.
- Discuss where the most crabs were found in relation to their position in the
intertidal zone (closer to or farther from the low tide/high tide mark; rockier
or sandier; other organisms in their area).
- What other animals live within this zone that might be impacted?
- Why is this species so highly competive? (Resistance to salinity and density,
plus an omnivorous diet)
- Describe the basis of this ecological problem – in other words, why are we
experiencing this problem?
The New York Times article gives a lot of information on potential impacts and
reasons why this species is so highly competitive. Take turns reading portions of
the article as you address these questions.
Day 2: Introduce Marine Invasive Species
Depending on the students, different levels of literature can be supplied. The
following two sources are thorough and informative:
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1. Hitchhikers by the MIT Sea Grant
http://massbay.mit.edu/exoticspecies/hitchhikers/index.html
2. The Science of Invasive Species by the Union of Concerned Scientists
http://www.ucsusa.org/invasive_species/
H. sanguineus was likely introduced by release of larvae in the ballast water
transported from the western North Pacific Ocean to the mid-Atlantic coast of
the United States. Following a discussion of marine invasive species the students
can be given a summary of a paper entitled, "Predation of Juvenile Lobsters by
the Asian Shore Crab." This summary is available in Appendix 3.
A teacher led discussion can relate the article to some of the potential impacts
of marine invasive species. For a homework assignment, the students can read
your chosen marine invasive materials, such as the ones listed above, and come
to class with 5 questions from the reading.
Day 3: Prepare for Field Work
The students should be introduced to sampling techniques for determining the
density of a species in an area (total catch, catch and release, random
sampling, and uniform sampling along a transect). You can then use the
sample population density problems below, and have the students practice
determining population densities. Following this, the students can practice
identifying, handling, sexing, and measuring live H.sanguineus crabs (caught
earlier in the morning).
Practice Problems:
1. Sampling was performed in a 20 m x 10 m area of rocky intertidal coastline
where 1-meter quadrats were placed in a pre-planned pattern. From a
sampling of 4-quadrats, 312 crabs were captured.
A. According to the sampling results, what is the density of crabs in the
sampled area?
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Answer: 312 crabs/4 quadrats = 78 crabs/m
B. How many crabs inhabited the area?
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Answer: 78 crabs/m x 200 m = 15,600 crabs
C. If in reality the crabs were not uniformly distributed throughout the
sampled area, but were clumped together under large rocks, how
could the results be different?
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Answer: The result of 78 crabs/m could be artificially high or low,
depending on where the quadrats were placed.
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Asian Shore Crab Population Study
D. How does having such a small sample size (n=4) affect the results?
Answer: With more samples, the results would be more accurate.
2. Random sampling was conducted in a 5 m X 16 m area of rocky intertidal
coast line. To determine where to place the samples, plastic balls were
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tossed randomly and 1 m quadrants were placed wherever a ball landed.
From a sampling of 50 quadrants, 188 crabs were captured.
A. What is the density of crabs in the sampled area?
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Answer: 188 crabs / 50 quadrats = 3.76 crabs/ m
B. How many crabs inhabited the total sampled area?
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Answer: 3.76 crabs/ m x 80 m = 301 crabs
C. Would the results for this example be more accurate or less accurate
than the results given in the example in Question 1?
Answer: More accurate because the number of samples is much
higher (n=50) and the area sampled is smaller.
3. Using a capture-mark-release approach (which offers a better estimation of
population that takes into consideration the movement of individuals, birth
and death rates) from a 10 m x 5 m area of rocky intertidal coastline, 112
crabs were captured, marked, and released. Two days later, the researchers
captured 88 crabs of which 32 were marked crabs.
N= (Number marked x total catch second time)
Number of marked recaptures
A. What is the population size?
Answer: 308 crabs
B. What is the density?
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Answer: 308 crabs / 50 m = 6.2 crabs/ m
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C. Upon examining the capture site, it is determined that 1/2 of the area is
sand and not inhabited by crabs. How will this influence the
determination of density?
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Answer: With half the area the density doubles to 12.3 crabs/m
The students can be sent home with the following protocol to copy into their lab
book and a data collection sheet.
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Asian Shore Crab Population Study
Research Method:
Protocol adapted from Nourse, 2003.
Please note: Students can either sex and measure the crabs outdoors during
the field experience, or take the crabs back to the classroom and do this as a
post- activity in the lab that day, or the next day. To euthanize the crabs once
you have finished the lab work, freeze them in plastic bags, then dispose of the
bags in the garbage.
1. Have the students work in pairs to perform the population study at low tide.
2. Run a transect line perpendicular to the shore within the intertidal zone.
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3. Place 0.5 m quadrats along transect line, alternating left and right. Record
the position of each quadrat as you work.
4. Have students estimate the amount of rock cover in each quadrat. Do not
allow them to proceed to step #5 until they have signed off with chaperone
or teacher to insure that the above tasks are complete first.
5. Students will now collect all the crabs in their quadrat by turning over rocks
and digging into the sediment to a 5 cm depth. Any crabs collected from a
quadrat should be placed in one of the buckets. Each bucket should have
one rock to provide some cover and to weight it down.
6. If removing crabs to identify, measure and sex back in lab, the students
should move the crabs they have collected from the bucket to labeled
containers.
7. If data collection is to be done in field, have the students take each crab, on
at a time, from the bucket to identify the species, measure the carapace of
each crab, and determine its sex. Return each crab to the other bucket so
students don’t recount the same crabs twice.
8. Make sure all the data is carefully recorded for each group.
A data sheet and crab identification handout is provided in Appendix 4.
9. For the activity extension below, students will need to mark their crabs as
they identify them using finger nail polish. A dot of color on the carapace will
suffice.
10. Once all the data is collected, the crabs can be returned to the beach or
euthanized as described above.
11. At the end of the activity, you can have the students note any additional
organisms and algae they observe in study site.
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Asian Shore Crab Population Study
Day 5: Data Analysis
Compile class data such that all the students have the same data. Have
students graph their data, displaying (1) numbers of crabs found for each
species, (2) sex of crabs found by species and (3) size of crabs by species.
Data Processing:
1. Compile data
2. Compute population density
3. Place data in Excel file
4. Prepare graphs of the data
5. Submit graphs
Extension: Return for Capture of released marked Hemigrapsus
One week later, have students return to the same site and perform a sampling
of the site looking for previously marked crabs. Determined the number of crabs
in the area and calculate the population density using the formula:
N= (Number marked x total catch second time)
Number of marked recaptures.
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C. Literature
Berrick, S. 1986. Crabs of Cape Cod. ISBN 0916275001.
Bourdeau, P. E. and N.J. O'connor. 2003. Predation by the nonindigenous Asian
shore crab Hemigrapsus sanguineus on macroalgae and molluscs. Northeastern
Naturalist. 10(3) 319-334.
Cone, M. 2001. From a subtle ecological change, big problems grow. The
Philadelphia Inquirer, 01/28/01.
Ledesma, M.E., and N.J. O'connor. 2001. Habitat and diet of the non-native crab
Hemigrapsus sanguineus in southeastern New England. Northeastern Naturalist.
8(1) 63-78.
Nourse, S. 2003. Marine Bioinvaders. Flotsam & Jetsam. 32 (2) 5-7.
Williams, A.B. and J.J. McDermott. 1990. An eastern United States record for the
western Indo-Pacific crab Hemigrapsus sanguineus (Crustacea: Decapoda:
Grapsidae). Proceedings of the Biological Society of Washington 103:108 -109.
D. Websites:
http://www.state.me.us/dmr/rm/asian_shore_crab.htm
http://massbay.mit.edu/exoticspecies/
http://massbay.mit.edu/exoticspecies/invaders/hemi.html
http://massbay.mit.edu/exoticspecies/hitchhikers/index.html
http://www.ucsusa.org/invasive_species/
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Appendix 1.
Crab Anatomy
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Appendix 2.
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Appendix 3.
"Predation of Juvenile Lobsters by the Asian Shore Crab"
Excerpts from a preliminary report by Anna Demeo, University of Maine graduate student, and
Dr. John Riley, University of Maine Professor of Marine Sciences and Lobster Institute
Cooperating Research Professor.
“The Asian shore crab (Hemigrapsus sanguineus) was first discovered on the east coast of the
U.S. in New Jersey in 1988. It is assumed that like many invasive species it was carried here in
ballast water from an ocean-going vessel. Since 1988, this crab has become abundant along a
large part of the mid-Atlantic and southern New England coast.
Asian shore crabs have been shown to eat red and green algae, small herbivores, mussels, clams,
snails, barnacles and polychaete worms. One study found that they consume an average of 6.8
juvenile mussels (9-20 mm in length) per day. This is fewer than the number consumed by
Green crabs. However, Asian shore crabs have become a more devastating predator because of
their sheer numbers. Average densities in southern New England are 60 to 90 crabs per square
meters – 60 times the density of Green crabs in many locations.
Until now, little attention has been given to the effects on the Maine lobster fishery. A
widespread colonization could affect juvenile lobsters in two ways: one as a competitor for
habitat, the other as a predator. Juvenile lobsters are found at or below the lower portion of the
Asian Shore crab intertidal range. If large populations of shore crabs colonize an area of the
intertidal zone where juvenile lobsters live, it could force these young lobsters from their shelter
making them vulnerable to predators. A more favorable scenario is that the juvenile lobsters
would migrate further down the tidal zone until they are beyond the vertical range of the shore
crab.
There is also the potential for the shore crab to prey on juvenile lobsters. It may be that this
would be an arduous task for the crab and therefore not profitable. However this and other
hypotheses on this subject have yet to be studied. A research project has recently been designed
to address this.
Field studies were conducted on 3 beaches on Mt. Desert Island to determine the presence or
absence of the Asian shore crab. These field surveys will be expanded this spring. While there
was an abundance of other species of crab there were no Asian shore crabs. A preliminary study
of the effect of these crabs on juvenile lobsters proved inconclusive. A larger competition
including space competition is currently underway. The goal of this study is to look at the Asian
Shore crab’s ability and desire to prey on juvenile lobsters.
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Asian Shore Crab Population Study
Appendix 4.
On the following pages please crab identification sheets that can be printed in
color, or black & white copies, and laminated back-to-back for reuse or
distributed to each student team in the field for reference.
In addition, you’ll find a sample data sheet that students can use in the field.
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Crab Identification Sheet
Narragansett Bay Research Reserve (www.nbnerr.org)
Hemigrapsus sanguineus
Asian Shore Crab
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3 teeth on each side of carapace
Smooth carapace
Banding pattern on legs
Callinectes sapidus
Blue Crab
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Sharp spine on each side of carapace
Olive green carapace with bright blue claws
Females have red-tipped claws
Rear legs are paddles, making blue crabs excellent
swimmers
Panopeus spp.
Atlantic Mud Crab
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Color is olive-brown to grey
Sometimes have red-brown spots
Tips of claws are black
5 points on each side of carapace
Carcinus maenas
Green Crab
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Carapace has five large teeth following the eye
Multicolored, mottled greenish or orange
5th pair of each legs is slightly flattened
Libinia emarginata
Spider Crab
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Six spines on center line of hairy carapace
Brown to dull yellow in color
Tips of claws are white
Often covered in algae or sponges
Take the measurement, in millimeters, along the widest
part of the crab's carapace.
Female (left) and male (right) green crabs
Left: Female crab abdomen
Right: Male crab abdomen
***You can use the following abbreviations when noting the species on your data sheet***
Species Common Name
Species Latin Name
Abbreviation
Asian Shore Crab
Hemigrapsus sanguineus
Blue Crab
Callinectes sapidus
Atlantic Mud Crab
Panopeus spp.
HS
CS
P
Green Crab
Carcinus maenas
Libinia emarginata
Spider Crab
CM
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Asian Shore Crab Population Study
Site:_______________________________________
Sex (M,F,J)
Name: _____________________________________
Species
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Date:__________________
Size (mm)