| Mitten Crab Teacher Activities |
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| Activities |
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A Crab with “Mittens”
Scientists still don’t know for sure the function of the mitten crab’s mittens. Horses have hoofs, ducks have webbed feet, and turtles have shells, but a crab with “mittens”? For most animals and plants, it is fairly easy to understand why they have certain structures. For example, hoofs help a horse rapidly escape danger, ducks use webbed feet to swim, a turtle’s shell provides protection and plants have a cuticle (waxy protective layer of epidermis or “skin”) to reduce water loss. But what about those “mittens” (actually tufts of fur) on the claw of the mitten crab? Think through what a crab needs to live and reproduce and come up with your own theory for what functions mittens provide for the mitten crab. How would you test your theory? Who knows, you could very well be right! We’d love to hear your ideas. Share your theories by emailing Oregon Invasive Species Council Coordinator Lisa DeBruyckere (lisad@createstrat.com).
The Curious “Catadromous” Life of the Mitten Crab
Mittens crabs have a very interesting life cycle in which the adults live in freshwater most of their lives, but migrate downstream to reproduce in the salty and brackish waters of estuaries. The juveniles then migrate away from the salt water estuary up stream (in some cases over 100 miles) into fresh waters to live their adult lives until it is time again to reproduce. This life cycle is called “catadromous.” The word "catadromous" means "downward-running," and refers to the seaward migration of adults.
1. Why do you think mitten crabs have evolved a catadromous way of life?
2. Think of some advantages and disadvantages that come with a catadromous life.
3. What other organisms have catadromous life cycles?
The opposite of catadromous is called anadromous or “upward-running.” Organisms with anadromous life cycles reproduce in fresh water and then migrate to the ocean as adults.
4. Do you know of any organisms that have an anadromous life cycle?
5. What are the advantages and disadvantages?
Mitten Crabs Town Hall Debate – Dine on a Delicacy to Divert a Disaster?
Written by Annie Pierpoint, Education Coordinator, West Coast Ballast Outreach Project
What if people enjoy eating Chinese mitten crabs and are will to pay lots of money to buy them? Debates over establishing a fishery for Chinese Mitten Crabs in California are highly contentious. Chinese mitten crabs in North America are undesirable because of their invasive harmful characteristics, but Chinese mitten crabs are also highly prized for food. Some suggest that heavy harvesting and eating Chinese mitten crabs is a perfect scenario for controlling their populations in areas they have invaded. The California Department of Fish and Game has not allowed a commercial fishery because this could lead to people introducing them in new areas. Indeed, the question of a fishery is a complex one and deserves careful consideration and research.
Should we establish a fishery for mitten crabs? Have students represent the different groups involved, like potential fishermen, potential buyers, consumers, the US Fish and Wildlife agency, a local “friends of the river” group, and biologists. Lead them in a “town hall” type meeting to decided if it is a good idea to establish a fishery, and if so how to regulate it.
Crab Quest
Go on a Crab Quest to look for and identify crabs on Oregon Coast. See how many native species and how many invasive species you can find. Do you think you will find a mitten crab on the coast?
This on-line guide from Oregon Sea Grant to is a good starting place to help you identify common native and invasive crabs in Oregon. http://seagrant.oregonstate.edu/sgpubs/onlinepubs/g05003.PDF
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| Answer Key and Resources |
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Answer Key and Resources for Teachers
A Crab with “Mittens”
Encourage your students to use their imagination to come up with unique and fun ideas. The most logical explanations will relate to how the “mittens” allow the crab to survive. What are some characteristics that allow the mitten crab to live and reproduce? Mitten crabs come into contact with many habitats and experience a number or critical and very different life stages. How could hair on the crabs claws help it survive in one or many of these habitats/life stages? Mitten crabs cause extensive damage to rivers by burrowing into the banks. How does a mitten crab create these burrows in a watery environment?
The Curious “Catadromous” Life of the Mitten Crab
Mittens crabs have a very interesting life cycle in which they reproduce in estuaries and then migrate up stream into fresh waters to live out their adult lives. This life cycle is called “catadromous.”
1. Why do you think mitten crabs have evolved a catadromous way of life? (Catadromous species generally occur in environments where freshwater productivity exceeds that of the ocean. Mitten crabs likely evolved where freshwater habitats provided more resources (nutrients, shelter, etc.) than saltwater or marine habitats. The mitten crab needs access to plenty of food and river banks to burrow. They create shelters in which to grow, survive, and reproduce. Also, a catadromous life cycle may allow species to escape harsh conditions, predators and “weather out” changes.)
2. Think of some advantages and disadvantages that come with a catadromous life.
Advantages: Access to freshwater and saltwater resources across their entire life cycle offers a wealth for resources available in each habitat. The estuarine environment is suitable for the mitten crab as larvae or juveniles since they are primarily planktonic and can disperse through ocean currents. Migrating upstream as adults provides additional habitats, protection, and food resources.
Disadvantages: Being dependent on so many habitats to complete their life cycle can be a disadvantage if one of those habitats is jeopardized or imperiled though pollution, destruction, over fishing, or invasive species. In addition, the crabs must expend energy to undergo a tiring migration as well as the physiological transition needed to be able to maintain the salt balance (osmoregulation) of its cells as the crab migrates from a salt to fresh water environment.
3. What other organisms have catadromous life cycles? (The best-known group of catadromous fishes are the true eels. In these species, females spend their lives largely in freshwater, while males live primarily in the brackish water of estuarine areas. Individuals breed in the seas and die after spawning once.)
The opposite of catadromous is called anadromous. In an anadromous life cycle, the organism will reproduce in fresh water then migrate to the ocean as adults.
4. Do you know of any organisms that have a anadromous life cycle?
(One of the best known examples is Pacific Salmon. Other examples include sturgeon, sea trout, three-spined stickleback and shad.)
5. What are the advantages and disadvantages? (see above answer to #3)
“The contrasting directions of migration can largely be explained by the relative availability of food resources in ocean and freshwater habitats. Oceans are more productive than freshwaters in temperate latitudes, and anadromous species predominate. In contrast, catadromous species generally occur in tropical latitudes where freshwater productivity exceeds that of the ocean.”
Aquatic Productivity and the Evolution of Diadromous Fish Migration. Gross et al. Science 2 March 1988: 1291-1293. DOI: 10.1126/science.239.4845.129)
Dine on a Delicacy to Divert a Disaster
Some important considerations for students as they debate:
It is important to determine if there is a viable market for the crabs. What kind of crabs are consumers looking for, and do we have them? Will people want to buy them?
Is there proof that a fishery will actually reduce the population of mitten crabs?
Mitten crab populations naturally fluctuate. How can the fisheries be maintained in years when mitten crab populations are low?
How can we insure that people won’t want to introduce the crab into un-invaded systems to begin a new fishery?
They do extensive damage and populations do need to be reduced. If we don’t start a fishery, what are other viable alternatives to mitten crab control?
Crab Quest
Go on a Crab Quest to look for and identify crabs on Oregon Coast. See how many native species and how many invasive species you can find. Do you think you will find a mitten crab on the coast? (It is unlikely that you will find a Chinese Mitten Crab on the Oregon Coast for two reasons:
1) they are not yet known to exist in Oregon, however they could easily be introduced! (People can help prevent the impacts by not transporting mitten crabs and looking out for and reporting them. It is very important that you report any findings of suspected mitten crabs through 1-800-Invader and the reporting website www.oregoninvasiveshotline.org )
2) Chinese Mitten Crabs spend most of their adult lives upstream in freshwater. They return in a mass migration once a year in the late fall or winter to breed). You may find the European Green Crab though!)
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| Printable format of activities |
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Mitten Crab Activities, Answer Guide, and Resources for Teachers (in pdf format)
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