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        Endangered Relationships

        This lesson uses videos from Nature’s "Crash: A Tale of Two Species" to explore the interrelationship between the horseshoe crab and a small migratory bird called the red knot.

        Lesson Summary


        This lesson uses video segments from the Nature film “Crash: A Tale of Two Species” to explore the interrelationship between the horseshoe crab and a small migratory bird called the red knot. Both species are in decline, and the red knot’s future, in particular, depends on the horseshoe crab making a comeback in the waters of the Delaware Bay.

        Students are introduced to the horseshoe crab via a video segment, learning that the species’ longevity (350 million years) makes the horseshoe crab a “living fossil,” an anomaly in terms of the Earth’s species. They watch several other video segments to explore the interconnectedness between the horseshoe crab and the red knot, following the videos with a discussion of the reasons for the decline of each species. Students then fill out an exit ticket to assess their comprehension of these concepts. An optional additional video segment can be used to explore how humans have come to depend on horseshoe crabs as well (their blood, which contains primitive antibacterial properties, is used to test intravenous drugs for contamination).

        As a culminating activity, students use an interactive online map to research endangered species in the region of the United States where they live. They discuss the environmental changes that have led to the species’ decline (often of human origin) and the strategies being employed to prevent their extinction.


        Students will:

        • describe characteristics of the horseshoe crab;
        • specify how “living fossils” like the horseshoe crab differ from most other species;
        • understand that over 99% of all species that have ever existed on earth are extinct;
        • name specific factors that can lead to species endangerment and extinction;
        • list several strategies that can be undertaken by humans to protect endangered species;
        • provide at least one example of species interconnectedness: how the population decline of one species adversely affects another species.

        Grade Level:


        Suggested Time

        One to two 45-minute class periods

        Media Resources

        The Living Fossil Video

        Horseshoe Crabs and Red Knots Video

        Protecting the Horseshoe Crab Video

        Horseshoe Crabs and Humans Video


        Endangered Relationships Exit Ticket

        Web Sites

        Endangered Species Map

        This interactive map provides information on key endangered species, including efforts currently being undertaken to protect them, in different regions of the United States.

        The Lesson

        Introductory Activity

        1. Frame the first video segment for students by telling them that they will be introduced to the horseshoe crab. As a focus, ask students to make a list of the facts they learn about horseshoe crabs by watching this video segment. For example, ask them to pay attention to the following information (you may want to make a list of the following on the board):

        • How old is the horseshoe crab species?
        • Where do most horseshoe crabs live?
        • What are their predators?
        • What are their closest relatives?
        • How many eyes do they have?
        • What color is their blood?

        2. Play The Living Fossil for the class. Follow up by holding a discussion about the facts the students learned about horseshoe crabs, including the information you asked before:

        • The horseshoe crab species is 350 million years old.
        • Most horseshoe crabs live on the ocean floor of the U.S.’s Eastern seaboard.
        • Predators include sharks and sea turtles.
        • Their closest relatives are spiders and scorpions.
        • Horseshoe crabs have ten eyes.
        • Horseshoe crab blood is blue.

        3. Ask if students noticed that the horseshoe crab was referred to in the video as a “living fossil.” Discuss the meaning of this term. (A typical species may inhabit the earth between 1 and 10 million years before going extinct – and the human species, homo sapiens, has only been around for approximately 200,000 years. Plants and animals that buck this trend, and have survived through multiple mass extinctions, are named “living fossils” for their close resemblance to their ancient relatives in the fossil record.)

        4. Poll your class onthe following question: Of all the species that have ever inhabited our earth, what percentage of species has gone extinct? (Scientists generally agree that 99.9% off all species that have ever existed are extinct.) The fact that thehorseshoe crab species has been on the earth for 350 million years makes it very special in this regard.

        Learning Activity 1

        1. Frame the next video segment by explaining that over time, other species have come to depend on the horseshoe crab. One species that relies on the horseshoe crab is a small bird called the red knot. This migratory shorebird makes one of the longest migrations of any bird, traveling from Chile, in South America, to the Arctic every year.

        2. Ask students for their predictions: in what way do they think red knots might depend on horseshoe crabs? (You may want to make a list of predictions.) Provide students with a focus for viewing by asking them to test their predictions as they watch the video.

        3. Play Horseshoe Crabs and Red Knots.

        4. Follow up with students by asking how the red knots depend on horseshoe crabs. (Horseshoe crab eggs in the Delaware Bay provide essential nutrition to the migrating red knots. When the birds arrive on the Bay, they are emaciated and must pack on muscle and fat before trekking to the Arctic. Without horseshoe crab eggs, they wouldn’t make it to their breeding ground to reproduce.)

        5. Frame the next segment by sharing that scientists are very concerned about declining red knot populations. There is fear that the bird will go extinct, largely because there are not enough horseshoe crab eggs making it to the beaches along the Delaware Bay to sustain the migrating birds. As a focus, ask students to determine a) the cause of the decline in horseshoe crabs along the Bay, and b) the strategies that are in place to try to protect the horseshoe crab population.

        6. Play Protecting the Horseshoe Crab.

        7. Follow up with a discussion about the role of overharvesting by humans in the dramatic decline in horseshoe crabs (and hence, in red knots). Also discuss the moratorium (temporary ban) that is protecting the horseshoe crab population while it recovers.

        8. Lastly, discuss why all the efforts being made to help horseshoe crabs thrive may still not be enough to save the red knot. (The horseshoe crab is not sexually mature until it is 9 or 10 years old. The protective measures might not allow enough time for the horseshoe crab population to recover in time to save the red knot from extinction.)

        Learning Activity 2 (Optional)

        1. Frame the final video segment by letting students know that another species that has come to depend on the horseshoe crab is the human. Ask for predictions: how do humans rely on horseshoe crabs? (You may want to make a list of predictions on the board or have students jot them down.) As a focus, ask students to test their predictions as they watch the video segment.

        2. Play Horseshoe Crabs and Humans.

        3. Follow up with a discussion about how humans rely on horseshoe crabs. (Every hospitalized American has probably benefited from the horseshoe crab, because every IV drug that comes to market in the US has been tested using an extract from horseshoe crab blood.) Explore this medical phenomenon with questions such as:

        • Why is horseshoe blood blue? (It contains copper as its oxygen-carrying agent, rather than iron-based hemoglobin.)
        • How does the blue blood help the horseshoe crab fight disease? (It is a primitive antibiotic that kills bacterial cells.)
        • How has this evolutionary adaptation proven useful to human medicine? (There are proteins in the blood that clot around bacteria – an extract from the blood is used to test every IV drug that comes to market for contamination, per FDA regulations.)
        • How much is a quart of horseshoe crab blood worth? ($15,000)
        • How has the practice of horseshoe crab harvesting changed as a result of the medical value of the blood? (Fishermen used to harvest the crabs for bait – but now fishermen with special licenses “borrow” the horseshoe crabs so medical laboratories can bleed them. The horseshoe crabs are then returned to the waterways.)

        4. Discuss the interdependent relationships that have been explored thus far in the lesson (horseshoecrab/human; horseshoe crab/red knot). Make sure students understand that interrelationships between species are the norm in every ecosystem – so if one link in the chain is disturbed, it affects many other species. If there were no more horseshoe crabs, what would happen to the red knot? (It would go extinct.) What would happen to humans? (While we won’t go extinct, we would have to revert to testing drugs on rabbits or invent another means of ensuring the safety of our pharmaceuticals.)

        Culminating Activity

        1. Direct students (in groups if desired) to visit the Endangered Species Map.

        2. This interactive map provides information on endangered species in different regions of the United States. Remind students that these animals and plants are highly threatened; some are on the verge of extinction. If any of these animals and/or plants were to go extinct, would there be an impact on other species? (Almost definitely yes – as students have learned in the case of the horseshoe crab and the red knot, the extinction or reduction in population of one species almost always affects other interdependent species. In some cases, we may not even know the species that will be impacted, yet.)

        3. Ask students to use the online map to make a list or summary of the endangered species in the area of the country where you live. For each species, they should include information on what has caused the population decline (if provided) and the efforts being undertaken to try to help them.

        4. Once students have had time to gather their information, have them report back to the class. Remind students that extinctions occur because an environmental change occurs that exceeds the adaptive capacity of the species. In our times, these environmental changes often have direct or indirect human causes.

        5. Use the students’ research as a jumping-off point to discuss some of the main strategies that can be undertaken to protect threatened and endangered species, such as:

        • Captive breeding programs
        • Predator control
        • Restricting human access (e.g., closing beaches and roads)
        • Limits or prohibitions on hunting and harvesting
        • Habitat protections, like establishment of a wildlife refuge
        • Endangered Species Act

        Note: The Endangered Species Act is rather complicated, but students can be told that it is the highest level of protection that can be bestowed on a species by the federal government, and ensures that a coordinated plan is made to help the species survive. The specific plan depends on the species being protected.

        6. To conclude the lesson, distribute the Endangered Relationships Exit Ticket. Allow students time to complete the questions and collect.


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