Invasive Species Council of British Columbia

Eelgrass Ecosystem – Spot the Difference 

Grade: 1 to 7

Duration: 30 mins

Setting: Indoor

Subject: Science

Physical Activity: No


Spark students’ curiosity by analyzing some differences between a healthy B.C. coastal eelgrass ecosystem and one invaded by European green crab, complete with engaging graphics and teacher reference key.  Extend the activity with a Kahoot Quiz to challenge students on some fun facts about European green crab. Thank you to the Pacific Salmon Foundation and the Port of Vancouver for their support in the development of this activity (see Additional Resources for links to their education and environment protection programs)


Why We Care About Invasive Species 

Habitat Hit – What Happened Here? 

Design an Invasive All-Star 

  • What are eelgrass habitats and why are they important?  
  • What is an invasive species and what impacts can they have on the environment? 
  • How do European green crabs harm coastal ecosystems in BC? 


Science Curricular Competencies  

  • Demonstrate curiosity and a sense of wonder about the world (Grades 1-4) 
  • Observe objects and events in familiar and unfamiliar contexts (Grades K-6)  
  • Compare observations with those of others (Grades 1-2) 
  • Make simple inferences based on their results and prior knowledge (Grades 3-4)  
  • Make observations aimed at identifying their own questions about the natural world (Grades 7-8) 

Science Big Ideas 

  • Living things have features and behaviours that help them survive in their environment (Grade 1) 
  • Living things have life cycles adapted to their environment (Grade 2) 
  • Living things are diverse, can be grouped, and interact in their ecosystems (Grade 3) 
  • All living things sense and respond to their environment (Grade 4) 
  • Multicellular organisms have organ systems that enable them to survive and interact within their environment (Grade 5) 
  • Multicellular organisms rely on internal systems to survive, reproduce, and interact with their environment (Grade 6) 
  • Evolution by natural selection provides an explanation for the diversity and survival of living things (Grade 7) 
  • Electronic devices (iPads or Tablets) to play Kahoot. 
Spot the Difference Images 
European Green Crab Quiz Questions:   

In this activity, learners will be able to spot the difference between a healthy eelgrass ecosystem and one that has been invaded by European green crabs (EGC). Two similar images are provided, with slight differences between the two that highlight the impact European green crabs have on an eelgrass environment when they are introduced. Learners can compare the image of the healthy eelgrass ecosystem and spot the changes to the ecosystem that European green crabs can cause.  

Importance of Eelgrass  

Eelgrass (Zostera marina) isn’t a seaweed or a grass, but is a flowering plant with long, green ribbon-like leaves that grows on muddy and sandy shores in shallow bays, lagoons and estuaries along the BC coastline and throughout the Northern hemisphere.  Eelgrass can form dense “meadows” or “beds” that have great ecological importance.  Some of the important traits of eelgrass meadows include the following:  

  • Calming the seas.  Eelgrass has dense roots systems that stabilize shorelines, buffer the force of waves during storms, and prevent coastal erosion and flooding. 
  • Food Factory.  Eelgrass supports the marine food web.  Diatoms, algae, bacteria and other microscopic organisms form a thin brown coating on eelgrass fronds, providing food for grazing marine animals. Over 400 species of algae grow on eelgrass!  Additionally, eelgrass leaves, roots, and seeds are important sources of food for seabirds, fish and numerous marine invertebrates.  The abundant variety of herbivores that are drawn to eelgrass meadows are in turn food for other consumers, supporting great biodiversity, from shellfish, sea slugs, and sand worms, to salmon and seals! (Try saying that 10 times fast!) 
  • A Protective Nursery.  Eelgrass meadows are like the nursery or preschool of the ocean.  Eelgrass creates calmer waters and forms an underwater forest with safe places for young marine animals to hide, eat, and grow before they move to the open ocean.  Eelgrass beds are important nursery and spawning sites for herring, lingcod, rockfish, and salmon, and are home to Dungeness crabs and many species of shellfish.  Many young fish spend the early part of their life cycle in eelgrass before moving to the open ocean.  It’s estimated that 80% of commercially important fish and invertebrates depend upon eelgrass for part of their life. 
  • Lungs of the Sea.  Eelgrass is a fast-growing, productive plant. As it grows and photosynthesizes, it captures atmospheric carbon (is a carbon sink).  It’s estimated that almost a third of the Earth’s oxygen that is produced in the benthic zone (ocean sediments) is from eelgrass photosynthesis.  
European Green Crab and Threats to Eelgrass  

Eelgrass ecosystems are in decline around the world due to pollution and habitat alteration.  Coastal development, such as dredging and dock building, rips up eelgrass and it often doesn’t recover.  Where eelgrass is lost, a decline in shellfish, fish, birds, and other biodiversity follows, while beach erosion increases.   

The invasive European green crab also poses a threat to eelgrass.  European green crabs are active and voracious predators.  They tear and uproot eelgrass and prey upon the species that live in this biodiverse habitat.  European green crabs alter ecosystems, and the differences between a healthy ecosystem and one impacted by European green crabs can be easy to spot. 

For a general overview on invasive species read Background on Invasive Species for Educators.   

For more information on European Green Crab see the Additional References section.  

  1. Download the 5 files for Spot the Difference. Either print copies of the images of Healthy Eelgrass and Eelgrass Invaded by European Green Crab, or present the images on a screen for a whole class/group participation 
  2. Review Background Information section about eelgrass habitats and European green crab (EGC).  
  3. To prepare for the EGC Kahoot, see the Additional Resources section for links to engaging stories and visuals on EGC and its spread in Puget Sound and British Columbia.   

Introduce the activity.  Ask if anyone has ever been to and sandy or muddy bay or has heard of eelgrass meadows.  Share their importance and introduce the invasive species that threatens them, the European green crab.  You may want to provide some background information on invasive species in general, such as by showing this short video (1:42). Invaders! Invasive Species in BC.  

Part 1. Spot the Difference 

Hand out or show on an overhead the Spot the Difference images to see how many differences can be found.  After finding differences between the images, be sure to discuss them and use the prompts, below, to help guide observations.   

Spot the Difference Observation Tips 

Image 1: Healthy Eelgrass Ecosystem 

Observe the following:

  • Who is using the eelgrass as a nursery?  Look for healthy salmonid and herring juveniles (important in food fisheries) and other young species.  
  • How many types of bivalves can you find? Look for smooth pink scallop, Butter clam, Soft-shelled clam, Blue mussel, and Littleneck clam 
  • Who is living and feeding on the sea bottom? Look for bottom-feeders like Dungeness and Red rock crabs, flat fish, and sea stars  
  • Who is sheltering and is camouflaged from predators (such as the harbour seal, perch and salmon)? Look for shrimp, sea slugs (Taylor’s sea hare and Hooded nudibranch) and the elongated fishes called the Penpoint gunnel and Kelp pipefish.  
  • Look for the ducks and geese that feed on eelgrass and the fish that live there, like Brant goose and the Red-breasted merganser. 

Image 2: Eelgrass Ecosystem Invaded by European Green Crab 

Observe the following:

  • How many animals are gone? Are there any species that have entirely disappeared? 
  • Missing patches of Eelgrass that provided shelter and nursery habitat for juvenile fish species like salmon and herring.  
  • Patches of eelgrass being mined and dug up by EGC causing disturbed ground and cloudy water.  
  • Compare and contrast.  How many European Green crabs are now in the picture compared to native crabs?  
Part 2:  It’s Quiz Time!  Take the European Green Crab Challenge  

After playing Spot the Difference, dive into learning more about the invasive European green crab, such as where it came from, its life cycle, and how it spreads.  Start out by sharing information with your learners about EGC (see the Additional Resources links).  Then challenge your group and learn even more by playing a fun game of Kahoot.  If you don’t have access to electronic devices to play Kahoot, split your class up into teams and read the questions aloud.  (Quiz questions are available in the Documents to Download section.) 

Additional Resources 
European Green Crab 
Share with us!  

We’d love to learn from you too and help us with our shared campaign to fight the spread of EGC in B.C. Share with us your students’ learning about the EGC. Send to for the opportunity to win resources and have your class have a virtual visit with an invasive species expert!