Episode 6: The Crab and the Urchin

Learn about the symbiotic relationship between the urchin and the crab with the Rookery Bay National Estuarine Research Reserve offsite link and a National Sea Grant Knauss Marine Policy Fellow. 

A single sea urchin and crab on the sea floor with the text "Episode 6: The Crab and Urchin" and logos for NOAA, Coastal Ecosystem Learning Centers, and Octonauts.

(Image credit: NOAA OKEANOS Explorer Program, 2013 Northeast U. S. Canyons Expedition)

Audio file

MUSIC: [Octonauts theme music] Octonauts, to your stations! Barnacles! Kwazii! Peso!

HOST: You’re listening to "NOAA & the Octonauts" — an episode-by-episode discussion of the children’s TV show The Octonauts, which features a crew of quirky and courageous undersea adventurers. Their mission: to explore the world’s ocean, rescue the creatures who live there, and protect their habitats. 

MUSIC: [Octonauts theme music] Explore! Rescue! Protect! Octonauts!

HOST: Our monthly podcast brings together experts from inside and outside of NOAA to help you — and the children you care about — learn more about the real-life versions of the Octonauts sea creatures and the ocean they call home. 

This podcast is hosted by NOAA’s Office of Education and the Coastal Ecosystem Learning Centers network. For today's podcast, we are talking about the Crab and the Urchin Octonauts episode. It's bedtime for the Octonauts until a knocking sound wakes up Kwazii and Captain Barnacles. A snail tells them that a green-blue crab and an orange sea urchin are arguing loudly and keeping all the sea creatures up. Shellington explains to Captain Barnacles that the crab and urchin are symbiotic, meaning that they have a mutually beneficial relationship, and need each other for food and protection. The urchin relies on the crab, just as the crab relies on the urchin.

Before we begin our chat, let’s set the stage for the deep dive. The two invertebrates featured have a large amount of diversity in the two groups with over 6,000 different species of crabs and 950 species of sea urchins. The crab in this episode is most likely a carrier crab, or it is also known as the “urchin crab”. This crab can carry fire urchins on their backs and when confronted with a predator, the crabs have been known to wield the spiny urchins, like a weapon. The crab is built with two back legs that are especially adapted to grab anything from debris, corals, and even sponges. In return for it’s protection, the sea urchin is able to find food more quickly and effectively when traveling with the crab. Now crabs are not the only marine creatures to benefit from the urchins' sharp spines. Sometimes smaller fish also use the urchins for safe shelter from predators while moving throughout the reef.

Our guests today are Dita O’Boyle from the Rookery Bay National Estuarine Research Reserve and Halle Berger, a 2021 Sea Grant Knauss Marine Policy Fellow. Thank you both for being here, Dita and Halle! Dita, can you maybe kick us off and tell us what you do at the Rookery Bay?

DITA O'BOYLE: Sure, estuaries are important areas where the river meets the sea and act as nurseries for numerous marine animals. I take the work that the researchers and staff do at the reserve and create activities and programming for the public. I work with a variety of audiences from kindergarteners to retirees sharing my passion about estuarine animals and their habitats. Our programming covers a huge range of topics from plankton to watersheds.  One of my favorite topics is crabs as they are such a diverse group of animals.  I love how you can look at their physical adaptations to figure what habitats they use or how they get their food.  We also keep different species of crab in our aquariums to discuss this with guests.

HOST: I am so happy you mentioned crabs, Dita, we are definitely going to come back to that. But first I want to hear from Halle. Halle, can you share a little bit about your work as a Knauss Fellow?

HALLE BERGER: As a Knauss fellow, I am working with NOAA program managers to help build a research community that will address the challenges of ocean acidification and harmful algal blooms, which are coastal stressors that may interact to negatively impact marine ecosystems. I am also currently a graduate student at the University of Connecticut, where I research the vulnerability of crabs and other shellfish to climate change using computer models and laboratory experiments.

HOST: Wow, You both come from really different backgrounds and I am curious which Octonauts’ job you think that you have? Do you think you are a researcher like Shellington, an oceanographer like Inkling, a systems analyst like Dashi, a leader like Barnacles?

DITA O'BOYLE: This is a toughie, as depending on the day I have tasks similar to different crew members. I think I’ll stick with Dashi as I do assist with capturing images of our animals on exhibit and love reading up on the animals in the reserve.   

HOST: And what about you, Halle? Which Octonauts character captures your current position?

HALLE BERGER: I would say I am a researcher like Shellington because we are both naturally curious and passionate about studying marine life.

HOST: Now, Halle you mentioned previously working with crabs and, Dita, you did as well, and you mentioned they are some of your favorite organisms to work with. Can you both expand a little bit more about the projects you currently have with crabs and why they are important?

DITA O'BOYLE: At our environmental learning center we have a series of aquariums and a touch tank. We have a permit to display local species, including crabs, I use these animals and others to introduce students to the estuaries in their backyard. Some of my favorites are the spider crab, and stone crab. They are all found in the same area but adapted to very different habitats. Spider crabs rely on camouflage by sticking things to their back to hide from predators while looking for soft food often near sea grass.  In comparison the stone crab is a predator and has very large strong claws to break into snail shells. 

HOST: It sounds like the spider crab also attaches different organisms and maybe different types of debris to its body the same way the urchin crab in this episode does. Halle what type of crabs do you work with for your research?

HALLE BERGER: I worked with Dungeness crab for my master’s research. This species can be found from the deep cold waters of Alaska to California. I was interested in learning about how the crab will be impacted by climate change in the future.

HOST: I know that before I had watched the Octonauts episode I had not previously heard of the crab and the urchin relationship and so I am wondering, and I think our listeners might be too, does the urchin actually hurt the crab in this symbiotic relationship? 

DITA O'BOYLE: Not in this case. The urchin is getting a free ride, easier access to food and the crab receives increased protection from predators!  This is a great example of symbiosis. 

HOST: So it sounds like this relationship is beneficial. I know there are different types of symbiosis, can you tell me the difference between mutualism, commensalism and parasitism? And which category the urchin and the crab would fall in?

DITA O'BOYLE: This would be an example of mutualism where both organisms are benefiting, the crab gets protection, the urchin more food. Another type of symbiosis would be commensalism, where only one animal benefits.  A good example would be a remora gets a free ride on a shark, and the shark gets nothing. These two are very different than parasitism where one animal is benefiting by harming the other animal. I believe this comes up in a later episode about the ocean sunfish and parasites. 

HOST: Halle, have you ever come across this type of relationship in your research?

HALLE BERGER: Not yet, because I have only studied one species of crab that does not participate in symbiosis. But I think it would be interesting to study how these types of symbiotic relationships change under stressful environmental conditions.

HOST: Well that sounds like a new graduate thesis if I have ever heard one! Can you expand on some of the threats that the crab and the urchin may be facing and if these threats impact their behavior or their ecology at all?

HALLE BERGER: Crabs and urchins can both be important parts of the marine food web. For example, crabs are great at cleaning up the seafloor and returning nutrients to the food web because they will eat pretty much anything. Urchins are especially important in coral reef ecosystems because they graze on algae that would otherwise overgrow the corals. Crabs and urchins face many threats including climate change, habitat destruction, disease, and overfishing. However, researchers at NOAA are helping to better understand and mitigate these threats.

HOST: Now, if I wanted to go and see the Octonauts episode first hand and see the urchin and the crab symbiotic relationship, where would I go? I have my scuba diving certification and I love snorkeling; is there a particular spot that I should head to see the urchin and the crab in person?

HALLE BERGER: You’d have to go snorkeling or diving in the Red Sea or Indian Ocean in order to see crabs carrying urchins like displayed in the episode. While that is far away for many folks, there are different types of crabs and urchins that are located all around the world, so it's likely you'll be able to spot them in the closest bay or ocean to you! And many times you can see at least crabs from the beach or in tide pools.

HOST: Now maybe one day I will be lucky enough to travel to the Red Sea but I don’t know if that is going to be anytime soon. Dita, in the meantime, if our listeners can’t get out to the Indian Ocean or the Red Sea, can you tell us about the work that's being done on crab research at the Rookery Bay National Estuarine Research Reserve?

DITA O'BOYLE: The Reserve helps crabs by conducting our own research onsite and starting conversations between local officials and agencies involved in fisheries management, which includes stone and blue crabs. The Reserve also welcomes visiting scientists to answer ecological questions in the area. As an educator, it is my job to spark curiosity about crabs and other animals in students and the general public. We want to encourage them to care about their environment. If you happen to be in Naples, Florida I encourage you to visit our Environmental Learning Center to see some of our local crabs and other animals to learn about their role in the estuary.  

HOST: This is really important research and we will make sure to link to the Rookery Bay reserve in this podcast. Once again, thank you again, Halle and Dita, for being here and answering every single question we had about sea urchins and crabs. We appreciate your time on our show!

MUSIC: [Captain Barnacles’ theme] [Octo-Alert]

HOST: That’s the Octo-Alert. We want to hear your questions. What would you want to ask an expert about crabs and urchins? Or may you have a few questions about other symbiotic relationships found in the ocean? We’ll make sure to pass these questions along to our NOAA scientists and aquarium educators. Just head to NOAA’s Office of Education Twitter, Facebook or Instagram to post your questions and we will post their answers!

Well, That’s it for today’s show! Be sure to check out our website and NOAA’s photo library, where you can find more resources on symbiotic marine organisms.
And to learn more about the local crabs and urchins found in Florida, check out Rookery Bay’s website at rookerybay.org

MUSIC: [Bubbles]

HOST: I’m Gabrielle Corradino and this has been NOAA & The Octonauts. See you next time. 

MUSIC: [Creature Report] 
Kwazii, activate creature report!
Creature report, creature report, creature report!

Creature report, creature report, creature report!

Crab has an urchin on his back
So urchin gets to share the snack
But there is more to their connection
The urchin gives the crab protection
Crabs and urchins help each other
Symbiosis means they depend on one another

Dance break!

Go crabs, go urchins, go crabs

We’re done with our mission. Octonauts, at ease, until the next adventure!