Sea Urchins

[00:00:00] Alex Re: Hello, welcome to On Wildlife. I'm your host Alex Re. On this podcast, we bring the wild to you. We take you on a journey into the life of a different animal every month, and I guarantee you, you're gonna come out of here knowing more about your favorite animal than you did before. Now this week's animal might not be one that you want to cuddle with, but they really are amazing.

[00:00:24] And these ocean dwellers are the definition of more than meets the eye. I think we'll learn a lot today because they're not the most widely talked about animals. So watch out for sharp objects as we dive underwater to talk about sea urchins.

[00:00:56] There are 950 sea urchin species in the world, so unfortunately I won't be able to talk about every single one today. But I will give you some insights about specific species and their overall characteristics. Sea urchins are invertebrates, meaning that they don't have a backbone like we do. Instead, they have a shell like structure called a test.

[00:01:22] Their test is made out of a calcium carbonate material called stereo, and this material makes up plates that interlock in the shape of a ball to form the sea urchins body. Sea urchins are classified as echinoderms. Derm means skin, and akino means spiny or prickly. This is really fitting because sea urchins are covered in spines, and the word urchin actually comes from an old English word, meaning hedgehog.

[00:01:53] Because of these spines, other animals that fall under the Echinoderm classification are starfish and sea cucumbers. So these are urchins closest relatives. And just like Starfish and other echinoderms, sea urchins have pento radios symmetry, meaning that their bodies can be divided into five equal parts.

[00:02:16] And sea urchins can be all kinds of different colors like purple, yellow, and red. One Red sea urchin species is actually called the Red Sea urchin, and it's also the world's largest sea urchin species. It can grow to be around seven inches in diameter and has spines that can be three inches long. Sea urchins live in the ocean and can be found in every ocean around the world, and they're only found in salt water.

[00:02:45] Depending on the species, they can live at various steps of the oceans as well. Some live in really shallow waters called intertidal zones. During high tide, these areas are underwater, but during low tide, they're exposed to the air. Sea urchin species that live in intertidal zones can live for short amounts of time outside of the water.

[00:03:08] The shingle urchin is a species that can live for a few hours outside of the water, but they're an outlier for most other species. Sea urchins that are a part of the Port Ide family are known to live in very deep waters reaching over 22,000 feet below the ocean surface, and sea urchins have existed on earth for an extremely long time.

[00:03:31] They first started to appear in the fossil record around 450 million years ago. That's over 60 million years before the first trees started popping up on earth. But what do sea urchins eat and how do they move? We'll find out right after the break

[00:04:00] on this episode. I wanna give a shout out to Anthony from Australia. His favorite animal is the Peacock mantis shrimp. And he actually gave me some fun facts about them that I'll share with you. They have the fastest punch in the animal kingdom and they see the world through 12 color channels, while humans only see the world through three color channels.

[00:04:23] Pretty crazy. Remember, if you wanna shout out or if you have an episode suggestion. You can send an email to on Wildlife dot podcast@gmail.com.

[00:04:40] Now back to the episode. When you're looking at a sea urchin from above, you really only see their spikes. But if you take a look underneath, you'll see their mouths. Their mouths are made of five sharp teeth in a circle, and this mouth structure is called Aristotle's Lantern. Their teeth are also self sharpening.

[00:05:05] Sea urchins are omnivores and their favorite thing to eat is algae. For the most part, they can be found scraping algae off of rocks with their teeth. One specific type of algae that they eat is kelp, which looks like really long leaves attached to the ocean floor. Sea urchins have an esophagus and intestines that help them digest the food, and they excrete waste through a small hole at the top of their body.

[00:05:32] Now you may be wondering how they move around well in between their spines. Sea urchins also have structures called tube. Feet. Tube, feet are controlled by the water vascular system. If you listen to our episode on sea stars, you may remember that the water vascular system is made up of canals throughout their bodies that they push water through.

[00:05:56] When the water is pushed through their tube feet, it helps them extend and contract allowing for movement. The water vascular system is also how sea urchins breathe. As they push water through their bodies, they're able to absorb the oxygen from the water and excrete carbon dioxide. Sea urchins tube feet aren't just for movement purposes, though.

[00:06:19] They're also able to create suction, which enables them to stick to surfaces. Not only that, but there's also evidence that their tube feet can detect light. Sea urchins don't have eyes like we do, but they may be able to use these tube feet to detect if there's something nearby like a predator. Speaking of predators, sea urchins have a good amount of them.

[00:06:43] Some of their top predators are sea otters, but other animals that'll eat them are seagulls, crustaceans like crabs and lobsters, and even sea stars and other sea urchins. The most obvious defense against these predators are their spines. Sea urchin spines come in all shapes and sizes. The spines of the black long spine sea urchin can be almost a foot in length, and some spines can also have venom in them for extra protection against predators.

[00:07:15] The flower urchin is known as the most venomous urchin in the world. They hide their spines underneath these structures called pedicellariae. That was a mouthful. Pedicellariae are similar to tube feet, but they're shaped kind of like a claw, and they're able to grab onto things. Some species like the sea potato don't have venomous spines.

[00:07:38] Instead, they burrow themselves underneath the sand in order to hide from predators. And many other species, such as the tan sea urchin cover themselves in rocks and shells. Scientists believe that they do this to protect themselves against predators and also against the harmful effects of UV rays from the sun, like their own way of wearing sunscreen.

[00:08:04] Sea urchins reproduce using a method called broadcast spawning. Basically, the female releases her eggs into the water and the male will then release his gametes to try and fertilize them. Sometimes instead of releasing the eggs, the female will hold them in between her spines in order to protect them.

[00:08:24] And they can release millions of eggs into the water at a time, relying on the current of the ocean to bring the male and female gametes together for fertilization. The Red Sea urchin can release 2 million eggs at once and sea urchins have a really interesting life cycle because their larva don't look anything like adult sea urchins.

[00:08:47] Their larva is called a plutus, and they kind of have the shape of really tiny squid. At the beginning of this larva stage, they'll have four arms. Then eventually they'll grow two more, and eventually two more. So their final form at the larva stage has eight arms. During this time, they don't really swim themselves.

[00:09:09] They float along with the ocean current and eat plankton. What's really interesting is that even though they float throughout the ocean, research suggests that most of the time they come back to the same shoreline where they were born. Nobody really understands how or why they do this yet. After about a month, they'll undergo a metamorphosis where a juvenile sea urchin will grow inside of the plutus and eventually burst out of it, taking a form that resembles what an adult sea urchin looks like.

[00:09:41] Just smaller. When sea urchins are juveniles, they're at a greater risk to being eaten by predators than adults. So sometimes they'll actually hide under the adult sea urchins for protection. When adults sense that there's a predator nearby, they'll send out a chemical signal that alerts juveniles to hide underneath them.

[00:10:03] Even though sea urchins don't have a brain, they do have a nervous system that allows them to sense the environment around them and respond to stimuli. Stimuli is just a fancy way to say anything that can trigger a response, whether it be physical or behavioral. And scientists have observed that some sea urchins can live to be up to 200 years old.

[00:10:28] A lot of research on this has been done on Red Sea urchins, so there may be sea urchin species out there that can live even longer, but we just don't have enough evidence of it yet. Now sea urchins can have large impacts on their ecosystems, both positive and negative. So we'll take our last break and then talk about it

[00:11:00] time for our trivia question. What is a group of hedgehogs called?

[00:11:17] The answer is a prickle.

[00:11:27] Sea urchins are very important to the ecosystems that they're a part of. Remember when we talked about how they eat algae, it's really important that they keep this algae in check. We've talked about algal blooms a bunch of times on this show, but to recap, when there are too many nutrients in the water, algae starts to grow exponentially.

[00:11:49] When this happens, it can be a risk to other organisms living in that water because they deplete the area of oxygen. Sea urchins can help prevent this in some cases because of all the algae that they eat. A specific type of algae that I talked about earlier is kelp. When sea urchins eat kelp, they break it down into pieces and some of them don't get eaten by the urchins themselves.

[00:12:13] There are other animals in the ocean that are Detroit DeVores, meaning that they're scavengers. These Detroit DeVores don't really eat the kelp by themselves, but they are able to eat it when it gets broken down by the sea Urchins. Detri Tri DeVores will also eat the sea urchins waste, which has a lot of nutrients.

[00:12:33] So not only are they feeding themselves, but sea urchins are also feeding other animals in the ecosystem just by eating kelp. They're also a food source for many predators whose populations are being depleted like sea otters. Now, this is where the problems come in when sea Otter populations decrease.

[00:12:53] There are less animals controlling the sea urchin population. This causes sea urchins to eat unchecked, and they can basically destroy an entire kelp forest, which is home to so many different organisms if they don't have predators to help reduce their populations. This is why it's so important not to disrupt the balance of the food web because it has a domino effect on every living thing in that area, despite their ability to take over an entire habitat if they're left unchecked.

[00:13:26] There are some sea urchin species that are in trouble. Species like the Stony Sea urchin and the European edible sea urchin are slowly declining. Scientists believe that this can be due to the rising temperature of our oceans, which is killing off coral reefs. And coral reefs are where a lot of sea urchins call home.

[00:13:48] Humans also eat the reproductive organs of sea urchins in the food world. This is called uni. So the hunting of sea urchins for food can also deplete their populations. And luckily there are organizations that are working to help remove sea urchins in areas where they've been left unchecked and working to help habitats that sea urchins live in.

[00:14:10] Some of these organizations are UR Genomics, the Sea Otter Foundation and Trust. And the Ocean Conservancy. Thank you so much for coming on this adventure with me as we explored the world of sea urchins. You can find the sources that we use for this podcast and links to organizations that we reference@onwildlife.org.

[00:14:32] You can also email us with any questions at On Wildlife dot podcast@gmail.com. And you could follow us on Instagram at On Wildlife, or on TikTok at On Wildlife. And don't forget to tune in next month for another awesome episode, and that's on Wildlife

[00:14:57] Jess Avellino: You've been listening to On Wildlife with Alex Re. On Wildlife provides general educational information on various topics as a public service, which should not be construed as professional financial, real estate, tax, or legal advice. These are our personal opinions only. Please refer to our full disclaimer policy on our website for full details.