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Crown-of-thorns starfish are large, spiny, and eat coral reefs and without enough natural predators to control their population, someone had to create one.

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Imagine for a second that you are a starfish. Instead of being bilaterally symmetrical, with right and left halves that are mirror images of each other. Now you’re generally radially symmetrical, like a snowflake. Instead of having two arms, you have more, sometimes, a lot more. Say somewhere between five and 21 arms. And instead of two feet, you’re equipped with thousands of tiny little tube feet. You are, by all accounts, a fairly standard starfish, living your life on a coral reef somewhere, just doing starfish things. Then, one day, you realize it’s a little crowded on the reef. Suddenly, there are way more other starfish than there were before. And you are being hunted by robotic assassins.

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 Unfortunately you have turned out to be a crown-of-thorns starfish, sorry about that. Now let’s just leave imaginary-starfish-you here, because it’s about to get a lot more stressful. See, crown-of-thorns starfish are relatively large, spiny, destructive predators. It’s not their fault, it’s just who they are. They can be found on reefs throughout much of the Indian and Pacific Oceans, including on Australia’s Great Barrier Reef. And they are bad news for these reefs, because they eat coral. How they do this is deeply weird for me, as a human, but it’s also actually pretty weird for starfish, too. They extend their stomach out through their mouth onto the coral, then digest its soft tissue and absorb it directly into their stomach walls,  leaving the stony part of the coral behind. Bleh. “Normal” starfish feeding usually just involves wrapping their stomach around things so they can pick it up or sticking it into things like clams, which is still pretty disturbing.

Now, their habit of snacking on coral wouldn’t be a major problem, if it wasn’t for two things. First, corals that are already stressed out by things like changes in water temperature are more vulnerable to being fed on. Warmer ocean temperatures are becoming more common, due to human-driven changes in the planet’s climate. And second, crown-of-thorns starfish have boom-and-bust population cycles, where all of a sudden there can be a huge outbreak of this species. Now, as for what causes these outbreaks, scientists aren’t exactly sure. They might be a natural part of the starfishes’ ecology. Females can produce more than 60 MILLION eggs during their six month spawning season, but we also might be making them worse by doing things like overfishing the predators that are willing to take them on. And there also just aren’t very many of those!

Along with being unpleasantly pointy, the starfishes’ spines also pack a nasty, stinging punch. Neither of these things seems to deter the giant triton though. This half-meter-long marine snail actively hunts down the starfish and uses its serrated, tongue-like structure called a radula to tear into the crown-of-thorns and deliver paralyzing saliva. And while they only eat about one starfish a week, their presence alone is enough to make the starfish avoid an area. Unfortunately, their shells have made them a target for collectors and, while they’re a protected species now, they’re still rare on the Great Barrier Reef. And for that reef, the combination of stressed out corals, outbreaks of starfish, and not very many giant tritons spelled serious trouble.

 Without enough natural predators to control the population, well, it was time for us to create one. Enter our robotic assassins: underwater autonomous vehicles capable of delivering a lethal dose of a starfish-killing toxin. Now, the first step in making these killer robots was creating a computerized system that could accurately identify the crown-of-thorns. They’re not the only starfish on the Reef, after all. Back in 2005, the designers were able to get the robot to ID the starfish about 66% of the time,  but that wasn’t good enough. So they started using machine learning techniques to train the system, showing it hundreds of images of starfish gathered from YouTube and from divers they equipped with cameras. That helped teach the robots what the crown-of-thorns looks like in its natural habitat. But finding and identifying the starfish was only the first challenge, once they’d been located by the robot, it also needed a way to get rid of them without, you know, killing everything else on the reef.

And it turns out, the crown-of-thorns is actually really hard to kill. Chop off an arm, they can regrow it. Cut them in half, they may even be able to regenerate from that. Inject them with poisonous chemicals, no big deal, says the starfish because you’d actually have to hit every arm, all 20 of them, for it to kill the starfish.

But then, in 2014, researchers discovered a substance that’s totally harmless to us, but is super toxic to the crown-of-thorns. In fact, you’re probably making a type of this substance inside your body right now. It’s bile, that weird greenish-yellow fluid that helps you digest fats. Now, the researchers didn’t test human bile on the starfish, there’s not a ton of it to go around. Instead, they used bile salts derived from cows and sheep, and found that a single shot of the bile was enough to kill a crown-of-thorns within 24 hours. Turns out, bile causes a serious immune response in the starfish, and damages the membranes of their cells and their mitochondria. And, in more good news, fish that ate the dead starfish after they’d been injected with the bile were totally fine! So, with the ID system trained and an environmentally-friendly toxin identified, it was time for the engineers to build their first robot assassin.

They named it the COTS-bot, after a common acronym for the crown-of-thorns. And it worked! It correctly identified the starfish 99.4% of the time! In lab tests, it was so good at picking out the real starfish that it would just ignore 3D-printed models the researchers were using to test its abilities. And it worked in real-life, too. The COTS-bot did several hundred successful runs out on the Reef. But there were some downsides. The COTS-bot was a large, expensive piece of equipment that needed an expert to deploy it. It was time for an upgrade to a more compact assassin: the Ranger-Bot, now with bonus features!

Along with its regular killer capabilities, the Ranger-Bot is more maneuverable and able to operate for longer periods than the COTS-bot, and it can run at night, which is when the starfish are more active. It also has water-quality sensors and the ability to help reseed reefs with coral larvae, so it’s not just a killer, it’s also an important tool for helping monitor and rebuild damaged reefs. Reef restoration is actually what it’s primarily being used for right now, in its “LarvalBot” configuration! The Ranger-Bot is also cheaper than the COTS-bot, which the researchers hope means that they’ll eventually be able to build a Ranger-Bot army large enough to take on a major starfish outbreak. And, while an army of killer robots doesn’t usually sound like a good thing, in the case of this particular bizarre beast, it might be our best hope of saving the thousands of other bizarre beasts that make their home on the Great Barrier Reef.

 The Bizarre Beasts pin club subscription window is open again, NOW through the end of May 9th. So you can snag yourself a starfish and help us keep the show going, no trip to Australia required. Sign up now to get the crown-of-thorns pin in the middle of the month and the pins after that right around the time each new video goes live. It’s very fun. I get my pin in the mail and then it's like, "Oooo! I should check and see if the new Bizarre Beasts is up!" This is the gar one, it’s very cute. Also we’ll be sharing extra facts about this starfish all month on Twitter @BizarreBeasts, and on Instagram and Facebook @BizarreBeastsShow! As usual, profits from the pin club go to support our community’s efforts to decrease maternal mortality in Sierra Leone.

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