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Pelagic red crabs are actually lobsters - and that’s not even the weirdest thing about them! They sometimes wash up on shore in droves, signaling large scale climate events like El Niños and serving as a warning to marine biologists of our changing climate.

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Thanks to the Monterey Bay Aquarium for partnering with us on this episode.

We hope you learn something crabulous! [♪ INTRO]. This beach isn't normally red.

You are looking at tens of thousands of pelagic red crabs that washed up on the shores of Monterey Bay. They're also known as tuna crabs because tunas love to snack on them, but these crimson creatures aren't really crabs. They're a kind of squat lobster — crab relatives that look like flattened lobsters with their long tails that curl under their bodies.

And they're kind of like little red weathermen, because their presence on the beaches of California indicates something unusual going on below the waves. Normally, red crabs are mostly found in the waters off Baja California — a Mexican state that borders the US state of California. There, they dine on phytoplankton — those tiny, microscopic marine algae suspended in the water that we've talked about many times before — as well as any other edible bits they can find.

Once they're adults, they start to hang out near the bottom in the benthic zone. But the larvae, juveniles, and young adult crabs live in the epipelagic zone, or upper part of the open ocean — hence their name. Of course, pelagic red crabs don't always stay in one spot.

And their movements have been pretty reliably linked to ocean and climate patterns. Sometimes, these movements are helpful to the crabs. Like, they'll move with their food during certain stages of their lives.

Take a 2004 study in the journal Deep Sea Research Part II, for example. Scientists recorded water temperatures and salinities at different depths and locations along the Baja California coastline. That let them map out ocean currents and areas of upwelling — where cooler, food-filled water gets pulled upward to the warm ocean surface.

Scientists also used sonar and fishing nets to measure how many red crabs there were. And they found that there were more crabs in areas of strong upwelling. Turns out the pelagic red crabs were moving with the upwelling, which makes sense since the plankton they eat flourish in that nutrient-rich water.

It's also no coincidence that the lobsters' breeding season is right around the time of the year that upwelling normally occurs, since being able to get more food means healthier breeding adults. But, the crabs don't get much of a say in where they end up. The thing about spending quite a bit of your time floating around in the top layer of ocean is that you can easily get carried away… literally carried away!

The crabs can swim. Like, sort of. They propel themselves backwards by flapping their tails and tucking their legs into their bodies.

But they aren't great swimmers, especially when they're young. So, they can get swept up in strong ocean currents which carry them northwards to California and even as far north as Oregon. And when this happens, hordes of them may wash up on shore.

You'll sometimes find red crabs covering entire beaches! These mass strandings have been linked to large-scale climate events like El Niño, where the permanent trade winds that flow around the equator weaken, which causes warm currents to flow from South America northward along the California coast. For example, a study published in 2015 in Fisheries Science examined the number of red crabs in their usual habitats off the coast of Baja California six times between October 2004 and March 2007.

And they found that there were more crabs in the area during cold water La Niña events and fewer during warm water El Niños — because the warm waters were carrying the crabs away from Mexico and north toward California. The upside of all this is that washed-up red crabs may be a nice source of food for seagulls and other hungry predators during. El Niño events, when their usual fare tends to be more scarce.

But the crabs don't just signal El Niños. They're also warning marine biologists and beach-goers alike of our changing climate. For example, a study published in 2019 in Scientific Reports linked the distribution of red crabs with another kind of ocean pattern: marine heatwaves.

In the winters of 2014 and 2016, areas of the northeast Pacific ocean warmed to two to four degrees Celsius above normal for months on end, creating what some people called the warm-water blob. The blob wreaked havoc on ocean life, killing marine mammals and triggering harmful algal blooms. But it also meant that more than thirty-five species of marine mammals, fish, seabirds and algae temporarily moved — or got carried — northward.

That included pelagic red crabs who made it as far north as Newport, Oregon! These northern shifts might not seem like a big deal now, but scientists think marine heatwaves and other ocean warming events are already more frequent than they used to be, and are going to happen more often due to climate change. That means coastal animal and plant communities will likely look really different in the future.

And the red crabs show that changes are already happening, as strandings are becoming more and more common. The first recorded one was in 1859, then 1959, 1969, and 1983. Then, there were a total of eight recorded strandings between 2015 and 2017 alone.

In a way, these little red crustaceans are acting as bellwethers for the ocean. The impressive sight of millions of beached squat lobsters is a clear signal that our ocean is being altered — one that's far more visceral than, say, satellite images or temperature readings. And by studying these little red drifters, scientists can gain a better sense of how ocean habitats are changing and what the effects of those changes will be.

Thanks again to the Monterey Bay Aquarium for partnering with us on this episode of SciShow. The Aquarium's mission is to inspire conservation of the ocean. So give them a follow on their social media accounts, which are all extremely worth it, visit their website at

They shore would love to sea you! [♪ OUTRO].