Between 2018 and 2021, tens of billions of snow crabs disappeared in the Bering Sea.

Like, somewhere between 10-47 billion of them. That’s a historical drop that  brought the number of snow crabs lower than ever recorded.

And it happened right after their  population hit a record high! We should be up to our elbows  in snow crabs right now! After some serious sleuthing,  we finally have an answer.

But you’ll want to come along for the whole ride. Here’s how the world’s leading crustacean experts solved the case of the missing snow crabs. [♪ INTRO] The last time you went to the beach, you probably didn’t happen to see any snow  crabs wandering the shore. And you wouldn’t, because they stow  away in the depths of the Bering Sea, where it’s so cold that for most of  the year, the water freezes over.

That makes them hard to get to,  which means marine researchers trying to check up on snow  crabs can only really do that in the summer when the water is navigable. They go out in a boat, look at  some crabs, and make an estimate of how the population is doing based on that data. That estimate is used to set  quotas for how many crabs can be taken out and eaten every  year so that we leave behind enough to keep the population  strong for next year.

We have to rely on just the summer  census and data from previous years to determine those quotas. But our window into the life of Alaskan  crabs was even more restricted in 2020, because they had to skip the annual crab survey that year due to the whole COVID thing. And really, that shouldn’t have  been a big deal because in 2018 there was a thriving ecosystem  full of more crabs than usual.

But in the summer of 2021, what they  found when they finally returned was devastating: they estimated  that billions of crabs were missing. So the researchers became detectives. They needed to figure out why they  saw so many extra crabs one year, and then a sudden loss of billions  of them just two years later.

The first question on everyone’s minds  was whether the count was even accurate. Considering that nobody had eyes  on these snow crabs for two years, it seemed possible that the  researchers’ calculations might have missed a few billion or so. But these studies didn’t just look at snow crabs, they looked at the other wildlife in the area too.

So if the drastically low  snow crab count was a mistake, then they probably also would have  had a drastically low count for other crabs in that area, like Tanner crabs. And that’s not what they saw at  all. Tanner crabs were doing fine.

Also, when they went back in the summer  of 2022 to sample the snow crabs again, the snow crab population was  still in pretty rough shape. So they couldn’t chalk this one up to error. Although there’s one other scenario  that would explain the missing crabs without going straight to crab apocalypse.

Crabopalypse? The crabs  could have moved to Russia! After all, it is right there.

And if you  like the cold, it’s a perfect habitat. So several boats went out  and looked for these crabs, both off the coast of Russia  and in the northern Bering Sea. And they didn’t find the missing  crabs hanging out in a new hip spot.

So it looked like the crabs probably hadn’t  traveled to another location after all. Things were looking bleak for the crabs, and the researchers had to consider  that they might not be alive anymore. So the new question on everyone’s minds  was what could have possibly killed that many crabs in such a short amount of time.

It seems like the kind of thing that  could only be the result of an act of God. Or maybe… an act of cod. See, there are these Pacific cod  that love to chow down on snow crabs.

So researchers started to  question whether a change in cod populations or diet led to  the demise of these snow crabs. A study in the journal Science  estimated that the cod were eating more crabs around the time of  the great disappearance than before. So the researchers might have  finally gotten a predator on the hook for the great snow crab massacre!

However, other researchers estimated  that cod weren’t eating any more crabs than usual, so this may not be the  slam dunk that it seemed to be. And even if cod were eating more  snow crabs, the researchers still questioned whether they were capable  of doing so much damage so quickly. Remember, the crab population was larger  than usual right before its collapse.

So even if the cod were eating more crabs, they probably weren’t doing  the kind of damage that would make an entire species disappear. And when they gave it more thought,  they realized that there was another hole in the cod theory. See, the few crabs that were left  post-masacre were mostly small, meaning all of the big ones went missing.

And cod tend to eat smaller  crabs when they have the choice. They wouldn’t have left  behind their favorite snack and eaten the cod equivalent of brussel sprouts! What’s even more surprising is that while  the crabs didn’t migrate, the cod did.

A bunch of them moved north where the  snow crabs don’t hang out as much. And that was enough evidence for the researchers to think the cod probably  weren’t their smoking gun. So with cod out, they needed another  predator to explain the disappearance.

And here’s where the researchers  may have jumped the shark, or in this case, the crab. They thought maybe the crabs became  cannibals and ate each other! Now, they had ruled out a lot of  other explanations by this point, so I could see them getting a little  desperate or losing some of their sense in the chase of whoever or  whatever caused these crabs to vanish.

But, as out there as this explanation  sounds, it isn’t so far-fetched. Snow crabs are kind of known to eat each other. So all of the snow crabs in the  Bering sea could have gotten wrapped up in a very literal food fight.

It should be easy enough to verify. Just look at the crabs’ stomach contents  like they looked at the cod’s stomachs. But that turns out to be difficult to do when the whole problem you’re trying  to solve is a lack of crabs.

One option is to look to the past,  when other scientists had checked out the stomach contents of  cannibalistic crabs in history. Yes, these records exist. From those reports, it appears that  cannibalistic crabs tend to only indulge once.

As in, most of them had remains of a  single individual in their stomachs. And, I mean, are you even a cannibal  if you’ve only done it once? Well, yeah.

But more importantly, that  means it’s not in their nature to suddenly go on a purge and eat every crab they meet. Basically, they didn’t think it was  likely for the sheer number of crabs to be eaten by other crabs in  such a short period of time. And they found another clue in the  records of historical crab cannibals.

Generally the bigger crabs were the ones  making a meal out of the little guys. Since the bigger ones went missing, there probably wasn’t a giant cannibal dinner party. Once scientists had ruled out cod and  crabs, the next possible culprit was… you!

Maybe not you specifically, but humans. More specifically, the humans that  fish for and eat these snow crabs. As the head investigator put it, overfishing  has kept marine researchers up at night.

Our love for those delicious crab morsels  could have run them out of existence. It could have. But it probably didn’t this time.

In the US and other parts of the world, we’ve done a lot of work to  make fishing more sustainable. So overfishing is much less of  a problem than it used to be. And at the time of the snow crab  population decline, even accidental bycatch of crabs as people fished for other marine goodies was significantly lower than  it had been in the past.

Which probably means that it’s  not such an important factor that it’d be capable of  causing this sharp a decline. So you’re off the hook…for now. Which put the detectives in a bit of a pickle.

All of the most likely crab killers had an alibi. And that points to natural causes. Maybe while we were all  dealing with COVID on land, the crabs were going through  their own pandemic in the ocean.

To be clear, nobody’s saying that  these crabs got literal COVID. But they could have gotten some other  infection that they also weren’t prepared for. For instance, they could have caught a fatal infectious disease called bitter crab syndrome.

And in the past, it’s done  a real number on snow crabs. Bitter crab syndrome tends to be  especially hard on small crabs because they molt more and in that process may  become more vulnerable to infection. It doesn’t always work that way.

But if the researchers expected the  small crabs to die in large numbers from bitter crab syndrome,  leaving behind only the big, sturdy snow crabs, that’s not what happened here. It might have contributed  to the population decline. But odds are it didn’t work alone.

Which brings us to the big reveal. Researchers finally figured out  what killed the missing snow crabs. Disease might have contributed.

But the main villain in this story  isn’t an infection at all. It’s heat. I mentioned earlier that crabs are more  susceptible to infection when they molt.

And they molt more when it’s hot. From 2018-2019, there was a marine heat wave. So the number of crabs suffering  from this disease may have increased in recent years because the  warmer waters led to more infection.

But that’s just the beginning. This heat wave could have  decimated the snow crab population in another even more critical way. If you’re anything like the marine detectives,  you might be thinking that the answer was in front of your eyes the whole time.

Climate change raised the  water temperature to be warmer than what this kind of crab can live in. They died from heat exposure. But here’s the thing - the temperature rose, but not enough to kill the crabs outright.

The researchers measured how hot the  water got at the bottom of the sea where snow crabs generally hang out, and they observed that it  rose to 3 degrees Celsius. And snow crabs in a laboratory setting  can handle water up to 8 degrees Celsius. So the crabs could have lived under those  conditions and it wasn’t too hot for them!

That means that heat wasn’t the  direct cause of their demise. But that marine heat wave was still why they died. Let’s go back to those laboratory  experiments where scientists discovered the upper limits of  how hot a snow crabs’ environment can get before bad things happen.

When they raised the temperature in  the lab environment from 0 degrees to 3 degrees, the crabs needed double the calories. That’s not the kind of increase that  an afternoon snack can take care of. So when the same thing happened in  nature, and their environment heated up, the crabs’ metabolisms increased  and they needed more calories.

Based on the amount of warming at  that time and measurements from the crabs’ bodies, they would have needed roughly 4x the calories they did the previous year. So this slight increase in ocean temperatures  probably led to billions of crabs starving to death, and the  remaining population was left malnourished from lack of food. And that’s where the detectives closed their case.

But it’s not where the story ends. Now that we know about this  historic loss of snow crabs, we have a responsibility to change  the guidelines around harvesting them. As hard as this event has been for people  in the Alaskan crab fishing industry to weather, and it has been  brutal, it’ll get even worse if we catch the last of the crabs before  they have a chance to rebuild.

Ultimately, Alaskan snow crabs  suffered this loss because of the human-caused climate crisis. And since people caused this  problem, we need to fix it. So the crab harvesting season  has been drastically limited for the last few years with  hope that the snow crabs may grow their population back to sustainable  numbers within about five years.

And the experts really do have hope  that the crabs can rebuild in that time. Until then, we’ll simply have to put our  crab bibs aside for regular old napkins. And ditch the tiny forks for normal sized  ones, even if they aren’t as much fun. [♪ OUTRO]