Learn about why we still don’t know how eels reproduce and how scientists solved a 150-year-old question about how sandcastles hold together.
Learn about why we still don’t know how eels reproduce and how scientists solved a 150-year-old question about how sandcastles hold together.
We still don't know how eels reproduce by Grant Currin
Scientists have solved a 150-year-old equation that governs how sandcastles hold together by Grant Currin
Subscribe to Curiosity Daily to learn something new every day with Cody Gough and Ashley Hamer. You can also listen to our podcast as part of your Alexa Flash Briefing; Amazon smart speakers users, click/tap “enable” here: https://www.amazon.com/Curiosity-com-Curiosity-Daily-from/dp/B07CP17DJY
Find episode transcript here: https://curiosity-daily-4e53644e.simplecast.com/episodes/scientists-finally-know-how-sandcastles-work
CODY: Hi! You’re about to get smarter in just a few minutes with Curiosity Daily from curiosity-dot-com. I’m Cody Gough.
ASHLEY: And I’m Ashley Hamer. Today, you’ll learn about why we still don’t know how eels reproduce; and how scientists solved a 150-year-old question about how sandcastles hold together.
CODY: Let’s satisfy some curiosity.
Here’s something I bet you didn’t know: scientists still don’t know how eels reproduce. No kidding.
See, eels have proven to be one of the most mysterious animals out there. Over the centuries, scientists have tried to catch them in the act of reproducing, to no avail. No one has ever found a single eel egg.
And that might come down to their weirdly complicated life cycle.
Pretty much every eel of the genus Anguilla [an-GWILL-lah / alt link] begins its life in the Sargasso Sea in the Atlantic Ocean. That’s when tons of tiny eel larvae start a perilous journey home along the same migration routes their ancestors have traveled for tens of millions of years — some head to Europe, others to North America, with many taking nearly a year to make the trip. Along the way, the eel larvae balloon from about the length of a sesame seed to the length of your thumb. At this stage they’re called glass eels. People actually thought glass eels were a completely different species for thousands of years.
At this point, something wild happens. These marine creatures swim from the salty seas straight into freshwater rivers. It’s a change that would make most marine animals swell up and die, but not the eels. Their kidneys morph to keep blood salinity at safe levels. Once they get into the river systems, they’re called freshwater eels.
As the freshwater eels swim upriver and mature, they transform again into “elvers.” Elvers are basically the goats of the river: they’re super-omnivores that eat anything they can get their jaws around. They spend ten years gobbling down every morsel they can before turning into yellow eels. Still the same species, but again, we only realized that recently.
Yellow eels grow to about the length of a baseball bat. It’s only at this point that they develop sexual organs — organs that scientists over the centuries struggled to find in all those other quote-unquote “different” species of eel.
But there’s still one step left in an eel’s life. Yellow eels eventually turn into silver eels, and that’s when they make the entire journey again, in reverse. A 2015 study successfully recorded one eel doing this exact thing: in just 45 days, it traveled 1,500 miles or 2,400 kilometers from Quebec to the Sargasso Sea.
Researchers have never witnessed what exactly the eels get up to when they meet back in the Sargasso Sea, but many scientists think the eels eject dense clouds of sperm and eggs that join together and create millions and millions of sesame-seed-sized larvae. Once they’re ready, those tiny creatures will attempt to make the same perilous journey as their ancestors. At which point they’ll just have to turn around and come right back. But for now, evidence to prove this theory once and for all is something that just keeps giving researchers the slip.
CODY: Yeah. [ham] THEY SURE ARE, ASHLEY!
Let’s take a little break from the cold winter weather. Close your eyes, if you’re not driving, and think about the beach. Feel that warm sun beating down on your sunscreen-covered skin, hear the tide rhythmically roll on and off the sand, and see kids plopping sand out of buckets and sculpting firm — yet fragile — sandcastles. And now (you can open your eyes again), prepare to feel the awe... of PHYSICS! Because researchers have just confirmed how sandcastles work at the very smallest scale. New research shows exactly how water and sand interact at the micro level to form such strong, solid shapes. And this research has been a long time in the making.
We’ve known since 2008 that wet sand sticks together when water molecules between the grains form structures called capillary bridges. It’s the same thing that happens when you get your fingers wet and put them really close together: a little bridge of water forms between them. Water likes to stick to things, and the rough edges of sand grains make for a particularly good surface. That’s why even a small amount of water can quickly travel through a bucket of dry sand to make the whole thing moist and sculptable. Even humidity in the air is enough to form these capillary bridges.
That’s because of the super tight spaces between the sand grains, which gives rise to a phenomenon called capillary condensation. That’s when water vapor automatically condenses into liquid if it finds its way into a very tight space. Sir William Thompson — later known as Lord Kelvin — was trying to figure out this phenomenon as far back as 1871. Experts still rely on the Kelvin equation to understand how molecules behave where a liquid and vapor meet.
The Kelvin equation has worked well for 150 years. But, researchers are working at smaller and smaller scales these days, and conventional physics tends to break down at really small scales.
So, researchers behind this new study tried to break the equation by testing it at the molecular level. They built a truly miniscule experimental setup by layering atom-thick crystals of mica and graphite on top of one another. Between each layer, they placed tiny strips of graphene as spacers. Imagine a sandwich: two slices of bread with three strips of bacon equally spaced out in the middle. The gaps between the graphene “bacon” were just wide enough for a layer of water one molecule thick.
After running some experiments, they concluded that Kelvin’s predictions held up, even at the smallest scale possible!
It was a big surprise for the researchers. Lord Kelvin would have been surprised, too: he also thought the equations wouldn’t hold up at small scales.
Is this big news for the sand castle engineers of the world? Probably not. But here’s a research-backed tip to take with you to the beach: one pail of water to eight pails of sand will give you the strongest sandcastle around.
Let’s recap the main things we learned today
[ad lib optional]
CODY: Today’s stories were written by Grant Currin, and edited by Ashley Hamer, who’s the managing editor for Curiosity Daily.
ASHLEY: Today’s episode was produced and edited by Cody Gough.
CODY: In honor of eels and sandcastles, we’ll leave you with this famous quote from Ralph Waldo Emerson: “Live in the sunshine. Swim in the sea. Drink in the wild air.” and join us again tomorrow to learn something new in just a few minutes.
...okay, so maybe he didn’t say that last part. But, you should still do all those things.
ASHLEY: And until then, stay curious!