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Every year, tiny shrimp do something strange on the banks of a river in Thailand: they get out of the water and walk on the land! Why do they take this risky path?

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Sources
http://doi.org/10.1111/jzo.12841

Image Sources:
https://www.inaturalist.org/observations/68719787
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Go to Brilliant.org/SciShow   to learn how you can take your  STEM skills to the next level this year! {♫Intro♫}. Every year in eastern Thailand,  at the height of the rainy season,   thousands of tourists flock to the edge of  the Lamdom river to see a remarkable sight.   All night long, streams of tiny shrimp march out  of the water and parade along the river’s edge.

And they’d have to have a really  good reason to take a risk this big. But until recently, no one knew why they did it!   Now, if you’re not familiar with shrimp,  walking is a weird way for them to get around. Shrimp are aquatic animals.

Like, they  have gills, and if they dry out, they die. Plus, there’s a slew of predators  waiting on land to eat them. Yet here these less than a  centimeter-long shrimps are,   leaving the water for 10 minutes or more  and crawling some 20 meters onto land!

The local lore was that it  had to do with breeding. Migrating upstream to breed is pretty  common for animals—like salmon,   and even other species of shrimp.  So, it’s a pretty good guess. But when scientists systematically  studied the parading shrimp, they found   that they were almost all juveniles.

That  means they were too young to mate or spawn.   Instead, the reason was much simpler and  cuter: it’s to avoid strong currents! Researchers realized two of the  most reliable places to see these   shrimp parades were next to a set of  rapids and a small artificial dam. So, for a 2020 article, they  tested this idea in the lab.   They set up tanks where  they could vary the current.

And, lo and behold, the smaller shrimp   got out whenever the current was too strong,  while the larger ones could still swim. That’s why, in the wild, these shrimp  that walk on land are so teeny. Bigger ones are also going  upstream, but they swim instead.

What we don’t know for sure is why  they’re so keen to go up the river. For clues, we had to look at other shrimp  that parade… Because yes, apparently,   other species do this as well! Including  ones in India, Australia, and the Americas.

Those species also seem to  parade to get around barriers. And the upstream areas they head  for often have fewer predators. So,   the behavior may be a survival tactic.

It may also shed light on how shrimp colonized  freshwater habitats in the first place. Their ancestors most likely came from the  ocean. So gaining the ability to walk around   barriers may have been a crucial part of the  move into resource-rich freshwater habitats.

But more practically, further research on shrimp   parades could help us take better  care of them and their habitats. On one hand, some freshwater  crustaceans are notorious   invaders. So, knowing how and why they move can  help us figure out how to keep them in check.

But on the other, many species are struggling, and  this sort of knowledge can help with conservation. These Lamdom shrimp likely  fall into that latter category.   It seems like fewer shrimp  show up to parade each year. And if that’s not bad enough,   as of yet unpublished research suggests  the animals themselves are getting smaller,   which means rapids and such might pose  even more of a problem in the future.

But, if we know what they can and can’t  handle, we might be able to help them out. Like, we could design structures that help these   cute little shrimp get around  dams or fast-moving water. That way, we can ensure these adorable  parades continue to amaze and delight us   for many years to come.

It’s pretty amazing that shrimp can tell when  they’re big enough to tough out some rapids.   They may have an instinctive  understanding of fluid dynamics…   something I definitely do not have.  Luckily I can turn to Brilliant! Fluid dynamics is notoriously complex,   but their differential equations course can  help you really understand how fluids flow,   and you can have some fun learning about math  because of their great interactive teaching style. Brilliant also offers courses in science,   engineering, and computer science.

And  with an annual premium subscription,   you get access to all of them — so you  can up your STEM skills all year long. Plus, as a thank you for watching  SciShow, you can actually get 20%   off the annual price! All you  have to do is go to sign up at   Brilliant.org/SciShow. {♫Outro♫}.