The science behind Thailand's mysterious shrimp parade

Research is beginning to uncover the secrets of a strange natural phenomenon in which thousands of freshwater crustaceans walk on land. 

Every year during the rainy season, northeast Thailand hosts a special parade.

From late August to early October, thousands of tourists flock to the banks of the Lamdom River to watch the nightly procession, but this parade doesn't feature the intricately carved wax figures of the Ubon Ratchathani Candle Festival or the explosions of the Bun Bang Fai Rocket Festival.

It's more of a shrimp parade.

For decades, locals have known that these shrimp emerge from the water and parade along the rocky shores.

There are statues, stories and even entire dances dedicated to crustaceans.

Fish biologist Watcharapong Hongjamrassilp first heard about prawn parades as a child in Bangkok, but as his research progressed, he began to wonder if scientists had actually studied this captivating natural phenomenon.

“I realized we didn’t know anything about it,” Hongjamrassilp, now a graduate student at the University of California, Los Angeles, told Elizabeth Preston of the New York Times.

There was almost nothing in the scientific literature about what type of shrimp they were or why they left the water to go upstream.

As a scientist, Hongjamrassilp was motivated to get to the bottom of what was happening with these shrimp, but his connection to Thailand and its people also kept him coming back.

"I wanted to do a project that could help people in Thailand and at the same time help the environment," he told National Geographic's Jake Buehler.

Hongjamrassilp and his colleagues spent two seasons observing and documenting hundreds of thousands of freshwater shrimp emerging from the Lamdom River in Thailand to walk on land.

Now, Hongjamrassilp is the lead author of a new paper published last month in the Journal of Zoology describing the phenomenon.

According to National Geographic, researchers believe these shrimp make their perilous journey to land, where they risk becoming food for frogs, snakes, and even spiders, to escape the river's strongest currents.

According to the Times, some shrimp traveled nearly 19 meters upriver and spent more than 10 minutes out of the water.

“I was so surprised,” Hongjamrassilp told the Times. “I never thought a shrimp could walk that long.”

To study what prompted these tiny shrimp to emerge from the water, the researchers brought the crustaceans into the lab.

After two years of perfecting the experiment, the team finally succeeded in getting the shrimp out of the water, according to National Geographic.

It turned out that increasing the current speed and using water directly from the river were essential to induce this unique behavior in a laboratory setting.

The researchers also found that darkness and cooler temperatures were also important factors for the shrimp to move to higher ground.

Finally, genetic analysis of the parading shrimp revealed that they were Macrobrachium dienbienphuense, according to the Times.

What remains a mystery is why exactly the shrimp are willing to risk everything to swim against the current.

Peter Novak, a freshwater ecologist with the Western Australian Department of Biodiversity, Conservation and Attractions who was not involved in the study, told National Geographic that the findings "raise interesting questions about why these animals are moving upstream if they don't need to be downstream in the first place."

In an interview with the Times, Alan Covich, an ecologist at the University of Georgia who was not involved in the study, said he was surprised by the number of tourists the shrimp parade attracts.

"We have crayfish festivals, we have all kinds of things," he says, "but usually it's people eating them, not people watching them walk."

Video: Shrimp Parade (Macrobrachium dienbienphuense)

Source: smithsonianmag.com