The Science Behind Thailand’s Great Shrimp Parade

New research begins to unravel the secrets of a strange natural phenomenon in which thousands of freshwater crustaceans march on land

Shrimp parade in Thailand
A group of shrimp that have left the safety of the water to parade upstream in Thailand. Watcharapong Hongjamrassilp

Every year during the rainy season, northeastern Thailand hosts a special parade. From late August to early October thousands of tourists flock to the region’s riverbanks to catch a glimpse of the nighttime procession, but this cavalcade doesn’t feature the intricately carved wax figures of Ubon Ratchathani’s Candle Festival or the explosions of the Bun Bang Fai Rocket Festival. Instead, it’s a parade of shrimp.

For decades, locals have known that these shrimp climb out of the water and march across the rocky riverbanks. There are statues, folktales and even entire dances devoted to the crustaceans. Fish biologist Watcharapong Hongjamrassilp heard about the parading shrimp during his childhood in Bangkok, but as his research progressed he started to wonder if scientists had actually studied this captivating natural phenomenon.

“I realized that we know nothing about this,” Hongjamrassilp, now a graduate student at the University of California, Los Angeles, tells Elizabeth Preston of the New York Times. There was almost nothing in the scientific literature about what kind of shrimp these were or why they left the water to move upstream.

The Science Behind Thailand's Great Shrimp Parade
A juvenile parading shrimp seen marching across the surface of a wet river rock. New research has identified these freshwater shrimp as a species known to scientists as Macrobrachium dienbienphuense. Watcharapong Hongjamrassilp

As a scientist, Hongjamrassilp was motivated to get to the bottom of what was going on with these shrimp, but his connection to Thailand and its people also drew him back. “I wanted to make a project that could help people in Thailand and at the same time help the environment,” he tells Jake Buehler of National Geographic.

Hongjamrassilp and his colleagues spent two seasons observing and documenting hundreds of thousands of freshwater shrimp emerging from Thailand’s Lamdom River 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.

Per National Geographic, the researchers think these shrimp make their perilous journey on land, where they’re at risk of becoming food for frogs, snakes and even spiders, to escape the river’s strongest currents.

The Science Behind Thailand's Great Shrimp Parade
When the shrimp leave the water they are easy prey for spiders, snakes, lizards and frogs that lay in wait along the riverbank. Watcharapong Hongjamrassilp

Some individual shrimp travelled nearly 65 feet up-river and spent more than 10 minutes out of the water, according to the Times. “I was so surprised,” Hongjamrassilp tells the Times. “I never thought that a shrimp can walk that long.”

To investigate what drove the tiny shrimp out of the water, the researchers brought the crustaceans into the lab. After two years of tweaking the experiment, the team was finally able to get the shrimp to leave the water, according to National Geographic. It turned out, increasing the speed of the current and using water straight from the river were key to inducing this unique behavior in a lab setting. The researchers also found that darkness and cooler temperatures were also important cues for the shrimp to head for higher ground.

Finally, genetic analysis of the parading shrimp revealed them to be Macrobrachium dienbienphuense, per the Times.

The Science Behind Thailand's Great Shrimp Parade
A large group of shrimp marching on land to make their way upstream alongside the Lam Duan Rapids of Nam Yuen city, Thailand. The crustaceans' journey is mostly relegated to the riverbank's splash zone, which researchers hypothesize may help keep the shrimps' gills wet during their journey. Watcharapong Hongjamrassilp

What remains mysterious is why exactly the shrimp are willing to risk it all to get upstream. Peter Novak, a freshwater ecologist at Western Australia’s Department of Biodiversity, Conservation and Attractions who was not involved in the study, tells National Geographic that the findings “raise interesting questions about why these animals are moving upstream if there is no need to be downstream in the first place.”

Speaking with the Times, Alan Covich, an ecologist at the University of Georgia who was not involved in the research says he was most surprised by how many tourists the shrimp parade attracts. “We have crayfish festivals, we have all kinds of things,” he says, “but generally it’s people eating them, not watching them move.”