These Shrimp Go away the Security of Water and Stroll on Land. However Why?
The shrimp stop swimming at dusk and gather on the river bank. After sunset, they begin to climb out of the water. Then they march. The centimeter-long crustaceans roam the rocks all night long.
The prawns on display in northeast Thailand have inspired legends, dances and even a statue. (The locals eat them too.) During the rainy season, between late August and early October, tourists crowd the shores with flashlights to watch the shrimp walk.
Watcharapong Hongjamrassilp first learned about the shrimp exhibiting about 20 years ago and the hundreds of thousands or more tourists who come to visit them each year. When he started studying biology, he returned to the subject. “I realized we don’t know anything about it,” he said: what types are they? Why and how do they leave the safety of water to walk upriver on dry land? Where you go?
Mr. Hongjamrassilp, a graduate student at the University of California at Los Angeles, decided to answer these questions himself. His results appeared in the Journal of Zoology this month.
Working with Wildlife Center staff, Mr. Hongjamrassilp staked out nine locations along a river in the Thai province of Ubon Ratchathani. They found shrimp in two places – a rapids and a low dam.
The videos they recorded showed that the shrimp were displayed from sunset to sunrise. They traveled up to 65 feet upriver. Some individual shrimp stayed out of the water for 10 minutes or more.
“I was so surprised,” said Mr. Hongjamrassilp, “because I never thought a shrimp could walk that long.” If they stay in the river’s splash zone, they can keep their gills moist so they can continue to take in oxygen. He also observed that the clam shells seem to trap a little water around their gills, like a reverse dive helmet.
DNA analysis of captured shrimp showed that almost all of them belonged to the species Macrobrachium dienbienphuense, which belongs to a genus of shrimp that lives mostly or entirely in fresh water. Many Macrobrachium species spend part of their lives migrating upstream to their preferred habitats.
Most of the demonstrating shrimp that Mr. Hongjamrassilp caught were young. Observations and laboratory experiments indicated that these shrimp are likely to leave the water if the river becomes too strong for them. Larger adult shrimp can handle a stronger current without washing them off, making them less likely to exit the water.
Going ashore is dangerous for the small shrimp, even under cover of darkness. Predators such as frogs, snakes and large spiders lurk nearby, says Hongjamrassilp. “They are literally waiting to eat them along the river.”
And the shrimp can only survive on land that long. If the performing crustaceans get lost, they can dry up and die before returning to the river. Several times, Mr. Hongjamrassilp came across groups of lost shrimp lying dead on the rocks, their once translucent bodies baked pink.
Most successfully navigate upriver, however, and scientists have discovered other freshwater shrimp around the world that perform similar feats, scaling dams and even climbing waterfalls.
Leaving the water when swimming became difficult could have helped these animals spread to new habitats as they evolved, Hongjamrassilp said. Today the number of showing shrimp in Thailand seems to be decreasing. He believes tourist activity could be a factor, and learning more about the shrimp could help protect them.
The study’s authors made “some really excellent observations,” said Alan Covich, an ecologist at the University of Georgia who was not involved in the research. But more research needs to be done to understand why the Ubon Ratchathani shrimp move upstream and how far they travel, he said.
“The most surprising thing to me was that it attracted so many tourists,” said Dr. Covich. He knows of no other example of people who gather to appreciate a crustacean in the same way.
“We have cancer festivals, we have all kinds of things,” said Dr. Covich, “but in general, it’s people who eat them and don’t watch them move.”