Aquatic species like shrimp can survive for decades in a state of suspended animation, only to revive under the right conditions. Now scientists have discovered, to their amazement, that tiny hatchlings have been born from 700-year-0ld water flea eggs in a Minnesota lake.
Photo of water flea courtesy of Paul D.N. Hebert/UNIVERSITY OF GUELPH
These water fleas, called Daphnia, are among the oldest-known to have revived after going into stasis. As Carl Zimmer reports in the New York Times, evolutionary ecologist Lawrence Weider began his search for tiny animals in lake sediment many years ago. At the time, he was trying to figure out whether the balance of species in the lake had changed over the past few decades of human habitation in the area.
To gather the animals, Dr. Weider and his colleagues took a boat out on the lakes. "It's a smaller version of a party barge, with a hole cut out of the deck," he said.
Through the hole, the scientists lowered a tube and pushed it about three feet into the sediment — deep enough, Dr. Weider thought, to gather water flea eggs a few decades old.
The scientists then went back to Oklahoma, sifted the cases from the mud, and started resurrecting the animals. They also extracted Daphnia DNA, giving them more data to analyze.
Only then did Dr. Weider get an estimate for the age of the sediment in South Center Lake from another lab.
"I said, 'Are you kidding me?'" said Dr. Weider.
The lab concluded that the bottom of the lake's sediment core was about 1,600 years old. The oldest eggs that Dr. Weider and his colleagues had successfully hatched were about 700 years old.
Luckily, Weider was already an expert in getting older eggs like these to hatch. He was able to study the 700-year-old creatures, comparing their DNA to those of species in the lake today. His study revealed that there had indeed been a dramatic transformation in the species of the lake over the past few centuries. Water fleas like his 700-year-olds were almost entirely gone, while a related species — rare hundreds of years ago — had become dominant.
Weider and his colleagues suggest that the shift is related to pollution. In the last hundred years, pollution from agricultural runoff filled the lake with phosphorous, and the water fleas that thrived were ones that didn't retain phosphorous in their bodies for long periods of time. The 700-year-olds had adapted to low-phosphorous waters, and their bodies retained tiny amounts of the mineral for weeks as a nutrient. Once the waters were rich in phosphorous, the best-adapted creatures were ones who didn't waste their energy holding on the mineral, and just chomped on more when needed.
Read more in the New York Times