Earthworms Shrink-wrap Themselves to Survive Long Periods of Drought

Earthworm researchers have learned that the garden-friendly creatures can survive periods of drought for as long as three weeks, an unexpected find for a creature that rely on water to keep their bodies from drying out and to create the slimy mucus that helps them move through soil.

When soil dries out, as it typically does in a place like eastern Colorado where water and organic matter is limited, earthworms go into a state of estivation, where their bodies bundle up into a tight knot to reduce the amount of surface area exposed to the soil.

"Then they'll seal themselves up in a chamber lined with their mucus. Inside that chamber, the humidity is higher so they don't dry out as the soil dries," said Jacob McDaniel, lead author of the earthworm study published in Soil Science Society of America Journal.

McDaniel and his colleagues suggest that the earthworm's ability to go into a state of estivation suggests they can survive for long periods in the soil.

For their study, the researchers sought to find out how long earthworms could survive in estivation and whether they would recover after an extended drought. McDaniel and his colleagues set up simulated drought conditions in pots filled with Colorado soil and worms.

Earthworms are naturally found in Colorado soil, but their distribution is limited mostly to areas close to water or with high levels of precipitation or irrigation. The researchers hypothesized that if the worms they collected from a well-irrigated alfalfa field near Fort Collins could survive a simulated drought, then they could survive in the dryland soils of eastern Colorado where their presence would improve soil conditions and promote mixing.

The experimental conditions put worms in four scenarios: constant water, and one, two and three weeks without added water. These conditions were based on rainfall patterns in the area where the soil was collected in eastern Colorado.

Over the course of the experiment the soil pots were surveyed for alive, estivated or dead worms. At the end of a simulated drought cycle, the dead worms were removed and water was added back to the soil and the drought cycle was reset.

The researchers learned that the length of drought stress affected the number of earthworms that died or went into estivation, with more worms entering estivation as the period of drought got longer.

In the three-week drought simulation, 14 percent of the worms died, significantly more than the other conditions. However, the earthworms that survived drought, even for three weeks, were able to recover after rewetting.

"If the soil did get rewetted, [the worms'] weight didn't change," said McDaniel. "They should be able to survive through and recover after a drought that matches our conditions."

The results led McDaniel to believe that earthworms in natural conditions could survive the same prolonged drought periods as those created in the experiment.

However, he noted that the conditions in the pots could turn out to be very different than the natural conditions.

"Future work needs to be done in the field setting with actual droughts instead of set time periods," he said.