Delaware University researchers report that invasive Asian Shore Crab numbers are declining and that there has been resurgence in native crab population in the region.

About ten years back, Delaware's mud crabs almost went extinct due to invasion of Asian crabs. The native population has now made a comeback and scientists are yet to figure out how.

"I was shocked," said Charles Epifanio, a scientist in the University of Delaware's College of Earth, Ocean, and Environment. The Asian crabs came to Delaware Bay in 1998 and soon took over the region.

 "When this (Asian shore) crab arrived here and took over the rocky intertidal habitat, it was remarkable. It wasn't gradual. It exploded," Epifanio, whose lab has been tracking Asian crab's population since several years, said in a news release.

The Asian shore crab Hemigrapsus sanguineus is indigenous to the western Pacific Ocean. Its territory ranges from Russia to Hong Kong and Japan. In the U.S. the crab has established its range from Maine to North Carolina.

Biologists conducted extensive research to understand how the invasive crabs rose to prominence in the U.S. They found that these crabs used water currents to reach different territories.

According to researchers, two out of three native mud crab species went extinct by 2001. The Asian shore crab population exploded and researchers thought that the native species would be wiped out in a few years.

A decade later, a follow-up project by a summer intern at the lab showed that the mud crabs were still living in Delaware.

"At first, I thought she had it wrong," Epifanio said about the intern's work.

Epifanio then re-conducted the research and found that the native crabs had indeed taken back their territory. Today, Asian shore crabs make up 25 percent of the crab population in rocky habitat near the mouth of Delaware Bay, which is a massive decline in population as these crabs accounted for 75 percent of crabs in the region in 2001.

According to Epifanio, the latest study doesn't just document the rise and fall of an invasive species, but also shows that there is a need for long-term monitoring efforts.

"We often close the book," Epifanio said in a news release. "What we're saying here is, if we come back a decade later, maybe they're not there any more."

The study is published in the Journal of Shellfish Research.