The invasive seaweed that clogs local shrimpers' nets probably came from oysters the U.S. bought from Japan after World War II, according to College of Charleston biology professor Erik Sotka.

Sotka is one of a team of professors who landed a $622,000 grant from the National Science Foundation to study the evolution of the "super strain" of the invasive seaweed species Gracilaria vermiculophylla.

Oysters had been over-harvested in the United States, Sotka said, and the country needed more of them. Unfortunately, he said, the Japanese oysters weren't cleaned before being shipped, and they may have brought the invasive seaweed with them.

Now the seaweed can be found in almost every estuary in the northern hemisphere, he said.

Sotka knows the species well. In 2010, he and a researcher at the University of Georgia landed another National Science Foundation grant to study its impact on the ecosystems of coastal Georgia and South Carolina.

They still are analyzing their findings, he said, but they already have learned some important things. For instance, he said, the seaweed actually is good for animals that eat it or live in it.

But the detritus from dead seaweed brings a lot of carbon and sediment to salt marshes. "And the carbon will fuel micro-algae that can create toxins." It could smother existing plants, he said.

"The shrimpers and the crabbers hate this thing," he said. "When they troll, they get ridiculous amounts of this invasive species."

It's especially bad off-shore, he said, where the seaweed gathers in clumps, like tumbleweeds under the water.

Vince Vierra, who worked for years on his stepfather Wayne Magwood's boat "Winds of Fortune," said seaweed is a big problem for shrimpers. It clogs a device on the net that ensures that sea turtles aren't caught in it, he said. That can reduce a 4-inch opening to a half-inch opening, which allows in fewer shrimp.

If there were no seaweed, "it would make it a lot easier for the guys on the boat doing the work," he said, "and it would make it a lot more profitable."

Sotka, postdoctoral researcher Stacy Krueger-Hadfield and College of Charleston biology professors Allan Strand and Courtney Murren will begin work on the new grant March 1. "It focuses on how we got here," Sotka said.

The researchers will travel to Asia to learn more about the species, and they will conduct historical research to learn more about where in the United States the Japanese oysters originally were dumped.

He thinks the strain likely came from the northern Pacific coast of Japan. People there farmed seaweed to make auger, a gelling agent used in foods and laboratories.

The team expects to find that the farmers cultivated a hearty strain that was resistant to herbivores, heat and bacterial infections, Sotka said. "We think it escaped from the farm."

Reach Diane Knich at 843-937-5491 or on Twitter at @dianeknich.