Marine biologist Paul Horsman of Greenpeace tramps through oil-clogged marshes on the east bank of the Mississippi River in Louisiana on Monday.

After weeks of staying mostly at sea, the Gulf oil spill is now washing up on the state's coasts—likely a devastating development, scientists say. (See pictures of ten animals at risk due to the Gulf oil spill.)

As the nurseries for much of the sea life in the Gulf of Mexico, coastal marshes are vital to the ecosystem and the U.S. seafood industry, according to Texas Tech University ecotoxicologist Ron Kendall.

It's much harder to remove the oil from coastal marshes, since some management techniques—such as controlled burns—are more challenging in those environments, Kendall said on May 12.

"Once it gets in there," he said, "we're not getting it out."

—Christine Dell'Amore

Pictures: Heavy Oil Seeping Into Louisiana Marshes

Thick pools of oil are now seeping into Louisiana's marshes, which are vital to the Gulf ecosystem and the U.S. seafood industry.