RENO, Nev. (AP) - Scientists already worried that Lake Tahoe's delicate ecosystem could be upset by foreign mussels also are studying whether another invasive mollusk is cause for alarm.

The number of Asian clams, first discovered on Tahoe's bottom in 2001, is "far more extensive" than previously thought, scientists said. Clam beds have been found along a stretch of Tahoe's southeast shore from South Lake Tahoe to the Zephyr Cove area.

Researchers are concerned that the clams could boost calcium levels in isolated areas of the lake, potentially allowing destructive quagga or zebra mussels to become established.

"It might be this existing invader is modifying the bottom environment," said Sudeep Chandra, an environmental science researcher at the University of Nevada, Reno. "One invader can facilitate another."

Officials at Lake Tahoe are increasingly concerned that quagga or zebra mussels, already thriving in parts of California and southern Nevada, could become established in the landmark lake. The quickly reproducing mollusks could profoundly alter the lake's ecosystem, clog water intakes and litter now-pristine beaches with sharp and stinking shells.

The threat was magnified in late August, when quagga mussels were discovered on the stern of a cabin cruiser about to be launched at South Lake Tahoe. The mussels apparently attached to the vessel while it was in Lake Mead earlier this summer.

"I think it's a pretty serious issue," said Mara Bresnick, chairwoman of the Tahoe Regional Planning Agency's governing board. "It could wreak havoc."

Unknown is whether the lake's waters contain sufficient calcium levels for the mollusks to grow their shells.

Chandra recently finished the first phase of a risk assessment into the potential mussel invasion of Tahoe. The work involved taking water samples from 83 locations along Tahoe's shoreline and measuring the water for calcium.

The next stage will involve laboratory tests to determine if mussels can survive in water taken from Tahoe.

Preliminary results of the assessment's first phase, Chandra said, are somewhat encouraging.

"There is some variability but overall, calcium levels seem relatively low," Chandra said.

But the few locations where calcium levels were higher were also some of the areas where Asian clams are found, Chandra said.

The finding raises the possibility that as the clams grow and die, they leach calcium into the water - making a suitable environment for quagga or zebra mussels if they are introduced.

"It creates a little hot bed zone," Chandra said. "Elevated calcium levels could be favorable to the quagga."

(Copyright 2008 by The Associated Press. All Rights Reserved.)