BOLTON LANDING — A thumbnail-sized clam blamed for clouding the azure bays of Lake Tahoe high in the Sierra Nevada has now turned up in a mountain-ringed Adirondack lake renowned for its limpid, spring-fed waters. The invasive Asian clam, Corbicula fluminea, is known as the “golden clam” in the aquarium trade and the “good luck clam” in its native southeast Asia. But in Lake George, scientists call it an unwelcome invader that could cause ecological and economic harm. An intensive search launched after a few tiny clams were found at a sandy beach in August turned up no additional infestations, suggesting the invasion was discovered before it had a chance to spread across the 32-mile-long lake, a popular vacation spot. “The next step is to determine their age and concentration by taking sediment core samples,” said Sandra Nierzwicki-Bauer, director of Rensselaer Polytechnic Institute’s Darrin Fresh Water Institute. “Then we’ll deci de what to do about them.