The culprit is the Vibrio parahaemolyticus (VIB'-ree-oh peh-ruh-hee-moh-LIT'-ih-kus) bacterium, which has occurred in Massachusetts waters since the 1960s. Why it's become a problem now, though, is a mystery.
Average monthly daytime water temperatures in the region rarely approach the 81 degrees believed to be the threshold that triggers dangerous Vibrio growth.
It has been only six years since Massachusetts was required to federally report Vibrio illnesses, so testing to predict and explain the problem is not fully developed.
The state's first closures were announced Aug. 30 for oyster beds along the shore south of Boston. The second closure, announced Monday, shut down oyster beds in Katama Bay at Martha's Vineyard.