A state-ordered shellfish closure due to toxic algae blooms was extended to Martha’s Vineyard late Sunday. All Island ponds and harbors are closed to shellfishing — except for bay scalloping, which is allowed.

Vineyard shellfish constables were notified by the state Division of Marine Fisheries late Sunday that the closure, initially ordered for the Buzzards Bay area, including Falmouth, Bourne, Fairhaven, Marion and New Bedford, had been extended to all areas south of Cape Cod. The Vineyard, Nantucket and Gosnold are all included in the closure.

The reason is a bloom of the diatom Psuedonitzchia. The plankton concentrates in the gut of shellfish and causes amnesiac shellfish poisoning, a severe form of gastrointestinal illness.

Bay scallops and sea scallops are exempt from the closure because the muscle of the shellfish is eaten, not the gut.

Oak Bluffs shellfish constable David Grunden said be believes it is the first closure in memory from this type of algae bloom.

“Certainly in the last 30 years or so,” the constable and biologist said. He said he had not done water tests to determine the concentrations of the algae, but he noted that it is present.

The cause of the bloom is unknown.

“If we knew that we would all be famous scientists,” Mr. Grunden said. But he said errant algae blooms have been occurring with more frequency on the Island in recent years.

The extensive shellfish closure comes at the very start of the bay scallop season on the Vineyard. Family scalloping opened in Edgartown and Oak Bluffs, with commercial scalloping due to begin in another week or two.