PORTLAND – State officials say heightened levels of red tide have triggered a ban on shellfish harvesting along sections of the southern Maine coast.

Clams are off-limits from Cape Elizabeth to the New Hampshire line, except for softshell clams in the Scarborough River. Mussels, European oysters and carnivorous snails are restricted from New Hampshire to Pemaquid Point in Bristol.

Red tide, toxic algae naturally present in offshore waters, appears along the coast most summers. The toxin accumulates in shellfish and can be potentially deadly to people who eat tainted shellfish.

Coastal states shut down shellfishing beds when the algae are detected at dangerous levels.

Scientists from the Woods Hole Oceanographic Institution in Massachusetts reported last month that conditions are right for a potentially extensive red tide bloom this year.