Scientists say an explosion of invasive green crabs that threatens Maine’s lucrative soft-shell clam industry appears to have tapered off, at least temporarily. The crabs took over hundreds of acres of Maine mudflats last year, pressing regulators and fishermen into action to slow the crustaceans’ rapid growth. But University of Maine at Machias marine ecology researcher Brian Beal said the number of crabs is at 10 percent of last year’s level at a key Freeport trapping site. He says anecdotes from around Maine’s coast suggest the same trend is happening elsewhere. Beal says the harsh winter may have culled population. The lower abundance coincides with a state effort to prompt crab harvesting. Maine changed rules last month to allow commercial fishermen to harvest and sell green crabs without a special license.