Purple and no bigger than a bottle cap, the Japanese shore crab doesn’t look threatening.

Yet Thursday’s announcement that this aggressive foreign invader had been found on the Maine coast as far north as Penobscot Bay should frighten biologists and fishermen alike, researchers said.

This tiny crab produces alarming numbers of young and feeds voraciously on immature shellfish, such as clams, mussels and perhaps even lobster.

“It’s been moving very rapidly and expanding into different habitats – I’m surprised that it’s gotten so far north, so fast,” said Robert Whitlatch, a professor of marine sciences at the University of Connecticut who has studied the crab for a decade.

Robin Hadlock Seeley, a marine biologist from Cornell University, discovered the most northerly crabs on Crescent Beach, near Owls Head, just last weekend.

The biologist is conducting a summer-long study to determine the Japanese shore crab’s northerly range. She already has recorded the crab’s presence at nine locations, including Biddeford Pool, Fort Popham and Port Clyde. Surveys of Hancock and Washington counties are scheduled for coming weeks, and Seeley isn’t hopeful.

“Last summer, I was shocked to see this alien crab … now, I’ve found them everywhere I’ve looked for them,” she said. “I’ve been very surprised at how abundant they are already.”

Maine scientists have feared the predator could make its way into the state since it first made news in 1988. It appeared in Long Island Sound, where biologists believe the crab was dumped into the Atlantic along with ballast water from an ocean liner.

The Japanese shore crab, also called the Asian shore crab, is common in the Pacific Ocean, with a native range that stretches from southern Russia to Hong Kong. Transferred to the Western Hemisphere, those latitudes are the same range as those from Cuba to the Gulf of St. Lawrence, Seeley said.

Japanese shore crabs have roughly square purple bodies and distinctly striped legs. They grow no larger than 31/2 inches across, but have no known predators. The tiny crabs are very aggressive and have chased larger species, such as green crabs, from prime habitat, Whitlatch said.

Seeley has been seeking the crab in its favorite habitat, cobble beaches with large boulders for shelter. In southern New England, however, scientists have found that the crab is able to adapt to a variety of environments.

“It likes rocks, but we’re also finding it in tidal areas and salt marshes,” Whitlatch said.

The Japanese shore crabs live only three years, but in an ecological twist to make up for their short lifetime, they can breed and lay eggs three or four times each year. A single female could lay as many as 450,000 eggs during her brief lifetime.

In Maine, population densities don’t yet exceed four crabs per square meter, Seeley said.

But in Connecticut, where the crab has been present for twice as long, densities of 200 crabs per square meter are not uncommon, Whitlatch said.

Every year, we find more of them,” he said.

Large populations of these crabs will threaten the shellfish aquaculture businesses along Maine’s coast, as their clam beds and mussel rafts provide a buffet for the ravenous crabs.

Laboratory tests have shown that a single Japanese shore crab will eat 13 mussels in a single day. The crabs prefer small prey, but could easily use their claws to crush the soft shells of rapidly growing farmed clams or mussels, Seeley said.

In Connecticut, local populations of blue mussels crashed once the Japanese shore crab established a large population, Whitlatch said.

“I’ve never seen them eat a lobster larvae, but I’m sure they could,” he added.

Questions about Japanese shore crab behavior abound. More data about this creature is needed to create a management strategy to save Maine from the ecological shifts that rocked Connecticut and Massachusetts once the crab moved in, Seeley said.