PORTLAND – In just over a decade, the Asian shore crab, a tiny purplish tidal-pool dweller that lives a peaceful existence in its native Japan, has established populations as large as 100 crabs per square meter on New England beaches.

A large green algae from Asia known as dead man’s fingers has spent the past 50 years overtaking native kelp beds where urchins and fish feed, developing entirely new ecosystems and often smothering native shellfish and other filter feeders.

If scientists have learned anything from these and the 33 other nonnative marine species found in Maine waters, it’s never to underestimate a creature bent on colonization, said scientists who led Maine’s Marine Invasion, a forum held at the University of Southern Maine Wednesday.

The event, sponsored by Maine Sea Grant and the Casco Bay Estuary Project, brought together researchers and policy-makers from throughout New England to discuss the marine invasive problem and seek solutions before it grows out of control.

“There’s an opportunity here. There isn’t yet a huge calamity, but there’s a threat,” said Karen Young, director of the Casco Bay Estuary Project.

Nationwide the introduction or at least discovery of invasive species has been on the rise since the 1980s, and $130 billion is spent annually to try to control the more than 400 species, said Judy Pederson of Massachusetts Institute of Technology Sea Grant in Boston and David Smith of Smith College in North Hampton, Mass.

But Pederson, whose research seeks to quantify the invasive problem in New England, said that what we know is probably only a small portion of what’s out there. Few invasive species draw public attention and research funding until they have become a liability, interfering with property values or commercial fisheries.

“It usually hits in the pocketbook,” she said.

Here in Maine, Pederson led a rapid assessment survey at three floating docks in Southern Maine last August and found 27 nonnative species. Many of the sea squirts, algae and crustaceans identified in this “snapshot research” are native to Europe or Asia.

Temperature and wave action can sometimes limit species spread, but marine invaders’ biological adaptations have proved as unpredictable as their arrivals, scientists said.

Shipping is the biggest means of species travel with creatures attaching themselves to the exterior of a ship’s hull or riding in the ballast water that large ships constantly take on and release to balance their weight.

More than 3 billion tons of ballast water is moved from port to port every year, and one Chesapeake Bay study indicated that more than 90 percent of ships had some hitchhikers in their ballast, Smith said.

“A single ship could dump more than a billion organisms in one shot,” he said.

Carelessness in the shipment of marine organisms for seafood sales and aquaculture operations also leads to more invaders. A recent small-scale study of seafood shipments indicated that a majority are transporting invaders of some kind, Smith said.

For example, an algae was introduced to New England on aquaculture oysters, and an invasive crab traveled cross-country hidden in the seaweed that cushioned a shipment of worms.

The aquarium trade, scientific research and ecological restoration projects also can spread invasive species on a smaller scale, Smith said, and all the potential vectors must be studied to give Maine any chance of preventing future introductions. Once a species has become established, it’s too late, he said. “These are biological pollutants,” Smith said. “They’re not like chemical pollutants that dissipate. They’re here to stay.”

Organizers of Wednesday’s forum hope to work together to promote research into Maine’s ecological vulnerability and the ecological consequences of marine invaders, Young said.

For more information, or to view the full forum proceedings, visit www.cascobay.usm.maine.edu