PORTLAND – Sea squirts have been found on 88 square miles of the ocean floor on Georges Bank but the colonies were not as thick as a year ago when they threatened to smother portions of the ocean floor, researchers said Monday.

Mats formed by the invaders covered double the amount of surface as last year’s survey, which was cut short by bad weather.

Scientists monitoring the spread of the sea squirts fear they could overrun the ocean’s bottom, covering everything from rocks to scallops and mussels. The mats could form a barrier between fish and their prey in the seabed.

Teri Frady, spokeswoman for the National Oceanic and Atmospheric Administration in Woods Hole, Mass., said there’s much to learn.

“We want to know the extent of the coverage. We want to know whether or not it’s colonizing other parts of Georges Bank. We want to know if it’s competing with species that rely on the [top] layer of the ocean floor,” she said.

Sea squirts are tunicates, a filter-feeding creature named for its flexible outer covering called a tunic. They’re found around the world, and they’re not always a threat.

However, the type of sea squirt called Didemnum found at Georges Bank off the Massachusetts coast tends to grow rapidly, and scientists don’t know how to stop its proliferation.

The U.S. Geological Survey and University of Rhode Island joined NOAA in conducting the latest survey over two weeks in August.

Researchers found sea squirts in one area that covered 67 square miles and another that covered 21 square miles. There were no sea squirts at three sites in Canadian waters with similar seabed conditions, researchers said.

When they colonize, sea quirts can form mats made of thousands of individuals covering the ocean floor with the potential to overpower ecosystems and overwhelm valuable shellfish beds, including oysters, clams, mussels and scallops.