St. Pete Beach, Fla. ? Six months after the rig explosion that led to the largest offshore oil spill in U.S. history, damage to the Gulf of Mexico can be measured more in increments than extinctions, say scientists polled by The Associated Press.

In an informal survey, 35 researchers who study the Gulf lowered their rating of its ecological health by several points, compared to their assessment before the BP well gushed millions of gallons of oil. But the drop in grade wasn’t dramatic. On a scale of 0 to 100, the overall average grade for the oiled Gulf was 65 — down from 71 before the spill.

This reflects scientists’ views that the spilled 172 million gallons of oil further eroded what was already a beleaguered body of water — tainted for years by farm runoff from the Mississippi River, overfishing, and oil from smaller spills and natural seepage.

The spill wasn’t the near-death blow initially feared. Nor is it the glancing strike that some relieved experts and officials said it was in midsummer.

“It is like a concussion,” said Larry McKinney, who heads the Gulf of Mexico research center at Texas A&M University-Corpus Christi. “We got hit hard, and we certainly are seeing some symptoms of it.”

Will the symptoms stick around or just become yesterday’s headaches? That’s the question that couldn’t be answered at a conference earlier this month of 150 scientists at a hotel on a Florida beach untainted by the spill. The St. Pete Beach gathering was organized by the White House science office to coordinate future research.

“There’s the sense that it’s not as bad as we had originally feared; it’s not that worst case scenario,” said Steve Lohrenz, a biological oceanographer at the University of Southern Mississippi. “There’s still a lot of wariness of what that long-term impact is going to be.”

Steve Murawski, the chief fisheries scientist for the National Oceanic and Atmospheric Administration, compared scientists’ research to a TV crime drama: “It’s the end of the story that counts, not all the steps along the way.”

We’re only at the 30-minute break in an hour-long drama, Murawski said.

And there’s a plot twist. Research findings already released have led scientists and the government to shift their focus from the sea’s surface to deeper waters and the ocean bottom.

A month-long cruise by Georgia researchers on the ship Oceanus reported oil on the sea floor that they suspect is BP’s but haven’t proven yet. Government officials still question whether there is oil on the sea floor, but the Georgia scientists say the samples smelled like an auto repair shop. They took 78 cores of sediment and only five had live worms in them. Usually they would all have life, said University of Georgia scientist Samantha Joye. She called it a “graveyard for the macrofauna.”

“The fact that there isn’t living fauna is a signal that something happened to these sites and these sediments,” Joye said in a phone interview Friday. “The horrible thing is they’ve been inundated with this oily material… There’s dead animals on the bottom and it stinks to high heaven of oil.”

University of South Florida’s Ernst Peebles said the oil on the floor “is undermining the ecosystem from the bottom up.”