NEW YORK ( TheStreet) -- Transocean ( RIG) and Anadarko Petroleum ( APC) shares were the biggest oil spill loser at the open on Thursday morning, with shares down roughly 3%.

On Wednesday afternoon, the Obama administration filed a civil lawsuit over the Gulf of Mexico oil spill, naming BP ( AIXG), Anadarko Petroleum ( VECO) and Transocean among the defendants.

While there were plenty of headlines on Thursday morning indicating that the suit was hitting BP shares hard -- and BP shares did fall in trading in London -- at the U.S. market open it was only Transocean and Anadarko taking a dive. BP share trade was exactly in line with the performance of the energy sector shortly after the open, close to flat.

Transocean was also downgraded to a hold by Morgan Stanley on Thursday morning. The rating change might explain some of the action in Transocean and Anadarko shares. The companies have seen their share prices double from oil-spill lows and some investors might be booking profits given the uncertainty of the oil spill litigation. Morgan Stanley noted the legal uncertainty in its downgrade of Transocean.

BP shares are up more than 40% since its oil spill low of $26.75.

Yet at a larger level, the lack of movement in BP shares on Thursday morning follows the logic that this opening legal move by the government was long expected. The fact that BP had long ago voluntarily waived its right to invoke the $75 million oil spill liability cap made the news that the government was asking the court to throw out that cap a non-issue, at least for BP.

Transocean had earlier this year tried to argue in Houston court using a 150-year old maritime law about sunk vessels that it's liabilities in the oil spill should be limited to $27 million, based on its insurance coverage for the value of the Deepwater Horizon rig when it sunk.

-- Written by Eric Rosenbaum from New York.

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